Myths and Truths of Stretching -Individualized Recommendations for Healthy Muscles
Ian Shrier, MD, PhD; Kav Gossal, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 28 - NO. 8 - AUGUST 2000
In Brief: Stretching recommendations are clouded by misconceptions and conflicting research reports. This review of the current literature on stretching and range-of-motion increases finds that one static stretch of 15 to 30 seconds per day is sufficient for most patients, but some require longer duration. Heat and ice improve the effectiveness of static stretching only if applied during the stretch. Physicians should know the demands of different stretching techniques on muscles when making recommendations to patients. An individualized approach may be most effective based on intersubject variation and differences between healthy and injured tissues.
_________________________________________________________________
Despite limited evidence, stretching has been promoted for years as an integral part of fitness programs to decrease the risk of injury (1-6), relieve pain associated with "stiffness" (5), and improve sport performance (4-6). Many different stretching recommendations have come out of the medical literature, and new research has challenged some long-held concepts about common stretching practices. As a result, misconceptions and misinterpretations are common--not just among patients, but among healthcare professionals, as well. Thus, many clinicians are at a difficult crossroads when making sound recommendations to patients.
Proposed Stretching Benefits
Proposed mechanisms are thought to be either
(1) a direct decrease in muscle stiffness (defined as the force required to produce a given
change in length) via passive viscoelastic changes or (2) an indirect
decrease due to reflex inhibition and consequent viscoelasticity changes
from decreased actin-myosin cross-bridging. Decreased muscle stiffness
would then allow for increased joint range of motion.
New evidence suggests that stretching immediately before exercise does
not prevent overuse or acute injuries (7,8). However, results from
animal studies suggest that continuous stretching (ie, 24 hours per day)
over days, compared with intermittent stretching of only minutes per
day, outside of exercise periods may produce muscle hypertrophy (9-11),
which could theoretically reduce the risk of injury (9,12). However,
clinical research on stretching minutes per day is still inconclusive
(13,14), and more research is needed before definitive conclusions can
be made.
With respect to alleviating the pain associated with stiffness, the
weight of the evidence suggests that the decrease in stiffness is not as
important as the increase in "stretch tolerance" (15-17). Briefly, an
increase in stretch tolerance means that patients feel less pain for the
same force applied to the muscle. The result is increased range of
motion, even though true stiffness does not change. This could occur
through increased tissue strength or analgesia; however, increased
stretch tolerance that occurs immediately after stretching must be
caused by an analgesic effect because tissue strength does not increase
during 2 minutes of stretching. Unfortunately, evidence of a possible
analgesic effect is recent, and the underlying mechanism is unknown.
After weeks of stretching, increases in stretch tolerance could
theoretically occur because stretch-induced hypertrophy may increase
tissue strength (9-11), and/or an analgesia effect may be present.
A Search for Answers
Despite the controversies mentioned previously, stretching still
decreases pain and may provide substantial benefits if used under
appropriate conditions. However, the problem remains on how to choose an
appropriate stretching protocol. Most authors now believe ballistic
stretching (ie, bouncing) is dangerous (4-6,18). Time recommendations
for holding a stretch vary between 10 and 60 seconds (5,19-24).
Clinicians are also faced with choosing a method that may improve the
effectiveness of stretching: superficial heat, superficial ice, deep
heat, and warm-up (25-30).
To determine which stretching techniques are most effective, we reviewed
all studies cited on MEDLINE and SPORTDiscus that compared stretching
protocols for increasing range of motion. We chose range of motion as
the end point because it is the tangible objective most people use when
they stretch and because most studies have not addressed true muscle
stiffness.
We addressed the following questions: (1) How long and how many times
should a stretch be performed to obtain maximum benefit?, (2) Does
temperature alter the effectiveness of stretching?, and (3) Which
stretching method is most effective: static, ballistic, or
proprioceptive neuromuscular facilitation (PNF) stretching?
Our review includes only studies of range of motion involving healthy
muscle-tendon units--not diseased or abnormal capsular or ligamentous
restrictions such as adhesive capsulitis that may require a different
duration and frequency of stretching to increase range of motion
(31,32). In addition, we could not find any papers that investigated the
effects of stretching on injured patients. Any extrapolations of our
review to injured patients should be performed with caution.
Duration and Frequency
Before discussing the evidence on how long to hold a stretch, it is
necessary to explain the concept of viscoelasticity. Stretches must be
held to obtain maximum range of motion because muscles are not purely
elastic, but rather viscoelastic. An elastic substance such as a rubber
band lengthens for a given force and returns to its original length
immediately upon release. The effect is not dependent on time. On the
other hand, the flow and movement of a viscous substance such as
molasses depends on time (33). A viscoelastic substance exhibits both
properties. Therefore, muscle length increases over time if a constant
force is applied (creep, figure 1A: not shown), or the force decreases
over time if the muscle is stretched to a constant length and held
(stress-relaxation, figure 1B: not shown). When the force is removed,
the substance slowly returns to its original length. This is different
from plastic deformation, in which a material such as a plastic bag
remains permanently elongated even after the force is removed (33). Note
that though stretches also affect tendons and other connective tissue,
within the context of normal stretching, the stiffness of the overall
motion is mostly related to the least stiff section (ie, resting muscle)
and is minimally affected by the stiffness of tendons.
Patients are given many common protocols on stretch duration. In
summary, for both the immediate (within 60 minutes) and long-term (over
weeks) range-of-motion increases, research shows that one 15- to
30-second stretch per muscle group is sufficient for most people, but
some people or muscle groups require longer duration or more
repetitions. For immediate effects, stretching increases range of motion
through both a decrease in viscoelasticity and an increase in stretch
tolerance (ie, the analgesic effect previously discussed). With
long-term stretching, viscoelasticity remains constant and the increased
range of motion occurs only because more force can be applied to the
muscle before the subject feels pain (ie, increased stretch tolerance).
Immediate effects. The immediate effects of stretching on range of
motion have been studied in animals and humans. In isolated rabbit
extensor digitorum and anterior tibialis muscles that were stretched for
30 seconds, viscoelastic effects increased muscle length until the
fourth stretch (34). These results are consistent with those of human
hamstring muscles that showed decreased stiffness with five repeated
stretches (35).
However, Madding et al (24) found that increased hip abduction range of
motion did not differ between 15, 45, or 120 seconds of stretching.
Although these results may appear contradictory, viscoelasticity may
vary by muscle group. In support of this theory, Henricson et al (27)
found that muscles differed in their response to heat plus stretching.
If true, the optimal duration and frequency for stretching may also vary
by muscle group. Alternatively, range of motion in humans might be
primarily limited by pain (15-17). If this theory is true, any smaller
benefits obtained from decreased viscoelasticity with longer-duration
stretches would be overshadowed by the large changes in range of motion
from stretch-induced analgesia (stretch tolerance).
Long-term effects
The long-term effects of stretching on range of
motion have been studied in humans only. After 6 weeks, individuals
randomized to stretch for 30 seconds per muscle each day increased their
range of motion much more than those who stretched 15 seconds per muscle
each day. (A small increase in range of motion in the 15-second group
was not statistically significant.) No further increase was seen in the
group that stretched for 60 seconds (19).
In another study conducted over 6 weeks, the same researchers (22) found
that one hamstring stretch of 30 seconds each day produced the same
results as three stretches of 30 seconds. However, the results of Borms
et al (36) appear to contradict these findings because 10-second
stretches were as effective as 20- or 30-second stretches. Closer
inspection of Borms' data, however, reveals large variation among
individuals, and the study was performed over 10 weeks instead of 6
weeks. If one examines the data for trends, it appears that the
20-second and 30-second groups reached a plateau after 7 weeks, but the
10-second group increased gradually over the entire 10 weeks. Therefore,
30-second stretches are likely to achieve the maximum benefit quicker
(within 6 to 7 weeks) than 10-second stretches, but the two programs
eventually achieve similar results by 10 weeks.
Rationale for individualized programs. The above studies support the use
of 30-second stretches as part of a general fitness program. This may be
appropriate for group exercise classes in which one would want to use a
duration that would benefit most individuals--similar to the recommended
dietary allowance for vitamins and minerals. However, physicians and
physical therapists usually treat individuals rather than groups.
In the animal study (34) that showed maximum benefit with four
stretches, response varied depending on the individual experimental
muscle. Consequently, some muscles must have achieved maximum benefit
after two to three stretches, whereas others required five to six
stretches. In human long-term studies, some individuals gained much
range of motion with only 15 seconds of stretching, while others gained
very little with 45 seconds (24).
Finally, all of the current research has been done on healthy tissue.
Because muscle fatigue decreases viscoelasticity (37), it is reasonable
to predict that injuries (with torn tissue, deposition of scar tissue,
tissue reorganization, and muscle atrophy and weakness) will also change
viscoelasticity, though the direction of the change is not clear.
Therefore, healthcare professionals should be cautious about
extrapolating these results to injured athletes, who may require longer
stretches to increase range of motion. (See "Safety Concerns in
Stretching," below.)
Rather than give everyone the same stretching recipe, we prefer to
individualize our prescription to account for intersubject variation and
differences between healthy and injured tissues. We advise patients to
stretch until they feel a certain amount of tension or slight pulling
associated with this length, but no pain. As the stretch is held,
stress-relaxation occurs, and the force on the muscle decreases. When
patients feels less tension because of changes in viscoelasticity and an
analgesic effect, they can then simply increase the muscle length again
until they feel the original tension. The second part of the stretch is
held until patients feel no further increase.
If patients return for follow-up and have not gained any range of
motion, and they are not overstretching (forcing a stretch, causing
muscle spasm or pain), intersubject variability cited above may be the
reason, and the clinician should consider recommending that the stretch
be held longer. The effectiveness of this approach, however, remains to
be tested.
Temperature Effects
In summary, passive warming of a muscle before stretching or icing
during the stretch can be used to increase the range of motion but will
not prevent injury. Patients who include an active warm-up period prior
to stretching obtain the greatest range of motion. Contrary to popular
belief, warm-up performed without stretching does not increase range of
motion.
Most of the research in this area has been done on animals using passive
warming devices such as heat lamps. Research in humans often uses
activity to warm the muscle, but activity affects the muscle in many
ways--for example, calcium release and motor unit recruitment
patterns--besides simply raising the temperature. This may explain the
different results observed in animals and humans.
Passive warm-up and icing. Several studies examined the effect of
temperature on range of motion. When applied before a static stretch,
neither heat nor ice significantly affected the range of motion during
active knee extension--a test of hamstring range of motion--when
compared with stretching alone (38). Though heat alone did not improve
range of motion, stretch plus heat was superior to stretch alone with
respect to increases in hip flexion, abduction, and external rotation
(27); shoulder range of motion (30); and triceps surae range of motion
(25). Ice applied during a static stretch was the most effective method
for increasing range of motion during a passive static stretch (29), but
only when applied during the earlier stages of the stretch (30). Cold
application during PNF stretching did not improve range of motion above
the normal PNF technique (26).
In summary, despite some conflicting results, applying either ice or
heat during a static stretch increases the range of motion compared with
static stretch alone, but it has no effect during PNF stretches. Because
ice and heat both increase range of motion and decrease pain, but have
opposite effects on stiffness, the mechanism for the increased range of
motion is probably analgesia rather than decreased stiffness.
Active warm-up. Most people believe that the light activity performed
during warm-up will increase muscle temperature, decrease muscle
stiffness, and increase range of motion. Animal studies consistently
show a decrease in stiffness if the muscle or tendon is preheated
(39-41). However, the range of temperatures studied is usually outside
the normal physiologic range in humans (39-41).
In humans, the effectiveness of active warm-up to decrease stiffness
appears to be related to the type of warm-up exercise and the muscle
tested. For example, running appears to decrease the stiffness of the
calf muscles (42) but not the hamstring muscles (43); running had no
effect on range of motion in these studies. Stretching added after
warm-up decreases hamstring muscle stiffness (range of motion not
reported); however, the effect lasts less than 30 minutes, even if
exercise continues after stretching (43). In the only study that
measured the effect of cycling, hamstring or quadriceps range of motion
did not change, although ankle range of motion increased (stiffness not
measured ) (44). In another study, 15 minutes of cycling increased
passive hip flexion and extension (stiffness was not measured) (45), but
the pelvis was not properly stabilized during range-of-motion
measurement.
Although activity by itself does not have a major effect on range of
motion, studies consistently show greater range-of-motion increases
after warm-up followed by stretching than after stretching alone
(42,44). This research has probably been the basis for the
recommendation to always warm up before stretching. The problem is that
most people interpret it to mean that stretching before exercise
prevents injuries, even though the clinical and basic science research
suggests otherwise (7,8). A more precise interpretation is that warm-up
prevents injury (46-49), whereas stretching has no effect on injury
(7,8). Therefore, if injury prevention is the primary objective (eg,
recreational athletes who consider performance a secondary issue) and
the range of motion necessary for an activity is not extreme, the
evidence suggests that athletes should drop the stretching before
exercise and increase warm-up.
Which Method Is Most Effective?
In general, PNF stretching has resulted in greater increases in range of
motion compared with static or ballistic stretching (26,50-56), though
some results have not been statistically significant (57-59).
Of the different types of PNF techniques, the agonist-contract-relax
method (the hip flexors, including quadriceps muscles, actively stretch
the hamstrings, followed by a maximal quadriceps contraction and passive
holding) appears superior to the contract-relax method (muscle
contraction followed by passive stretching) (50,54-56), which appears
superior to the hold-relax technique (isometric contraction with
resistance gradually applied over 9 seconds) (50,54-56,60).
For those who prefer the simplicity of static stretching, one study (61)
reported that static stretching (continuous stretching without rest) is
superior to cyclic stretching (applying a stretch, relaxing, and
reapplying the stretch), whereas two studies (62,63) suggested no
difference. All of these studies involved stretching the hamstring
muscles, and methodological reasons for the discrepancy were not
apparent. More research is needed before definitive conclusions can be
made.
Take-Home Points
Many of the different proposed protocols for stretching have some
support from the published literature. The major points for clinical
practice are:
•Heat, ice, and warm-up all increase the effectiveness of stretching to
increase range of motion, but only warm-up is likely to prevent injury.
•Although one 30-second stretch per muscle group is sufficient to
increase range of motion in most healthy people, it is likely that
longer periods or more repetitions are required in some people,
injuries, and/or muscle groups.
•Individuals should determine a strategy
for themselves by simply holding a stretch until no additional benefit
is obtained.
•Though PNF stretching is the most effective technique for
increasing range of motion, the mechanism is an increase in stretch
tolerance, and the muscle actually undergoes an eccentric contraction
during the stretch. The increased analgesia may aid in performance but
theoretically increases the risk of injury when compared with static
stretches.
Safety Concerns in Stretching
Although the main objective of this article was to compare the
effectiveness of different stretching regimens to increase range of
motion, we also feel it is important to discuss safety. Follow-up
studies have not investigated the safety of different stretching
modalities, so all comments here and in the medical literature are
theoretical.
Some clinicians believe ballistic stretching is dangerous because the
muscle may reflexively contract if restretched quickly following a short
relaxation period (ie, eccentric or lengthening contraction) (1), and
eccentric contractions are believed to increase the risk of injury
(2,3). We agree with this concern, but it is important to add that
ballistic stretching is more controlled than most athletic activities.
Therefore, it is likely to be much less dangerous than the sport itself
if performed properly and not overly aggressively.
The original theory that proprioceptive neuromuscular facilitation (PNF)
techniques increase range of motion through reciprocal muscle
inhibition, thereby decreasing electromyographic activity, was first
disproved in 1979 (4,5) and again more recently (6,7). Muscles are
electrically silent during normal stretches until end range of motion
nears. Surprisingly, PNF techniques increase electrical activity and
muscle stiffness during the stretch (4,5,7), despite the observed
increase in range of motion. This means that the muscle eccentrically
contracts during the PNF stretch, which most clinicians would consider
more dangerous than electrically silent muscle. PNF and ballistic
stretching both cause an eccentric contraction, but PNF stretching
appears to have a more pronounced analgesic effect. From a safety
viewpoint, it does not seem prudent to "anesthetize" a muscle during or
immediately before it is required to perform higher-risk eccentric
contractions. The benefits of the greater increase in range of motion
should be balanced against a theoretical increase in the risk of injury.
(There are no data on risk of injury with PNF stretching.)
Suggested Readings
•Bandy WD, Irion JM, Briggler M: The effect of time and frequency of
static stretching on flexibility of the hamstring muscles. Phys Ther
1997;77(10):1090-1096 •Borms J, Van Roy P, Santens JP, et al: Optimal
duration of static stretching exercises for improvement of coxo-femoral
flexibility. J Sports Sci 1987;5(1):39-47 •Halbertsma JP, van Bolhuis
AI, Goeken LN: Sport stretching: effect on passive muscle stiffness of
short hamstrings. Arch Phys Med Rehabil 1996;77(7):688-692 •Hartig DE,
Henderson JM: Increasing hamstring flexibility decreases lower extremity
overuse injuries in military basic trainees. Am J Sports Med
1999;27(2):173-176 •Henricson AS, Fredriksson K, Persson I, et al: The
effect of heat and stretching on the range of hip motion. J Orthop
Sports Phys Ther 1984:110-115 •Leterme D, Cordonnier C, Mounier Y, et
al: Influence of chronic stretching upon rat soleus muscle during
non-weight-bearing conditions. Pflügers Arch 1994;429(2):274-279 •
Magnusson SP, Aagaard P, Larsson B, et al: Passive energy absorption by
human muscle-tendon unit is unaffected by increase in intramuscular
temperature. J Appl Physiol 2000;88(4):1215-1220 •McNair PJ, Stanley SN:
Effect of passive stretching and jogging on the series elastic muscle
stiffness and range of motion of the ankle joint. Br J Sports Med
1996;30(4):313-318 •Osternig LR, Robertson R, Troxel R, et al: Muscle
activation during proprioceptive neuromuscular facilitation (PNF)
stretching techniques. Am J Phys Med 1987;66(5):298-307 •Pope RP,
Herbert RD, Kirwan JD, et al: A randomized trial of preexercise
stretching for prevention of lower-limb injury. Med Sci Sports Exerc
2000;32(2):271-277 •Shrier I: Stretching before exercise does not reduce
the risk of local muscle injury: a critical review of the clinical and
basic science literature. Clin J Sport Med 1999;9(4):221-227
References
1.Stark SD: Stretching techniques, in The Stark Reality of Stretching.
Richmond, BC: Stark Reality Publishing, 1997, pp 73-80 2.Newham DJ,
McPhail G, Mills KR, et al: Ultrastructural changes after concentric and
eccentric contractions of human muscle. J Neurol Sci 1983;61(1):109-122
3.Hunter KD, Faulkner JA: Pliometric contraction-induced injury of mouse
skeletal muscle: effect of initial length. J Appl Physiol
1997;82(1):278-283 4.Markos PD: Ipsilateral and contralateral effects of
proprioceptive neuromuscular facilitation techniques on hip motion and
electromyographic activity. Phys Ther 1979;59(11):1366-1373 5.Moore MA,
Hutton RS: Electromyographic investigation of muscle stretching
techniques. Med Sci Sports Exerc 1980;12(5):322-329 6.Magnusson SP,
Simonsen EB, Aagaard P, et al: Mechanical and physical responses to
stretching with and without preisometric contraction in human skeletal
muscle. Arch Phys Med Rehabil 1996;77(4):373-378 7.Osternig LR,
Robertson R, Troxel R, et al: Muscle activation during proprioceptive
neuromuscular facilitation (PNF) stretching techniques. Am J Phys Med
1987;66(5):298-307
_______________________________________________________________________
Dr Shrier is director of the Consultation Service Centre for Clinical
Epidemiology and Community Studies at Sir Mortimer B. Davis-Jewish
General Hospital in Montreal. Dr Gossal is a staff physician in the
Department of Family Medicine at Saint Mary's Hospital at McGill
University in Montreal. Address correspondence to Ian Shrier, MD, PhD,
Centre for Clinical Epidemiology and Community Studies, Lady Davis
Institute for Medical Research, SMBD-Jewish General Hospital, 3755 Côte
Sainte Catherine Rd, Montreal, QB H3T 1E2; e-mail to
[email protected].
THE PHYSICIAN AND SPORTSMEDICINE - VOL 28 - NO. 8 - AUGUST 2000
In Brief: Stretching recommendations are clouded by misconceptions and conflicting research reports. This review of the current literature on stretching and range-of-motion increases finds that one static stretch of 15 to 30 seconds per day is sufficient for most patients, but some require longer duration. Heat and ice improve the effectiveness of static stretching only if applied during the stretch. Physicians should know the demands of different stretching techniques on muscles when making recommendations to patients. An individualized approach may be most effective based on intersubject variation and differences between healthy and injured tissues.
_________________________________________________________________
Despite limited evidence, stretching has been promoted for years as an integral part of fitness programs to decrease the risk of injury (1-6), relieve pain associated with "stiffness" (5), and improve sport performance (4-6). Many different stretching recommendations have come out of the medical literature, and new research has challenged some long-held concepts about common stretching practices. As a result, misconceptions and misinterpretations are common--not just among patients, but among healthcare professionals, as well. Thus, many clinicians are at a difficult crossroads when making sound recommendations to patients.
Proposed Stretching Benefits
Proposed mechanisms are thought to be either
(1) a direct decrease in muscle stiffness (defined as the force required to produce a given
change in length) via passive viscoelastic changes or (2) an indirect
decrease due to reflex inhibition and consequent viscoelasticity changes
from decreased actin-myosin cross-bridging. Decreased muscle stiffness
would then allow for increased joint range of motion.
New evidence suggests that stretching immediately before exercise does
not prevent overuse or acute injuries (7,8). However, results from
animal studies suggest that continuous stretching (ie, 24 hours per day)
over days, compared with intermittent stretching of only minutes per
day, outside of exercise periods may produce muscle hypertrophy (9-11),
which could theoretically reduce the risk of injury (9,12). However,
clinical research on stretching minutes per day is still inconclusive
(13,14), and more research is needed before definitive conclusions can
be made.
With respect to alleviating the pain associated with stiffness, the
weight of the evidence suggests that the decrease in stiffness is not as
important as the increase in "stretch tolerance" (15-17). Briefly, an
increase in stretch tolerance means that patients feel less pain for the
same force applied to the muscle. The result is increased range of
motion, even though true stiffness does not change. This could occur
through increased tissue strength or analgesia; however, increased
stretch tolerance that occurs immediately after stretching must be
caused by an analgesic effect because tissue strength does not increase
during 2 minutes of stretching. Unfortunately, evidence of a possible
analgesic effect is recent, and the underlying mechanism is unknown.
After weeks of stretching, increases in stretch tolerance could
theoretically occur because stretch-induced hypertrophy may increase
tissue strength (9-11), and/or an analgesia effect may be present.
A Search for Answers
Despite the controversies mentioned previously, stretching still
decreases pain and may provide substantial benefits if used under
appropriate conditions. However, the problem remains on how to choose an
appropriate stretching protocol. Most authors now believe ballistic
stretching (ie, bouncing) is dangerous (4-6,18). Time recommendations
for holding a stretch vary between 10 and 60 seconds (5,19-24).
Clinicians are also faced with choosing a method that may improve the
effectiveness of stretching: superficial heat, superficial ice, deep
heat, and warm-up (25-30).
To determine which stretching techniques are most effective, we reviewed
all studies cited on MEDLINE and SPORTDiscus that compared stretching
protocols for increasing range of motion. We chose range of motion as
the end point because it is the tangible objective most people use when
they stretch and because most studies have not addressed true muscle
stiffness.
We addressed the following questions: (1) How long and how many times
should a stretch be performed to obtain maximum benefit?, (2) Does
temperature alter the effectiveness of stretching?, and (3) Which
stretching method is most effective: static, ballistic, or
proprioceptive neuromuscular facilitation (PNF) stretching?
Our review includes only studies of range of motion involving healthy
muscle-tendon units--not diseased or abnormal capsular or ligamentous
restrictions such as adhesive capsulitis that may require a different
duration and frequency of stretching to increase range of motion
(31,32). In addition, we could not find any papers that investigated the
effects of stretching on injured patients. Any extrapolations of our
review to injured patients should be performed with caution.
Duration and Frequency
Before discussing the evidence on how long to hold a stretch, it is
necessary to explain the concept of viscoelasticity. Stretches must be
held to obtain maximum range of motion because muscles are not purely
elastic, but rather viscoelastic. An elastic substance such as a rubber
band lengthens for a given force and returns to its original length
immediately upon release. The effect is not dependent on time. On the
other hand, the flow and movement of a viscous substance such as
molasses depends on time (33). A viscoelastic substance exhibits both
properties. Therefore, muscle length increases over time if a constant
force is applied (creep, figure 1A: not shown), or the force decreases
over time if the muscle is stretched to a constant length and held
(stress-relaxation, figure 1B: not shown). When the force is removed,
the substance slowly returns to its original length. This is different
from plastic deformation, in which a material such as a plastic bag
remains permanently elongated even after the force is removed (33). Note
that though stretches also affect tendons and other connective tissue,
within the context of normal stretching, the stiffness of the overall
motion is mostly related to the least stiff section (ie, resting muscle)
and is minimally affected by the stiffness of tendons.
Patients are given many common protocols on stretch duration. In
summary, for both the immediate (within 60 minutes) and long-term (over
weeks) range-of-motion increases, research shows that one 15- to
30-second stretch per muscle group is sufficient for most people, but
some people or muscle groups require longer duration or more
repetitions. For immediate effects, stretching increases range of motion
through both a decrease in viscoelasticity and an increase in stretch
tolerance (ie, the analgesic effect previously discussed). With
long-term stretching, viscoelasticity remains constant and the increased
range of motion occurs only because more force can be applied to the
muscle before the subject feels pain (ie, increased stretch tolerance).
Immediate effects. The immediate effects of stretching on range of
motion have been studied in animals and humans. In isolated rabbit
extensor digitorum and anterior tibialis muscles that were stretched for
30 seconds, viscoelastic effects increased muscle length until the
fourth stretch (34). These results are consistent with those of human
hamstring muscles that showed decreased stiffness with five repeated
stretches (35).
However, Madding et al (24) found that increased hip abduction range of
motion did not differ between 15, 45, or 120 seconds of stretching.
Although these results may appear contradictory, viscoelasticity may
vary by muscle group. In support of this theory, Henricson et al (27)
found that muscles differed in their response to heat plus stretching.
If true, the optimal duration and frequency for stretching may also vary
by muscle group. Alternatively, range of motion in humans might be
primarily limited by pain (15-17). If this theory is true, any smaller
benefits obtained from decreased viscoelasticity with longer-duration
stretches would be overshadowed by the large changes in range of motion
from stretch-induced analgesia (stretch tolerance).
Long-term effects
The long-term effects of stretching on range of
motion have been studied in humans only. After 6 weeks, individuals
randomized to stretch for 30 seconds per muscle each day increased their
range of motion much more than those who stretched 15 seconds per muscle
each day. (A small increase in range of motion in the 15-second group
was not statistically significant.) No further increase was seen in the
group that stretched for 60 seconds (19).
In another study conducted over 6 weeks, the same researchers (22) found
that one hamstring stretch of 30 seconds each day produced the same
results as three stretches of 30 seconds. However, the results of Borms
et al (36) appear to contradict these findings because 10-second
stretches were as effective as 20- or 30-second stretches. Closer
inspection of Borms' data, however, reveals large variation among
individuals, and the study was performed over 10 weeks instead of 6
weeks. If one examines the data for trends, it appears that the
20-second and 30-second groups reached a plateau after 7 weeks, but the
10-second group increased gradually over the entire 10 weeks. Therefore,
30-second stretches are likely to achieve the maximum benefit quicker
(within 6 to 7 weeks) than 10-second stretches, but the two programs
eventually achieve similar results by 10 weeks.
Rationale for individualized programs. The above studies support the use
of 30-second stretches as part of a general fitness program. This may be
appropriate for group exercise classes in which one would want to use a
duration that would benefit most individuals--similar to the recommended
dietary allowance for vitamins and minerals. However, physicians and
physical therapists usually treat individuals rather than groups.
In the animal study (34) that showed maximum benefit with four
stretches, response varied depending on the individual experimental
muscle. Consequently, some muscles must have achieved maximum benefit
after two to three stretches, whereas others required five to six
stretches. In human long-term studies, some individuals gained much
range of motion with only 15 seconds of stretching, while others gained
very little with 45 seconds (24).
Finally, all of the current research has been done on healthy tissue.
Because muscle fatigue decreases viscoelasticity (37), it is reasonable
to predict that injuries (with torn tissue, deposition of scar tissue,
tissue reorganization, and muscle atrophy and weakness) will also change
viscoelasticity, though the direction of the change is not clear.
Therefore, healthcare professionals should be cautious about
extrapolating these results to injured athletes, who may require longer
stretches to increase range of motion. (See "Safety Concerns in
Stretching," below.)
Rather than give everyone the same stretching recipe, we prefer to
individualize our prescription to account for intersubject variation and
differences between healthy and injured tissues. We advise patients to
stretch until they feel a certain amount of tension or slight pulling
associated with this length, but no pain. As the stretch is held,
stress-relaxation occurs, and the force on the muscle decreases. When
patients feels less tension because of changes in viscoelasticity and an
analgesic effect, they can then simply increase the muscle length again
until they feel the original tension. The second part of the stretch is
held until patients feel no further increase.
If patients return for follow-up and have not gained any range of
motion, and they are not overstretching (forcing a stretch, causing
muscle spasm or pain), intersubject variability cited above may be the
reason, and the clinician should consider recommending that the stretch
be held longer. The effectiveness of this approach, however, remains to
be tested.
Temperature Effects
In summary, passive warming of a muscle before stretching or icing
during the stretch can be used to increase the range of motion but will
not prevent injury. Patients who include an active warm-up period prior
to stretching obtain the greatest range of motion. Contrary to popular
belief, warm-up performed without stretching does not increase range of
motion.
Most of the research in this area has been done on animals using passive
warming devices such as heat lamps. Research in humans often uses
activity to warm the muscle, but activity affects the muscle in many
ways--for example, calcium release and motor unit recruitment
patterns--besides simply raising the temperature. This may explain the
different results observed in animals and humans.
Passive warm-up and icing. Several studies examined the effect of
temperature on range of motion. When applied before a static stretch,
neither heat nor ice significantly affected the range of motion during
active knee extension--a test of hamstring range of motion--when
compared with stretching alone (38). Though heat alone did not improve
range of motion, stretch plus heat was superior to stretch alone with
respect to increases in hip flexion, abduction, and external rotation
(27); shoulder range of motion (30); and triceps surae range of motion
(25). Ice applied during a static stretch was the most effective method
for increasing range of motion during a passive static stretch (29), but
only when applied during the earlier stages of the stretch (30). Cold
application during PNF stretching did not improve range of motion above
the normal PNF technique (26).
In summary, despite some conflicting results, applying either ice or
heat during a static stretch increases the range of motion compared with
static stretch alone, but it has no effect during PNF stretches. Because
ice and heat both increase range of motion and decrease pain, but have
opposite effects on stiffness, the mechanism for the increased range of
motion is probably analgesia rather than decreased stiffness.
Active warm-up. Most people believe that the light activity performed
during warm-up will increase muscle temperature, decrease muscle
stiffness, and increase range of motion. Animal studies consistently
show a decrease in stiffness if the muscle or tendon is preheated
(39-41). However, the range of temperatures studied is usually outside
the normal physiologic range in humans (39-41).
In humans, the effectiveness of active warm-up to decrease stiffness
appears to be related to the type of warm-up exercise and the muscle
tested. For example, running appears to decrease the stiffness of the
calf muscles (42) but not the hamstring muscles (43); running had no
effect on range of motion in these studies. Stretching added after
warm-up decreases hamstring muscle stiffness (range of motion not
reported); however, the effect lasts less than 30 minutes, even if
exercise continues after stretching (43). In the only study that
measured the effect of cycling, hamstring or quadriceps range of motion
did not change, although ankle range of motion increased (stiffness not
measured ) (44). In another study, 15 minutes of cycling increased
passive hip flexion and extension (stiffness was not measured) (45), but
the pelvis was not properly stabilized during range-of-motion
measurement.
Although activity by itself does not have a major effect on range of
motion, studies consistently show greater range-of-motion increases
after warm-up followed by stretching than after stretching alone
(42,44). This research has probably been the basis for the
recommendation to always warm up before stretching. The problem is that
most people interpret it to mean that stretching before exercise
prevents injuries, even though the clinical and basic science research
suggests otherwise (7,8). A more precise interpretation is that warm-up
prevents injury (46-49), whereas stretching has no effect on injury
(7,8). Therefore, if injury prevention is the primary objective (eg,
recreational athletes who consider performance a secondary issue) and
the range of motion necessary for an activity is not extreme, the
evidence suggests that athletes should drop the stretching before
exercise and increase warm-up.
Which Method Is Most Effective?
In general, PNF stretching has resulted in greater increases in range of
motion compared with static or ballistic stretching (26,50-56), though
some results have not been statistically significant (57-59).
Of the different types of PNF techniques, the agonist-contract-relax
method (the hip flexors, including quadriceps muscles, actively stretch
the hamstrings, followed by a maximal quadriceps contraction and passive
holding) appears superior to the contract-relax method (muscle
contraction followed by passive stretching) (50,54-56), which appears
superior to the hold-relax technique (isometric contraction with
resistance gradually applied over 9 seconds) (50,54-56,60).
For those who prefer the simplicity of static stretching, one study (61)
reported that static stretching (continuous stretching without rest) is
superior to cyclic stretching (applying a stretch, relaxing, and
reapplying the stretch), whereas two studies (62,63) suggested no
difference. All of these studies involved stretching the hamstring
muscles, and methodological reasons for the discrepancy were not
apparent. More research is needed before definitive conclusions can be
made.
Take-Home Points
Many of the different proposed protocols for stretching have some
support from the published literature. The major points for clinical
practice are:
•Heat, ice, and warm-up all increase the effectiveness of stretching to
increase range of motion, but only warm-up is likely to prevent injury.
•Although one 30-second stretch per muscle group is sufficient to
increase range of motion in most healthy people, it is likely that
longer periods or more repetitions are required in some people,
injuries, and/or muscle groups.
•Individuals should determine a strategy
for themselves by simply holding a stretch until no additional benefit
is obtained.
•Though PNF stretching is the most effective technique for
increasing range of motion, the mechanism is an increase in stretch
tolerance, and the muscle actually undergoes an eccentric contraction
during the stretch. The increased analgesia may aid in performance but
theoretically increases the risk of injury when compared with static
stretches.
Safety Concerns in Stretching
Although the main objective of this article was to compare the
effectiveness of different stretching regimens to increase range of
motion, we also feel it is important to discuss safety. Follow-up
studies have not investigated the safety of different stretching
modalities, so all comments here and in the medical literature are
theoretical.
Some clinicians believe ballistic stretching is dangerous because the
muscle may reflexively contract if restretched quickly following a short
relaxation period (ie, eccentric or lengthening contraction) (1), and
eccentric contractions are believed to increase the risk of injury
(2,3). We agree with this concern, but it is important to add that
ballistic stretching is more controlled than most athletic activities.
Therefore, it is likely to be much less dangerous than the sport itself
if performed properly and not overly aggressively.
The original theory that proprioceptive neuromuscular facilitation (PNF)
techniques increase range of motion through reciprocal muscle
inhibition, thereby decreasing electromyographic activity, was first
disproved in 1979 (4,5) and again more recently (6,7). Muscles are
electrically silent during normal stretches until end range of motion
nears. Surprisingly, PNF techniques increase electrical activity and
muscle stiffness during the stretch (4,5,7), despite the observed
increase in range of motion. This means that the muscle eccentrically
contracts during the PNF stretch, which most clinicians would consider
more dangerous than electrically silent muscle. PNF and ballistic
stretching both cause an eccentric contraction, but PNF stretching
appears to have a more pronounced analgesic effect. From a safety
viewpoint, it does not seem prudent to "anesthetize" a muscle during or
immediately before it is required to perform higher-risk eccentric
contractions. The benefits of the greater increase in range of motion
should be balanced against a theoretical increase in the risk of injury.
(There are no data on risk of injury with PNF stretching.)
Suggested Readings
•Bandy WD, Irion JM, Briggler M: The effect of time and frequency of
static stretching on flexibility of the hamstring muscles. Phys Ther
1997;77(10):1090-1096 •Borms J, Van Roy P, Santens JP, et al: Optimal
duration of static stretching exercises for improvement of coxo-femoral
flexibility. J Sports Sci 1987;5(1):39-47 •Halbertsma JP, van Bolhuis
AI, Goeken LN: Sport stretching: effect on passive muscle stiffness of
short hamstrings. Arch Phys Med Rehabil 1996;77(7):688-692 •Hartig DE,
Henderson JM: Increasing hamstring flexibility decreases lower extremity
overuse injuries in military basic trainees. Am J Sports Med
1999;27(2):173-176 •Henricson AS, Fredriksson K, Persson I, et al: The
effect of heat and stretching on the range of hip motion. J Orthop
Sports Phys Ther 1984:110-115 •Leterme D, Cordonnier C, Mounier Y, et
al: Influence of chronic stretching upon rat soleus muscle during
non-weight-bearing conditions. Pflügers Arch 1994;429(2):274-279 •
Magnusson SP, Aagaard P, Larsson B, et al: Passive energy absorption by
human muscle-tendon unit is unaffected by increase in intramuscular
temperature. J Appl Physiol 2000;88(4):1215-1220 •McNair PJ, Stanley SN:
Effect of passive stretching and jogging on the series elastic muscle
stiffness and range of motion of the ankle joint. Br J Sports Med
1996;30(4):313-318 •Osternig LR, Robertson R, Troxel R, et al: Muscle
activation during proprioceptive neuromuscular facilitation (PNF)
stretching techniques. Am J Phys Med 1987;66(5):298-307 •Pope RP,
Herbert RD, Kirwan JD, et al: A randomized trial of preexercise
stretching for prevention of lower-limb injury. Med Sci Sports Exerc
2000;32(2):271-277 •Shrier I: Stretching before exercise does not reduce
the risk of local muscle injury: a critical review of the clinical and
basic science literature. Clin J Sport Med 1999;9(4):221-227
References
1.Stark SD: Stretching techniques, in The Stark Reality of Stretching.
Richmond, BC: Stark Reality Publishing, 1997, pp 73-80 2.Newham DJ,
McPhail G, Mills KR, et al: Ultrastructural changes after concentric and
eccentric contractions of human muscle. J Neurol Sci 1983;61(1):109-122
3.Hunter KD, Faulkner JA: Pliometric contraction-induced injury of mouse
skeletal muscle: effect of initial length. J Appl Physiol
1997;82(1):278-283 4.Markos PD: Ipsilateral and contralateral effects of
proprioceptive neuromuscular facilitation techniques on hip motion and
electromyographic activity. Phys Ther 1979;59(11):1366-1373 5.Moore MA,
Hutton RS: Electromyographic investigation of muscle stretching
techniques. Med Sci Sports Exerc 1980;12(5):322-329 6.Magnusson SP,
Simonsen EB, Aagaard P, et al: Mechanical and physical responses to
stretching with and without preisometric contraction in human skeletal
muscle. Arch Phys Med Rehabil 1996;77(4):373-378 7.Osternig LR,
Robertson R, Troxel R, et al: Muscle activation during proprioceptive
neuromuscular facilitation (PNF) stretching techniques. Am J Phys Med
1987;66(5):298-307
_______________________________________________________________________
Dr Shrier is director of the Consultation Service Centre for Clinical
Epidemiology and Community Studies at Sir Mortimer B. Davis-Jewish
General Hospital in Montreal. Dr Gossal is a staff physician in the
Department of Family Medicine at Saint Mary's Hospital at McGill
University in Montreal. Address correspondence to Ian Shrier, MD, PhD,
Centre for Clinical Epidemiology and Community Studies, Lady Davis
Institute for Medical Research, SMBD-Jewish General Hospital, 3755 Côte
Sainte Catherine Rd, Montreal, QB H3T 1E2; e-mail to
[email protected].