Stretching out

[Guest guest]

BioMechanics October 2004

http://www.biomech.com/showArticle.jhtml?articleID=51201058

Stretching......out?

By: Cary Groner

B. Thacker, MD, isn't used to getting hate mail. As director of

the epidemiology program office at the Centers for Disease Control and

Prevention (CDC) in Atlanta, Thacker spends most of his time assessing

research, evaluating its implications for policy, and writing the

occasional paper. So he was taken aback this year when, as lead author

of a meta-analysis of studies on the impact of stretching on injury

risk,1 he started getting flamed.

People who advocate stretching were outraged at the paper's conclusion

that stretching appeared to be neither particularly helpful nor harmful,

according to Thacker. On the other side were those who felt the CDC had

gone weak in the knees and should have slammed the practice.

" At CDC we encourage physical activity, " Thacker said. " We want people

to do things that have been documented to prevent injury, which includes

interventions that improve balance, strength, and conditioning. We just

don't want people depending on stretching, thinking they'll be all

right. "

Healthy skepticism

Even defining stretching can be complex, because physical therapists and

trainers promote different approaches depending on their own

preferences, experience, and perceived needs of the athlete (see table,

page 22).

Thacker's paper makes clear why athletes and performers should be

skeptical of stretching's alleged benefits. For example, several

investigators found little evidence to support injury prevention by

stretching immediately before or after events, and determined that the

practice may negatively affect performance.2-5

Other studies have found that stretching decreases muscle strength for

anywhere from 10 minutes to 24 hours,6,7-a drop that increases injury

risk in itself-and that passive stretching adversely affects jumping

ability and plantar flexion.8,9 Increased flexibility also appears to

decrease running economy and peak performance.10-12

Duane Knudson, PhD, a professor of biomechanics at Chico State

University in Chico, CA, has conducted extensive research into

stretching and comes down on the side of the naysayers, even though

several of his own studies suggest that stretching has little effect one

way or the other.14-16 Knudson raised questions about the purported

merits of stretching in a 1999 paper in the Journal of Physical

Education, Recreation & Dance, as well.17 There he pointed out the

difference between static flexibility-measured by the limits of joint

motion-versus dynamic flexibility, which refers to how quickly

resistance (tension) increases in stretched muscles.

Regular stretching does increase static flexibility, which is important

in activities such as dance or gymnastics, where performers exceed

normal motion ranges. However, the gain may be due more to increased

" stretch tolerance, " or the ability to be comfortable in those extended

ranges, than to actual decreases in muscle stiffness, Knudson reported.

He also noted that the literature doesn't support the notion that

increases in static flexibility prevent injury. For one thing, more

mobile joints tend to be less stable, and the most flexible athletes

have higher injury rates.18 Some stretching techniques may also increase

risk by stretching ligaments or creating hazardous loading patterns. And

no research has documented ranges of motion related to minimized injury

risk.

Although little is known about the long-term effects of stretching on

dynamic flexibility, it does affect a muscle's viscoelastic properties

in the short run. What remains unclear is whether this is beneficial,

neutral, or harmful. Overall, Knudson concluded, " light to moderate

muscle actions of gradually increasing intensity are more appropriate

than stretching as warm-up activities for most sports. " He added,

however, that for those who need a range of motion beyond the

norm-gymnasts, dancers, or divers-stretching during the warm-up may be

necessary.

" I'm generally of the belief that unless you're doing a sport where you

need a lot of flexibility-or you're a very inflexible person-you don't

need to stretch, " Knudson said recently. " There is just an overwhelming

amount of evidence that you make yourself weaker. "

Eccentrics stand out

Other researchers have had similar results but are somewhat more

equivocal in their conclusions. Cramer, PhD, assistant professor of

kinesiology at the University of Texas at Arlington, has investigated

the effects of static stretching on the vastus lateralis and rectus

femoris, two muscles in the quadriceps group.19,20 " We found that static

stretching seems to decrease the muscle's ability to produce force at

both slow and fast velocities, " Cramer said.

According to new data he and his team presented in June at the annual

meeting of the American College of Sports Medicine, the acute effects of

static stretching may be mode-specific, affecting isometric and

concentric force production, but not eccentric force production.21

(Eccentric force would be, for example, extending the arm while holding

a barbell; concentric force would be raising the barbell to the

shoulder; isometric force would be holding it in place.)

" We know that there is this decrease in concentric and isometric force

production as a result of static stretching, " Cramer added, " but what we

really want to know is why. "

The question arises due to the intriguing discovery that stretching one

leg weakens both, implying that more than mechanical forces are at play.

One theory is that a central nervous system mechanism is invoked.

Cramer doesn't feel as if he has enough information to recommend

sweeping changes in training methods, regardless.

" Our studies suggest that these decreases in force production are so

small that this may be a nonissue in actual practice, " he said. " This

fall we're going to conduct a longer study to see if regular static

stretching (versus preexercise stretching alone) may avoid some of these

deleterious effects. "

Different approaches

Ian Shrier, MD, PhD, a past president of the Canadian Academy of Sport

Medicine and currently director of the epidemiology consultation service

at the Sir Mortimer B. Jewish General Hospital in Montreal, has

earned a reputation for speaking bluntly about the issue.

" Most people believe that if you stretch immediately before exercise, it

prevents injury and improves your performance, " Shrier said. " Both of

those are wrong. Lots of studies show that stretching right before

exercise decreases the amount of force you can produce and how high you

can jump. "

Though it doesn't seem to have much effect on running speed, he added.

In a 2000 article in the British Journal of Sports Medicine concluding

that preexercise stretching didn't prevent injury unless it was combined

with an overall warm-up,23 Shrier made several key points. For one, most

injuries occur during eccentric contractions rather than concentric

ones-and eccentric actions typically cause damage within the normal ROM,

suggesting that stretching isn't likely to prevent such injuries. He

also pointed out that stretching often increases pain tolerance, which

in itself can increase injury risk for the simple reason that athletes

may not be aware when they're hurting themselves.

But Shrier acknowledged that when stretching is done as part of a

comprehensive program, the situation changes.

" Where most people mess up is by lumping stretching before exercise with

stretching in general, " he said. " If you stretch regularly, but not

immediately before exercise, you actually increase your force, increase

the amount you jump, and increase your speed. My guess is that if you

stretch three or four times a week, you'll see benefits, and I

personally believe that in the future people will say that it prevents

injury-though the jury's out on that. "

Fitting the stretch to the activity

It's illustrative of the tenor of the broader argument that Malachy

McHugh, PhD, claims friendship with Shrier, then laughingly claims to

disagree with most of what Shrier says, then proceeds to agree with him

on several issues including this last one.

McHugh, director of research at the Institute of Sports

Medicine and Athletic Trauma at Lenox Hill Hospital in New York, has

published several studies of the effects of stretching on muscle

elasticity. One found that muscle stiffness may be a risk factor for

postexercise damage,24 but others have suggested that the relationship

of flexibility to performance may depend on which sport is studied.25

Nevertheless, McHugh thinks preexercise stretching is valuable as part

of an overall warm-up. He noted that most people stretch to avoid muscle

strains, and that little research has focused on its effects in sports

with a high incidence of strains, such as soccer or football.

The rationale for strain prevention is that stretching makes the muscle

more compliant, he said, which has implications for force production and

injury prevention.

" We think a more compliant muscle has a greater functional range of

motion, meaning the longer muscle should be able to produce more force, "

he said. " Usually at longer muscle lengths you lose strength because

there is less overlap of your cross-bridges-the force-generating part of

the muscle. But if you make a muscle a little more compliant, you can

get more cross-bridge overlap and generate more force at the longer

length. The muscles adapt rapidly, which is why a workout that makes you

sore one week doesn't do so the next. "

McHugh also offered an intriguing theory about the nature of strength

loss after an acute bout of stretching. In some sports, such as

sprinting, athletes must push their muscles almost to the point of

failure.

" Maximal performance and injury risk might be complementary, " he said.

" The safety window might get smaller and smaller. As a result, if

there's a small decrease in the amount of force you can produce, it

might have a protective effect. "

However, viewed in the context of reports from Cramer (that static

stretching doesn't reduce eccentric force production) and Shrier (that

most injuries occur during eccentric contractions), McHugh may need some

evidence to back this up.

Overall, McHugh believes that the activity should determine the

flexibility required-and that in many cases, stretching in some form is

essential.

" In a lot of sports, dance and gymnastics in particular, you have to

have the range of motion to perform your task, " he said. " If dancers

don't warm up and stretch, they won't be able to get their bodies into

the positions required. Hurdlers have to have flexible hamstrings or

they're not getting over the hurdle. But for a long-distance runner,

tighter hamstrings are actually beneficial. A lot of other sports fall

in between, and that's where the controversy lies. "

Another kind of performance

When it comes to activities such as dance, performance doesn't just mean

power and speed, of course; it carries connotations of artistry. And in

this, consensus emerges among the factions.

" Say a ballet dancer has a vertical jump of 23 inches, " Shrier said. " If

she stretches before her performance and it drops to 22 inches, nobody

in the audience is going to notice. But she might feel that it is

easier, less strenuous, and that she can hold her form longer. So even

though she isn't jumping as high, her performance is actually better.

And though I don't think stretching needs to be part of most warm-ups,

the rest of warm-up is extremely important. I'm not saying the ballerina

should go out there cold. "

Ruth agrees. Professor emeritus at the University of California,

Santa Cruz, has been a dancer and dance trainer all of her

professional life. She has published dozens of books, monographs, and

journal articles about training and injury prevention, and is a member

of the board of the International Association for Dance Medicine &

Science.

She is often shocked when she walks into dance studios to teach for the

first time and sees dancers stretching on a cold floor.

" I say, 'Please don't do that!' and explain that we'll stretch in the

middle and at the end of class, " she said.

According to , stretching must be an integral part of the warm-up

process.

" As long as the blood is coursing through the body, the oxygen is

flowing through the muscles, and the muscles are warm-then you can

stretch, " she said. " But not before. If you don't stretch and strengthen

together, you'll have a weak muscle. The strength must balance the

stretch if you want to control your movements. "

explained that dancers are at risk for injury partly because

dance demands such extended ranges of motion. Moreover, ballet dancers

typically do exercises such as developpes and grand battements that

develop their quadriceps, but may neglect the hamstrings. The resulting

strength imbalance puts extra stress on the knee joint.

" If the muscles are really stretched out, the ligaments may not be able

to protect the joints, " she said. " So you get unstable joints,

particularly knees, and you may get hyperextension and ligament tears. "

Proprioceptive neuromuscular facilitation stretches are now favored in

the dance community because they both strengthen and lengthen muscles,

said.

A holistic view

Because dancers do get injured, however, it's helpful to have physical

therapists available who understand the injuries, how best to rehab

them, and how to prevent recurrence. Rocky Bornstein, PT, was a dancer

for 25 years before going into practice at Westside Dance Physical

Therapy in New York.

Although Bornstein understands the necessity of isolating muscle groups

to measure biomechanical forces, as in the studies described earlier,

from a practical perspective she must consider her patients more

holistically.

" Dancers tend to have a lot of laxity in their joints, a lot of range of

motion, so in some cases strengthening may be more of an issue than

stretching, " she said. " If you have a joint that is not biomechanically

lined up, the muscles that move it will be working overtime to

compensate. Stretching the muscle without addressing the joint won't

help. "

For Bornstein, it's also critical to address how certain muscle groups

affect the whole body. These data are typically missing from clinical

trials because they are hard to measure, but perplexing results such as

those reported by Knudson (where complex motions seemed to nullify the

weakening effects of stretching) or by Cramer (where unilateral

stretching had bilateral effects) highlight the issue's importance.

" Muscle lengths affect other joints in the body, " Bornstein said.

" People with short hamstrings who don't stretch them are going to break

down somewhere else, probably in the lower back. We stretch our

pectorals not just to lengthen them, but to alleviate upper back or

cervical strain. It's allowing joints to move in the best way

possible-and that's not necessarily the joint directly attached to the

muscle. "

Consensus begins to emerge when it comes to long-term stretching

regimens.

" Dancers should stretch when their bodies are warm, " Bornstein said.

" That would not be right before you go out to perform. For that, you

want to increase your circulation, be warm and ready and viable.

Afterward, when the muscle has been worked really hard, is a better time

to stretch. "

As noted, Shrier supports this notion. " Think of stretching like weight

training, " he said. " If you do it regularly, you get stronger. It's just

that nobody does an exhausting workout right before they compete. "

" I think he's exactly right, " Knudson said when told of this remark.

" Studies of strength and weight training in combination with stretching

show that stretching doesn't diminish the effects. Some people who

stretched did a little better. "

It's apparent that the extent to which stretching is incorporated into

warm-ups will depend on the individual and the activity, but it's

reassuring to know that professional opinions may be converging. And who

knows?-as time goes on, Thacker at the CDC may even get a little

less hate mail.

Cary Groner is a freelance writer based in Northern California.

References

1. Thacker SB et al. The impact of stretching on sports injury risk: a

systematic review of the literature. Med Sci Sports Exerc

2004;36(3):371-78.

2. 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 Sports Med 1999;9:221-27.

3. Weldon SM and Hill RH. The efficacy of stretching for prevention of

exercise-related injury: a systematic review of the literature. Man Ther

2003;8:141-50.

4. Herbert RD and M. Effects of stretching before and after

exercising on muscle soreness and risk of injury: a systematic review.

Br Med J 2002;325:468-70.

5. Knudson DV, Magnusson P, McHugh M. Current issues in flexibility

fitness. Pres Council Phys Fitness Sports 2000;3:1-6.

6. Fowles JR, Sale DG, MacDougall JD. Reduced strength after passive

stretch of the human plantar flexors. J Appl Physiol 2000;89:1179-88.

7. Kokkonen JA, AG, Cornwell A. Acute muscle stretching inhibits

maximal strength performance. Res Q Exerc Sport 1998;69:411-15.

8. Cornwell AG et al. Acute effects of passive muscle stretching on

vertical jump performance. J Hum Mov Stud 2001;40:307-24.

9. McCue BF. Flexibility measurements of college women. Res Q

1953;24:316-24.

10. Craib MW, VA, Fields KB. The association between

flexibility and running economy in sub-elite male runners. Med Sci

Sports Exerc 1996;28:737-43.

11. Glem GW, Stachenfeld NS, JA. The influence of flexibility

on the economy of walking and jogging. J Orthop Res 1990;8:814-23.

12. Rosenbaum D and Henning EM. The influence of stretching and warm-up

exercises in Achilles tendon reflex activity. J Sports Sci

1995;13:481-90.

13. Pope RP et al. A randomized trial of preexercise stretching for

prevention of lower-limb injury. Med Sci Sports Exerc 2000;32(2):271-77.

14. Noffal GJ, Knudson D, Brown L. Effects of stretching the upper limb

on throwing speed and isokinetic shoulder torques. (Presented at ACSM

2004)

15. Knudson D et al. Acute effects of stretching are not evident in the

kinematics of the vertical jump. J Strength Cond Res 2001;15(1):98-101.

16. Knudson D, Mache M, Kotte J. Stretching has no effect on free throw

shooting accuracy. (Presented at ACSM 2004)

17. Knudson D. Stretching during warm-up: do we have enough evidence?

JOPERD 1999;70(7):24-27.

18. Knapik JJ et al. Strength, flexibility and athletic injuries. Sports

Med 1992;14(5):277-88.

19. Cramer JT et al. Acute effects of static stretching on peak torque

in women. J Strength Cond Res 2004;18(2):236-41.

20. Cramer JT et al. The acute effects of static stretching on peak

torque, mean power output, electromyography, and mechanomyography. Eur J

App Phys 2004 (in press).

21. Cramer, JT et al. Acute effects of static stretching on peak torque,

mean power output, EMG, and MMG during eccentric muscle actions. Paper

presented at 2004 ACSM meeting.

22. 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 Sports Med 1999;9:221-27.

23. Shrier I. Stretching before exercise: an evidence based approach. Br

J Sports Med 2000;34:324-25.

24. McHugh, MP et al. The role of passive muscle stiffness in symptoms

of exercise-induced muscle damage. Am J Sports Med 1999;27(5):594-99.

25. Gleim GW, McHugh MP. Flexibility and its effects on sports injury

and performance. Sports Med 1997;24(5):289-99l.

Stretching methods

Method-Description

Passive-Slow, sustained muscle lengthening with a partner

Static-Slow, sustained muscle lengthening held by subject for 15 to 60

seconds

Isometric-Static stretching against an immobile force

Ballistic-Rapid lengthening of the muscle using bouncing movements -(now

largely discredited due to increased injury risk)

Dynamic-Use of an antagonist muscle to help stretch another-(e.g.,

activating the quadriceps to help stretch the hamstrings)

PNF*-Stretching with or without a partner by contracting, then relaxing,

-the muscle to be lengthened

*Proprioceptive neuromuscular facilitation

Source: Adapted from reference 1, with changes based on information from

other sources.

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