Stretching Debate Overview (02:10)
Drs. Malachy P. McHugh and Ian Shrier will debate the safety and function of stretching. Stretching can have negative effects on performance, and may not play a role in injury prevention. View definitions for stretching, flexibility, passive stiffness, passive compliance, and extensibility.
Muscle Viscoelasticity (02:01)
In a stress relaxation study, Dr. McHugh found holding hamstrings at a fixed ROM decreased resistance over time. Another study found maximum ROM is related to stiffness, not hysteresis. Stiffness measures the muscle’s elastic component; hysteresis measures the viscous component.
Acute vs. Chronic Adaptations in Flexibility (00:43)
Pre-participation stretching affects viscous properties, while inherent flexibility is a function of the elastic properties.
Acute Effects of Stretching (01:15)
Dr. McHugh presents data showing muscle groups need four to six minutes of stretching for effects to last at least 30 minutes.
Effect of Warm-Up vs. Stretching on Passive Muscle Tension (01:34)
In an animal model, a warmed up muscle reduced passive resistance to stretch by 52% while stretching reduced it by 42%. Chronic stretching increased the maximum ROM but did not decrease passive muscle stiffness. Chronic stretching somewhat decreased stiffness in plantar flexors.
Acute and Chronic Stretching Effects on Viscoelasticity (01:11)
Dr. McHugh concludes that acute viscous effects can be sustained if stretching time exceeds four minutes. Chronic elastic effects are difficult to achieve in the hamstrings; chronic stretching can lengthen the muscle during maximal hamstring contractions.
Stretching and Force (03:56)
Dr. Ian Shrier cites a bullfrog study showing that overstretching occurs with 20% of stretch and a Japanese quail study where stretch-induced hypertrophy increased muscle mass, force, and velocity. Acute MVC weakens human muscles while chronic stretching strengthens them and increases ROM.
Effects of Stretching on Injury Risk (02:46)
Dr. Shrier cites three studies showing a 20% injury reduction in men when stretching before exercise as part of an injury prevention program. Other research shows no gender difference.
Passive Warming Effects (01:08)
Dr. Shrier presents data showing muscles are easier to stretch after warming, but absorb less force. Any tissue tears when the force applied to it is more than it can absorb.
Fastest Man in the World (03:31)
Dr. Shrier describes the 150 meter race between Michael Johnson and Donovan Bailey where Johnson sustained a hamstring injury short of maximal ROM. This illustrates how absorbing force during eccentric activity can tear muscle.
Injury at the Sarcomere Level (01:11)
During eccentric activity, many sarcomeres shorten. Those lengthening are at the muscle tendon junction, where most muscle strains occur.
Zen Bamboo Paradox (01:39)
Dr. Shrier argues that the paradigm that flexibility prevents breaking is irrelevant to muscle function. Longitudinal force bends bamboo, but applying longitudinal force does not affect muscle direction. Similarly, stretching does not prevent damage in other materials.
Regular Stretching: Injury (00:44)
Data shows stretching on a regular basis reduces injury rates by 27%, but not stretching before exercise.
Effect of Stretching on Injury Risk (01:10)
Dr. McHugh points out a lack of hypotheses for why stretching might decrease injury risk. He proposes that stretching increases muscle-tendon unit compliance, decreases passive resistance to elongation, increases active ROM, and decreases susceptibility to muscle strain.
Acute Effects of Stretching on the Length-Tension Relationship (01:05)
Dr. McHugh cites three studies showing strength loss is only apparent at short muscle lengths.
Effect of Stretching and Warm-Up on Risk of Muscle Strain Injury (04:40)
Dr. McHugh discusses four studies looking at the effects of combining stretching and warm-up. He believes it can reduce muscle strains, if sufficient intervention time is allowed. The Ekstrand study involving soccer players showed 74% fewer strains in the intervention group.
Experimental Evidence for Warm-Up Benefit (01:03)
A study using isometric contractions to warm up rabbit muscles found increases in force to failure and length to failure. Dr. McHugh concludes that stretching and warm-up likely alter viscous properties and may protect against muscle strain.
Fatigued Muscle Absorbs Less Energy (01:30)
Dr. Shrier believes warm-up is important in injury prevention; Dr. McHugh believes stretching is important. Research shows fatigued muscles absorb less shock in multiple types of sports. Muscles pulled to failure at different rates represent different types of sports.
Acute Stretching: Force (Isokinetic) (01:34)
Dr. Shrier shows how Dr. McHugh used a graph lacking data to prove a point about injury occurring within normal ROM. Acute stretching absorbs less force while regular stretching absorbs more force.
Muscle Injuries vs. All Injuries (01:56)
Dr. Shrier criticizes Dr. McHugh's failure to explain why other injuries remain the same in studies while muscle injuries decrease. He argues that stretching damages and weakens muscles, increasing risk of injury to ligaments and tendons.
Dynamic Stretching (03:58)
Dr. Shrier argues that dynamic stretching more closely resembles a warm-up, and that warm-ups increase strength and prevent injury. Studies show time to peak force decreases more with dynamic than with static stretching.
Does Dynamic Stretching Decrease MTU Stiffness? (01:50)
Dr. McHugh found little data on whether dynamic stretching decreases muscle-tendon unit stiffness or injury risk. A muscle's greater capability of energy absorption is better able to avoid the amount of stress that produces injury.
Stretching: Conclusions (03:07)
Drs. McHugh and Shrier agree that stretching can cause strength loss; improper stretching can cause muscle strains; and regular stretching may increase strength. They disagree whether stretch-induced strength loss is significant; stretching is relevant to injury prevention; and pre-game stretching increases injury risk.
QA: Randomized Controlled Passive Stretching Trial (01:57)
Research shows longer time spent in passive stretching yields the greatest permanent improvement in ROM restoration. Dr. McHugh doubts he could find athletes willing to participate in a study. Dr. Shrier argues that dancers spend equal time stretching and warming up.
QA: Stretching in Growing Athletes (01:22)
Dr. Shrier argues that sitting in school causes a lack of flexibility in boys, rather than growth spurts. He recommends stretching and strengthening growing athletes.
QA: Adding Benched Players (01:28)
Dr. McHugh is unaware of any study looking at injury risk to basketball players put in the game "cold." A weak study found half time stretching decreased high school football injuries in the second half.
QA: Losing Muscular Protection (03:09)
Besides movement, muscles protect joints as shock absorbers. Dr. Shrier believes this accounts for injuries besides strain. Dr. McHugh says that in stretch shortening cycle movements, muscles work isometrically close to optimal sarcomere length to provide maximal force for joint stabilization.
Credits: Stretching: the Debate (00:25)
Credits: Stretching: the Debate
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