Flexibility refers to a joint’s ability to move through its normal range of motion. This is important for carrying out daily activities as well as successful participation in many types of sports. Flexibility is an inherited trait that can be improved with training, and population-level differences in flexibility have been observed by gender (women are more flexible than men), race/ethnicity (blacks are more flexible than whites), and age (flexibility decreases with age).
Stretching before practicing or playing a sport has long been recommended to athletes on the grounds that it improves performance and prevents injuries. However, these benefits were asserted without supporting evidence. Recent research has suggested that pre-exercise stretching has an ambiguous effect on performance and may even be harmful. Stretching after exercise or outside of the exercise period may confer benefits.
The range of motion available without injury is determined by the flexibility of bodily tissues. It is influenced by factors, including inherent flexibility, injury history, and the presence or absence of a warm-up. Flexibility can be increased through a variety of techniques, including static, isometric, and ballistic stretching, as well as proprioceptive neuromuscular facilitation (PNF).
- Passive stretching is a type of slow, sustained muscle lengthening practiced with a partner
- Static stretching is slow, sustained muscle lengthening in which a stretch is held for 15 to 60 seconds
- Isometric stretching is static stretching against an immobile force
- Ballistic stretching is the use of a jerking or bouncing movement to rapidly increase the length of a muscle
- PNF is passive muscle lengthening practiced with a partner after an antagonistic muscle has been contracted
All of these stretching methods have supporters, but there is insufficient evidence to recommend one over another. However, injury concerns have pushed ballistic stretching out of favor, while passive and PNF stretching both necessitate the assistance of a skilled partner. Some actually believe that PNF stretching may increase the risk of injury.
Effects of Stretching on Flexibility and Performance
Some studies have found that PNF stretching improves flexibility more than other methods, while others have not. Passive stretches held for 15 or 30 seconds prove more effective in increasing flexibility than stretches held for shorter periods and as effective as stretches held for longer periods. In some studies, passive stretching was found to be more effective than dynamic stretching, but not in others.
Increased flexibility after stretching has been found to last 6 to 90 minutes. Some stretching programs lasting several weeks have produced flexibility increases that may last several weeks. Animal studies have shown that regular stretching can increase muscle hypertrophy (an increase in cross-sectional area). However, the conditions in these studies were far more severe than would be possible in a human workout program.
Stretching has also been linked to some negative side effects, such as a temporary decrease in strength and power (e.g., jumping ability). Some studies have found that increased flexibility reduces running economy, whereas other studies have found the opposite.
Stretching, Flexibility, and Injury
There is no clear relationship between flexibility and injury in general. In some cases, injury can be attributed to either too little or too much flexibility, or to a flexibility imbalance. A study of 303 military trainees discovered that the most flexible and least flexible men were the most likely to be injured. Individuals who are hypermobile (increased range of motion than normal) may be more vulnerable to injury. However, hypermobility can be advantageous in certain sports, such as gymnastics and ballet. As a result, there may be a trade-off between selecting and training those with the greatest ability to perform at a high level and the desire to minimize injury risk.
Stretching before exercise is frequently promoted as a way to avoid injury, but the evidence to support this recommendation is ambiguous at best. However, there is a substantial body of evidence supporting warm-up routines, which increase blood flow to the muscles and prepare the body for vigorous activity. Warming up, even if done passively, has been shown to improve performance. Previous studies have sometimes failed to separate the effects of warming up in general from those of stretching. This is because warm-up routines frequently include multiple activities, including stretching.
A review by Wood and colleagues revealed that, when it came to muscular injuries, stretching did have a positive effect on injury prevention. Amako and colleagues investigated the effects of static stretching on injury prevention in military recruits. They discovered that the difference in injury rates between the stretching and control groups was not statistically significant. However, when only muscle and tendon injuries were considered, the stretching group had a statistically significant reduction in injuries.
Hartig and colleagues also looked at military recruits and the effectiveness of flexibility exercises in reducing overuse injuries. They discovered that trainees assigned to protocols designed to improve hamstring flexibility had significantly greater flexibility than the control group. They also had a lower rate of lower extremity overuse injuries.
Pope and colleagues randomly assigned 1538 military recruits to either an intervention (warm-up and stretching before exercise) or a control group (warm-up before exercise) and followed them through 12 weeks of basic training. After controlling for a variety of variables such as age, height, weight, and date of enlistment, they discovered that while the stretching group had fewer injuries, the difference was not statistically significant. Pope and colleagues found similar results in a previous study that focused solely on calf stretching.
Although all studies were well designed, they had two limitations. First, military recruits are not representative of the general population. Second, military recruits may have increased their physical activity after enlistment, which may not apply to elite or recreational athletes.
Many aspects of flexibility and stretching are still unknown. For example, it is well known that some people are naturally flexible but never engage in stretching exercises, whereas others may engage in frequent stretching exercises but still show little improvement in flexibility, and the reasons for these differences are unknown. Furthermore, some people experience significant gains in range of motion after stretching, while others do not. This effect has been observed in both human and animal studies, and the change in range of motion after stretching varies depending on the muscles involved and the age of the participants.