Although it is commonly used the science of flexibility training is probably the least understood of all the fitness components. Why should it be done? How should it be done? When should it be done? This article will discuss the latest findings and recommendations to answer these questions.

What is Flexibility?

Flexibility is defined as the static maximum range of motion (ROM) available about a joint with the largest limiting factor being the structure of the joint itself. Therefore the ROM at each joint is dependant upon the individual joint but can also vary between each person.

The variability in the ROM is due to the elastic properties of the muscle and tendons attached across the joints. 'Stiff' muscles and tendons reduce the ROM while 'compliant' muscles and tendons increase ROM.

When stretching it is these elastic properties of the muscle that are altered and the passive tension of a muscle decreases, which means that the muscle lengthens and ‘gives’ a little. Over time regular static stretching will therefore increase the ROM at a joint, which is associated with the decrease in passive tension.

Experimentally, Toft et al (1989), found a 36% decrease in passive tension of the gastrocnemius (calf mauscle) after three weeks of regular calf stretches. This showed a relationship between static stretching and pssive tension which was further supported by McHugh et al (1998).

These researchers found maximum static hip flexion ROM was inversely correlated with the passive tension of the hamstrings during the mid-range of hip flexion. This therefore suggests that the muscle can be easily stretched through the mid ROM if maximum static ROM is improved.

This therefore suggests that static stretching is beneficial to sports performance.

Improving Performance Through Flexibility
Increased flexibility has been shown to be related to increased force production during stretch shortening movements (SSC). Stretch shortening movements are plyometric exercises involving an eccentric (lengthening) contraction followed immediately by a concentric (shortening) contraction.

However, in contrast, some studies have shown flexibility has little effect on running performance, which is quite odd because running is a kind of SSC movement.

Examples have been that pre-stretching increased static ROM in sprinters but had no effect on speed during the 100-yard dash and that stiffer leg muscles in endurance athletes may make them more economical in terms of oxygen consumption at sub max speeds.

The reason for these converse findings is probably related to the principle of specificity, which seems to underlie all sports training. The sprint and running studies above compared static ROM and stretches with performance, while the SSC research compared active stiffness with performance.

Holding a static stretch is a completely different action to those performed in sports, where joints are moving at fast speeds with muscle contracting while they are changing length. Therefore static ROM may not be an appropriate flexibility measurement to relate to performance.

However, active stiffness is a measurement of the force required to stretch a previously contracted muscle, and is therefore more sports-specific as the ease with which the muscle can change length will have an impact of the performance of a SSC movement.

Iashvili (1983) found that active ROM and not passive ROM was more highly related with sports performance. Active ROM is defined as the ROM that athletes can produce by themselves, which will usually be less than the passive ROM.

Passive ROM is the amount of static ROM available when assisted manually of by gravity. For example active ROM would be the height an athlete could lift their own leg in front of them whilst passive ROM would be the height a partner could lift this leg.

Athletes must be able to generate the movement themselves, which suggests that for improving sports performance it is active ROM that should be developed and not passive ROM. A sprinter must be able to achieve full knee lift and hip extension at toe-off in running to ensure a good technique and full stride length. This is achieved by having enough active ROM of the hip flexors and hamstrings.

Having further passive ROM, developed through static stretching, will not provide any extra benefit, especially since the joint angular speeds during sprinting are very high.

Improving ROM
It is suggested that, to improve sports performance, active stiffness should be reduced and active ROM should be improved. This is more specific than just static stretching, since sports involve both movement and muscle contractions.

No studies have been found that look at training methods to reduce active stiffness but studies have shown ways of improving active ROM. Alter (1996) suggests that the active ROM can be improved by any kind of active movement through the available active range of motion. Therefore by performing the movement through the full ROM.

For instance, weight-training exercises have been shown to improve active ROM (Tumanyan & Dzhanya, 1984) as well as ballistic stretches that can be performed at sport specific speeds. However ballistic stretches must be performed carefully as they can cause injury. Start with slow and small ROM gradually increasing speed and ROM.

Flexibility follows the specificity principle as with all other types of training. Therefore to improve active ROM you must use active and ballistic mobility exercises and not static stretching. Therefore activities such as shoulder circles and high knee skip are commonly used in warm ups.

These exercises are utilised in what is known as a dynamic warm up as they actively take the joints through all ROM and prepare them and muscles for the subsequent activity. It is believed this will be more beneficial for sports performance and injury prevention than static stretching.

Unfortunately there is little research to support this. Nevertheless, based on the fact that these exercises will be more specific than static stretches and that, through experience, I have found them to be very beneficial, I would strongly recommend them.

An example for warming up the lower leg before any running activity is to: Perform a 20 yard walk on the toes with legs straight, then walk 20 yards on the heels to warm up the tibialis anterior (muscle loacted at front of lower leg). This is then followed by 20 ankle flexion exercises on each leg (involves holding one leg up so ankle is free to move and then first fully flexing the ankle bringing the toes right up and then fully extending the ankle pointing the toes away).

This would then be followed by an exaggerated walk involving pulling the toes up on heel contact and pushing right up on to the toes at toe-off. Then finally, do the same while skipping, ensuring the full ankle movement is performed at sports-specific speed.

This warm up protocol can then be followed for the rest of the body taking joints through the full range of motion, starting slowly and then building the speed. These kind of exercises not only provide an effective warm up but will improve your active ROM and mobility for your sport.

Flexibility Effects on Injury
Insufficient ROM is more than likely to cause injuries such as muscle strains. Gleim & McHugh et al (1997) reviewed studies relating flexibility measures or stretching habits to injury incidence and found flexibility is extremely important for reducing injury in soccer players.

Three examples are that soccer players who stretched regularly suffered fewer injuries, tighter players suffered more groin strain injuries and there is a relationship between muscle tightness and knee pain.

Therefore there is a strong correlation between muscular tightness and increased muscle strain risks. Yet studies of endurance runners have not shown the same results. For instance in a study by Jacobs & Berson (1986), it was found that those who stretched beforehand were injured more often than non-stretchers.

Other studies have also found no relationship between flexibility or stretching on injury. However one study of sprinters found that 4° less hip flexion led to a greater incidence of hamstring strain.

The reason for these findings is the nature of each sport. Sprinting and soccer involve greater ROM and depend on good flexibility compared to endurance runners where ankle, knee and hip joints stay within the mid ROM throughout the full running cycle and therefore static ROM will have no effect.

Other biomechanical relationships between flexibility and injury also exist. An example is poor hip flexor flexibility which may lead the pelvis to tilt down. This increases strain on the lower back which will tighten the lower back muscles and possibly lead to back injuries.

A flexibility/injury relationship also exists for young adolescents. During the pubertal growth spurt, the tendons and muscles tighten dramatically due to the rapid bone growth. For young athletes this poor flexibility may lead to injury problems, especially tendinitis-type injuries such as Osgood Schlatters (knee problem).

Thus regular stretching is essential for young athletes. Remember it is biological age that counts, so children in the same team or squad may need to pay extra attention to flexibility at different times.

Too Much Flexibility
To prevent injury athletes should have a normal ROM of all the major muscle groups and correct postural alignment in the back. An example is hamstring mobility should allow for 90° of straight-leg hip flexion. Any further ROM should be developed only if analysis of the sport's movements suggests that extra mobility is required e.g. gymnastics.

Gymnasts require extreme ROM and for example a footballer with the flexibility of a gymnast would be at a greater injury risk. This relationship has been shown in American football players, with those who have over-developed hamstring flexibility suffering more from ACL strain. A likely reason is that the flexible hamstrings allow the knee to hyperextend (extended beyond full extension) more readily.

To protect against injury it is recommended to keep a normal ROM in each muscle group. However specific movements may require extra ROM in certain sports. A sprinter should have greater hamstring flexibility than an endurance runner but would not need as much ROM in the groin muscles as a tennis player, who perform lots of lateral movements.

Stretching
Coaches and physiotherapists need to know the normal ROM for each joint in relation to their sport. This will ensure athletes are not as suspecitble to injury.

To develop flexibility, research suggests (Alter, 1996) that static stretches should be held for at least 20 seconds, possibly up to 60 seconds, to gain a benefit and performed every day, or even twice a day. The stretches should not be painful and you should feel a mild stretch and maintain that position. If the tension eases, taking the stretch a little further and holding the new position will help gains in ROM.

Partner assisted stretches or PNF stretching will also increase flexibility. A PNF stretch involves applying an isometric (no movement) contraction against the stretch to invoke a greater relaxation response and thus enable further ROM to be reached. The protocol for PNF stretching is:

Partner takes the stretch to the initial point and holds that position for 20 about seconds.
The athlete then provides a strong 10 second isometric contraction pushing against the partner.
The athlete then relaxes, breathes out, and the stretching muscle should relax, allowing the partner to take it further.
This is repeated.
Research is however mixed on this method of stretching from being a very effective method to no real benefit is gained.

Mechanics of Stretching
The most important aspect of stretching is to choose an exercise with the correct mechanics. Static stretching’s purpose is to improve or maintain ROM of a particular muscle and therefore the mechanics of the exercise must ensure that the target muscle is being stretched effectively.

For example, the hamstring can be stretched by a toe touch position but this also requires back flexion and so the effectiveness of the stretch for the hamstrings is compromised. The best way to stretch the hamstrings mechanically is to place one foot in front of the other, lean forward from the hips and support your weight through your rear leg feeling the stretch in the front leg. Hamstrings are lengthened optimally in this stretch.

The message here is that you must ensure that any static stretching exercise you perform allows the target muscle to be lengthened effectively, without being limited by other structures.

Also make sure you are stable and there is no stress on any other joints. During the hamstring stretch discussed above, it is important to support one's weight with the hands on the rear leg so that the lower back is protected - leaning forward unsupported from a standing position places a great strain on the back.

Flexibility Conclusions
Methods of stretching still need researching further before definite answers can be given. However we should look at the methods and techniques we currently use and ask why do we do them?

One major and common technique used is static stretching as part of a warm up. Research and common sense suggest that static stretches will do little to help prevent injuries or improve muscle function before an activity. Instead a dynamic warm up involving mobility exercises that take the muscles through a full ROM at sport specific speeds should be used.

Static stretches are still necessary to develop maximum static ROM needed to avoid muscle strain injuries. Static stretches should be used in separate training sessions or in cool downs after training when muscles are warm.

These stretches must be effective, safe and stable in terms of their mechanics. As mentioned, a normal ROM in all muscle groups, plus any sports-specific ROMs, should be developed or maintained with static stretches following the above guidelines. If flexibility is well below normal, then PNF stretches may be considered to improve flexibility more quickly.

As in all aspects of training the principle of specificity is also important in flexibility training. For instance to develop strength no one would consider using only isometric contractions. Instead, coaches try to devise strength exercises that are as specific as possible, both in terms of speed and mechanics, to the sports-specific condition. That said, why do so many people use only static stretches at the maximum ROM to develop flexibility for sport which involves active motion through various ROMs depending on the movements?

Therfore it is important to consider both dynamic and static stretching to develop flexibility to reduce the risk of injury and improve performance.