The stretching or changing the length of a muscle and the rate of changing the length of the muscle are sensed by muscle spindles. These spindles are embedded in the regular (extrafusal) muscle fibres and are covered by connective tissue. When the muscle is lengthened or shortened so is the muscle spindle. The muscle spindles are innervated by Gamma motor neurons and the larger extrafusal fibres are innervated by Alpha motor neurons. There are also receptors that sense the stretch of tendons that will work with the muscle spindles.

When the stretch reflex is activated, the gamma nerve fibres send a signal to the spinal column and to the brain. To relieve the stretch, in simplistic terms, the gamma motor neurons activate the alpha motor neurons to contract the muscle (an example of this is your doctor testing your reflexes by tapping a little hammer on the tendon in front of the knee when you are seated). This stretch reflex obviously has an impact on how we stretch to improve flexibility. If an individual is bouncing vigorously when stretching, as often seen in Karate, the gamma nerve fibres may be triggered and actually activate the alpha motor nerves in the muscle to shorten and hence relieve the stretch. By vigorously bouncing an individual may be triggering a spinal column reflex that will work against a stretch and not result in the desired outcome.

Factors that influence flexibility
There are a range of genetic, physiological and external factors that influence an individual’s flexibility. These include;

• Muscle and joint specificity
• Age
• Gender
• Ethnic origin
• Temperature
• Reflex activity
• Central nervous system disease processes
• Antagonist muscle strength (dynamic flexibility)

The type of muscle, location and the type of joint will influence flexibility. Obviously when talking about muscles and flexibility we are looking at skeletal muscle and not cardiac or visceral muscles. A number of muscles will act over two joints (hamstrings, quadriceps, biceps, etc) and can be stretched by either joint. Some muscles work only one joint. The shape and type of muscles will also influence flexibility, such as fusiform (parallel or straplike), pennate (resembling the make up of a feather) and convergent (triangular) muscles.

The age of an individual is an interesting factor in regards to flexibility. What is known is that age does impact on flexibility. Young children and adolescents can lose flexibility during growth spurts, as children’s muscles and bones grow at slightly different rates. It is also known that children’s trunk-to-leg length ratio and arm length-body length ratio will change as they grow. A sit reach test for hamstring flexibility is not a very good indicator of flexibility for children if their scores are being measured in the same manner as an adult (i.e. how many cm past their feet they can reach). It should also be noted that these changes in a child’s proportions will effect their motor coordination.

When a young student Karateka is going through growth spurts their body proportions will alter. The student will continue to change their body shape until they have reached maturity. A student who is the tallest in a class may become the shortest in the class when fully matured and vice versa. It is likely that young Karateka will appear to struggle with stretches during their growth spurts. This may not be the entire story as their muscles and tendons may be just as flexible but their bones may have grown longer during a growth spurt or alternatively their legs or arms may have grown longer giving the impression of poorer reaching flexibility. The Karateka should be simply encouraged to continue with their stretching program within comfortable limits where the young student feels the stretch without it being painful.

It’s reasonable to believe that as an individual becomes older that they become less flexible. This may not be necessarily the case and there appears to be little research into this relationship to provide an evidence based conclusion. Most studies are investigating how stretching can improve older sedentary individuals. There appear to be no studies that investigate the flexibility of very active individuals or individuals who are already flexible. However there is research showing that that as an individual ages, it is beneficial if they hold a static stretch for longer. Generally a static stretch is maintained for 15 to 20 seconds, in older persons this stretch could be maintained for up to 30 to 40 seconds. They will still improve their flexibility by stretching daily with a shorter stretch, but the outcomes may not be as good as with a longer static stretch.

Gender can also influence flexibility. Women are generally more flexible than men, but this does not apply to all joints for example, studies have found no difference in flexibility of the soleus muscle - one of the muscles that makes up the calf - between men and women. There are a number of reasons for this variation in flexibility between men and women. In general terms there are several skeletal differences between men and women, yet it can be difficult to tell the sex of an individual by their skeleton due to the huge range of variations in both men and women, which includes; the carry angle (the position of the forearms when arms are by the side with palms facing forwards), hip width (most women have wider hips, but there will always be individual variations), the angle of the femur (upper leg bone) from the hip, etc. In general men have a greater muscle mass and a greater proportion of lean muscle than females. There are also hormonal variation between men and women; this is very obvious during pregnancy when the hormonal balance changes as does joint laxity.

Muscle and joint temperature affect flexibility. Heating will improve the extensibility of collagen, a major component of tendon and joint capsules. Heat will also facilitate the response of the major spinal reflexes by decreasing the sensitivity of muscle spindle reflex and the tendon stretch reflex. It is also believed that elevating a muscles temperature will improve its viscosity allowing it to stretch more easily. Heat will also improve the viscosity of synovial fluid and bursa (fluid that lubricates, provides oxygen and nutrients and provides some shock absorption to joints).

The balance between the body’s agonist (the muscle contracting) muscles and the antagonist is important for flexibility. Research has shown that it is important for the body’s agonist and antagonist muscles (e.g. flexor and extensor muscles like the quadriceps and hamstring muscle groups) to be proportionally balanced to avoid injury and optimise flexibility. Using the legs as an example it is important that the quadriceps are proportionally balanced to the hamstrings (this does not mean that they should be the exact same strength as the quadriceps are more powerful), as if they are imbalanced this is likely to an increased chance of musculoskeletal injury or overuse type injuries. In elite athletes, rectus femoris (one of the quadriceps muscles) tightness is associated with anterior knee pain overuse syndromes.

If a Karateka is going to participate in a weight training program as an adjunct to their karate then they would be advised to think carefully about maintaining a balance between the body’s muscles. A classic example of this is a mentality in some gyms to build massive pectoral (chest) muscles but to completely ignore the muscles that pull the shoulder blades back towards the spine; this will eventually give the person a gorilla-like appearance as their shoulders are pulled forward by these over developed muscles. Such a poorly developed program is likely to leave a Karateka at a disadvantage as they will eventually not be able to perform certain arm and hand techniques. It is also important that if a Karateka is participating in a gym program that they also increase their flexibility program. A strengthening program will increase muscle size and also tendon thickness which can cause inflexibility if stretching is not included in such a program.

As well as the balance between the agonist and the antagonist muscle groups, it is important to maintain balance between the