Physical fitness refers to a set of attributes, that a person has or can achieve, which relate to their ability to perform a physical activity (Pate et al., 1995). The degree to which people have these attributes can be measured using specific tests (Casperson et al., 1985).
It is important to be physically fit in order to reduce the risk of various diseases such as diabetes mellitus and cardiovascular disease (Pate et al., 1995).
To assess a person's physical fitness it is necessary to examine various elements of the person's health. These include cardio-respiratory fitness, body composition including regional fat distribution, muscular strength and endurance and flexibility (Pollock et al., 1998).
Flexibility is defined as the ability of a muscle to lengthen and allow a joint to move through its range of motion (Nelson and Bandy, 2004). Range of motion (ROM) is used to quantify joint mobility. It is the maximal range of the joint angle (Chung and Wang, 2008). Flexibility is an important aspect of physical fitness as good flexibility provides multiple benefits such as improving joint ROM and in enhancing muscular performance (Pollock et al., 1998). It has significant importance in functional performance (Singh et al., 2006).
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Another important aspect of physical fitness is muscular strength which refers to the amount of external force that a muscle unit can exert (Casperson et al., 1985).
1.2. Changes in flexibility and strength with age
Aging has an effect on the properties of muscultendinous units (Hewett, 2000). Skeletal musculotendinous units consist of both the muscle and the tendon component. The muscle is made up of parallel, multinucleated cells called fibres (See fig.1). Each of these fibers contains myofibrils which are also arranged parallel to each other. A myofibril is composed of sarcomeres which are the contractile units of the muscle. Zhong et al., (2007) reviewed the contractile units of the muscle fiber. The contractile unit is made up of the functional proteins actin and myosin. These interact with each other to generate force within the unit. Tendons form an integral part of the musculotendinous unit. Their function is to transmit the tensions, generated in muscles, to bones (Raab et al., 1988).
Collagen solubility in the musculotendinous unit declines with age, which results in reduced tensile strength and increased tendon rigidity and reduced parallel orientation of fibers (O'Brien, 1992). With age, the elastic properties of tendons change i.e. stretch and recoil function. This causes the muscle tendon to become stronger, stiffer and less resilient (Shadwick, 1990).
Aging can also have an effect on myosin filaments. Changes can occur to the head of the myosin structure which reduce its ability to bind to the myosin binding site on the actin filament. This will have an affect on myosin and actin interactions and thus on the contractile function of the musculotendinous unit (Zhong et al., 2007).
Bassey et al., (1988) studied the ROM of the shoulder joints of both older and younger subjects. They reported that older subjects had a shoulder flexibility of ~30° less than that of the younger subjects. The rate of reduction in shoulder flexibility with age amounted to approximately 10° per decade (Bassey et al., 1988).
Aging often results in a loss of tendon flexibility and reduced joint ROM - in shoulder, hip and ankle joints (Raab et al., 1988). This decrease in flexibility is related to both biochemical changes in the musculotendinous unit and mechanical factors in the underlying skeletal structure (Pollock, 1998).
Aging has also been associated with the condition sarcopenia. This is characterised by a loss of muscle mass due to a decrease in muscle fibre numbers and fibre atrophy. Sarcopenia also contributes to decreased muscle function with age (Deschenes, 2004).
Muscle strength - the force generating capacity of muscle - is another aspect of muscular ability that is reduced with age (Jan et al., 2005). This is due to the changes that occur in the muscle fiber such as a decrease in the amount of muscle fibers, shrinkage in their size, decrease in the number of functional motor neurones and a slowdown of reaction rates (Jan et al., 2005).
Another reason for the decline in muscle strength could be due to a reduction in Growth Hormone (GH) secretion by the pituitary gland. The highest levels of GH secretion occur during puberty which is when the increase in muscle mass and strength appears to occur (Hulthén et al., 2001). This GH secretion is said to decline around the age of 30 years. This results in a reduction in lean body mass and muscle strength (Zachweiga and Yarasheski, 1999).
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Fig.1.Shows the structure of skeletal muscle unit down to the sarcomere level
(Zhong et al., 2007)
1.3. Influence of gender on flexibility and strength
There are differences reported in the flexibility of the musculoskeletal system between genders. Flexibility patterns of males and females change during and after puberty (Hewett, 2000). Females have been shown to be more flexible than males. Singh et al., (2006) reported that women were significantly more flexible than men regarding functional performance, while men appeared to be stronger. They found that men were stronger regarding elbow and knee extension strength while women showed greater flexibility of their hamstrings, hips and spines.
Female hormones, such as progesterone, estrogen and relaxin may have an influence on muscle flexibility. Hewett (2000) found that estrogen can have an effect on the properties of the musculotendinous unit. It can cause increased tensile strength and reduce stiffness of tendons, making the muscle unit more flexible.
Strength of muscles differs between genders, as females have a smaller physiological cross-sectional area within their muscles (Jan et al., 2005). Males and females have similar muscle fiber distribution; however, females appear to have fewer and smaller muscle fibers (Pollock et al., 1998).
Hewett (2000) found that girls showed an increase in lower back, hip and hamstring flexibility after puberty, whereas the flexibility of these regions showed a steady decline when compared to their flexibility before puberty.
In contrast, differences in flexibility appear to be joint specific as Bassey et al., (1988) found that women had poorer shoulder flexibility when compared with their male counterparts. Men appeared to have greater flexibility of the shoulder joint.
1.4. Influence of stretching on flexibility
Warming up before a physical activity is also said to reduce the risk of occurrence of injuries to the musculoskeletal system (Shellock and Prentice, 1985). Stretching exercises are generally included as part of warm-up and cooling-down exercises (Witvrouv et al., 2004). Warm-up exercises are an essential part of physical preparation before an athletics event (Papadopoulos et al., 2005). There are many benefits associated with such warm-up exercises e.g. reduction in muscle viscosity, which increases the suppleness of the muscle and enhances its mechanical ability. Smith (1994) reported that warm-up exercises increased muscle and tendon suppleness, increased body temperature and aided free and coordinated movement.
There are three forms of stretching - static, dynamic and ballistic. Static stretching is performed by stretching the muscles to their greatest possible length and holding that position for a period of time - usually about five seconds (Anderson, 1991). Dynamic stretching involves moving the limb from its neutral position to end range, where the muscles are at their greatest length and then moving the limb back into its original position (Murphy, 1994). Ballistic stretching involves using the momentum created by repetitive bouncing movements to produce a stretch on the muscle (LaRoche et al., 2006). The bouncing uses the momentum of the extremity to lengthen and stretch the muscles (Nelson and Bandy, 2004).
Stretching itself is a preparatory activity which can improve flexibility and thus physical performance (Shellock and Prentice, 1985). When the muscle fibers are stretched there is an increase in sarcomere length (Edman et al., 1978).Stretching reduces the stiffness of the musculotendinous unit and allows for an increase in joint ROM (Kubo et al., 2001). This results in both an increase in the musculotendon unit length, resulting from actin-myosin complex relaxation, and a lasting increase through alteration in the surrounding extracellular complex relaxation (Pollock, 1998).
The visco-elastic property of a muscle refers to its stress-relaxation ability. This occurs when the unit is stretched and held at a constant length. The force at that length gradually declines causing the muscle to relax (Kubo et al., 2001). Stretching causes a change in the visco-elastic properties of the musculotendinous units which causes a decrease in muscle stiffness and thus improves flexibility. Less energy is required to move the limb and this can lead to faster muscle contraction (Shrier, 2004).
The primary effects of stretching involve the viscoelastic properties of tendons (Pollock, 1998). When the musculotendinous unit is stretched and held at that constant length, the passive tension at that length declines, also known as stress-relaxation. Repeated stretching of the unit can significantly reduce the peak passive tension (Kubo et al., 2001).
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Wiktorsson-Moller et al., (1983) found that performing stretching exercises resulted in increased flexibility with regard to hip, knee and ankle ROMs while O'Sullivan et al., (2009) showed that gentle aerobic warm-up increased hamstring flexibility. They showed that when static stretching was combined with aerobic warm-up the increased in hamstring flexibility was considerably higher. This increase in hamstring flexibility was short-term and decreased after 15 minutes of rest. Even though there was a decrease after 15 minutes, flexibility still remained significantly higher than at baseline after 25 minutes (O'Sullivan et al., 2009).
1.5. Influence of flexibility on sport
Flexibility is considered an essential element of normal biomechanical functioning in sport. Benefits of flexibility in sport include improved athletic performance, reduced injury risk, prevention or reduction of post-exercise soreness and improved co-ordination (Hopper, 2004). Sports, such as football or basketball, involve jumping and bouncing, so in order to participate properly and efficiently in these sports, the muscle tendon units need to be able to store and release a high amount of elastic energy (Witvrouw et al., 2004). Elastic energy refers to the energy that is reserved in order for the muscles to perform the stretch-shortening cycles (Komi and Bosco, 1978).
Witvrouw et al., (2003) reported that flexibility in the hamstring and quadriceps muscles were important factors in the development of muscle injuries. They found that those with hamstring flexibility of less than 90° had a higher risk of sustaining a musculoskeletal injury than those that had a greater flexibility (Witvrouw et al., 2003).
1.6. Influence of flexibility on injury prevention
A relationship between poor flexibility and subsequent injury has been established in several musculotendinous units. These include the hamstring tendons, Achilles tendon and plantar fascia. Injuries in these units have been related to either a decreased or low musculoskeletal flexibility (Pollock, 1998).
The most common injury in sport is muscle strain. Inflexibility has been identified as one of the primary factors that contribute to this muscle strain (Cross and Worrell, 1999). Muscle strain involves either partial or the complete tear of the muscle tendon unit (Weldon and Hill, 2003).
General pre-performance stretching programs have been shown to be effective in reducing both the severity and frequency of injuries (Smith, 1994). The ability of muscles to prevent excessive sarcomere lengthening and hence injury is dependant on contractile strength and is greatly reduced when the muscle is fatigued (Weldon and Hill, 2003).
Cross and Worrell (1999) found that there was a decrease in muscle strains, caused by athletic activity, when stretching exercises had been performed prior to the activity. They reported a significant decrease (48.8%) in the occurrence in lower extremity strains e.g. hamstring muscle strains, in subjects that performed stretching exercises. Increasing flexibility of the muscle tendon unit promotes better performance in sport and decreases the number of injuries sustained (Witrouw, 2004). When the muscle is stretched, less tension is applied to the musculotendinous unit. Therefore, the potential for muscle strain can be reduced by the elongation of the musculotendinous unit (Cross and Worrell, 1999).
1.7 Influence of flexibility on health
Lower back pain is a common health issue that can be associated with musculoskeletal inflexibility. Low back pain is pain that occurs at a frequency of at least once a week within a period of six months (Mikkelson et al., 2006).
Decreased muscle flexibility and trunk strength have been shown to be risk factors for lower back pain (Feldmann et al., 2001). The main muscle group related to lower back pain are the hamstrings. Poor hamstring flexibility and strength have been associated with the problem in both studies involving adults and adolescents. Feldmann et al., (2001) found that poor quadriceps flexibility also contributes to low back problems. However, tight hamstrings are the main contributors (Feldmann et al., 2001).
Mikkelson et al., (2006) investigated whether adolescent flexibility could predict the later occurrence of recurrent lower back pain. Hamstring flexibility, hip and back motility were all examined. The results of the 25 year follow up study showed that adolescents who had been physically active and had a high flexibility predicted a low occurrence of lower back pain in both males and females. This study suggested that enhancing strength and flexibility may be important components in the rehabilitation of patients with chronic low back pain (Mikkelsson et al., 2006).
Feldmann et al., (2001) also investigated the problem of lower back pain in adolescents to determine whether a previous history of back pain can be predictive of future back problems. They examined the flexibility of the quadriceps, hamstrings and the lumbar flexors and found a connection between tight hamstring muscles and low back pain.