Humans can be described as adapting-evolving living machines. The basic componenet of adaptation and evolution is skelatal muscle which is vital for human motion and a primary target for hormonal interactions.
The vast diversity in muscle strentgh and endurance reflects evolutuanary genetic diversity in the human populatio and can be attributed to exercise which are mediated by endocrine system.
We can summarize basic structure of skeletal muscle. Each end of the muscle is connected to a tendon, which connects to the bone mediating the body’s lever system. The muscle is made up of fasciculi containing the muscle fibres, which are made up of myofibrils and ultimately the myofilaments actin and myosin. Connective tissue plays an important role at every level of organization, providing structural stability and contributing the elastic component to human movement when utilizing the stretch-shortening cycle.
A single motor neuron can activate multiple muscle fibres; a motor neuron and all the fibres that it innervates are collectively referred to as a motor unit. The signal for muscle contraction originates in the motor cortex and travels down the motor neuron.
Muscle fibres are classified according to their functional capabilities and enzymatic profiles. Historically, fibres have been referred to as ‘slow twitch’ and ‘fast twitch’ based on their contractile properties. More recently, histochemical techniques have been developed to classify fibres according to their myosin ATPase isoform.
Slow forms of myosin ATPase (type I) are associated with slow contraction and relaxation times and are more resistant to fatigue. Alternately, fast forms of myosin ATPase (type IIA and type IIX) are associated with fast contraction and relaxation times and high fatigability.
The strength of a contraction can be increased in two ways: (1) by increasing the neural firing frequency; and/ or (2) by increasing the number of muscle fibres recruited.
The latter is dictated by a concept called the ‘size principle’. The size principle ensures that motor units are recruited in a particular order during exercise. This order ensures that the correct amount and type of motor units are recruited to produce a force for a given task. For example, during a low-force activity (e.g. distance running), primarily type I motor units are recruited. However, if a
runner begins sprinting, additional motor units are recruited to provide sufficient power.
