PCSA of a muscle is calculated using the formula
Muscles with small pennation angle and long muscle fiber size (e.g. hamstrings and tibialis anterior) provide large excursion and higher contractile velocity. However muscles with greater pennation angle, small muscle fiber length and large muscle physiological cross-sectional area (e.g. soleus) produce higher force at the expense of excursion and contractile velocity [13].

An alpha motor neuron in the anterior horn of the spinal cord and the muscle fibers innervated by the axon of this neuron form a motor unit. The motor unit is the smallest subunit that is under neural control in a muscle. The number of the muscle fibers in a motor unit is related with the function of that muscle. Muscles contributing fine motor movement have less muscle fibers in a motor unit than the muscles exerting crude movement. The alpha motor neuron axon gives branches to synapses with the muscle fibers of a motor unit. When an alpha motor neuron is evoked, the electrical signals pass to the muscle fibers via neuromuscular junctions. These electrical signals may generate an action potential in the membrane of the muscle fibers. The sum of the action potentials of the muscle fibers forms a motor unit action potential (MUAP). The result is a mechanical twitch of tension. Tension increase in a muscle occurs via consecutive mechanisms. First, stimulation rate increases in the related motor unite, and then the other motor units are activated (recruitment). Reverse sequences cause decreases of tension in the muscle. While performing movements the human body tries to spend little energy as possible to conserve energy expended. Entire motor units in a muscle are not necessarily activated during performing a movement. Needed tension force for a movement determines the amount of motor unit recruitment via neuromotor control mechanism. When tension force requirement increases, a greater number of motor units will be activated. First, the smallest motor units are recruited, and then large motor unites are recruited. This is called as “size principle” [2].

Motor units are divided into two main types. The alpha motor neuron innervating the muscle fibers determines the type of motor unit. Smaller slow-twitch motor units (type 1) are called tonic units. They produce twitches with lower peak tension in a long time. The large fast-twitch motor units (type 2) are called phasic units. They have larger peak stress with a short time to peak. Muscles performing strong and phasic contractions have type 1 and type 2 muscle fibers equally. The gastrocnemius is an example to this type of muscles. During the terminal stance phase of the gait, gastrocnemius muscle contracts strongly in a phasic manner. Type 1 fibers especially exist in the tonic postural muscles. An example of such muscles is soleus. It creates tonic contraction during the second rocker of the gait [13].

As noted above, muscles contain contractile active cells (muscle fibers) and passive elements of connective tissue. The force-length curve of a muscle consists of a combination of the force-length characteristics of both active and passive structures in the muscle (Fig. 6.2). Interactions between myofibrillar structures at sarcomere level determine the shape of the force-length curve. Resting sarcomere length is 2.5 μm. At this situation, maximum number of cross-bridges occurs between the filaments; hence, maximum tension is generated by the active elements. As the muscle undergoes lengthening or shortening, cross-bridges between the filaments are reduced, and the tension produced by muscle fibers decreases. Connective tissue of the muscle whether in parallel or series has significant effects on the tension generated. Connective tissue that surrounds the contractile elements is called as parallel elastic component and behaves like an elastic band. It is in a slack state when the muscles are at rest or in a shorter position. In this situation parallel elastic component does not constitute to total tension. As the muscle length increases, elastic component causes increases in the tension. This increase occurs first slowly then quickly. All connective tissues, including tendon, in series with the contractile elements, are called the series elastic component. During the isometric contraction the series elastic component is under tension and undergoes limited stretching. Total length of the muscle and tendinous structure does not change as the muscle shortens during this kind of contraction. Shortening of the muscle fibers stretches the serial elastic component of the muscle. This is called internal shortening. When the muscle isometrically contracts, stretched series elastic component stores large amounts of energy. As an example of this, the gastrocnemius muscle contracts and stores energy in the terminal stance phase of the gait to generate propulsion power during push off. Series elastic component stores large amounts of energy at the terminal stance phase during contraction of the gastrocnemius muscle. This contraction causes only little plantar flexion movement at the ankle, because the body weight is still supported by the ipsilateral lower extremity and there is considerable amount of dorsiflexor moment generated by ground reaction force. In the preswing phase of the gait, considerable part of the body weight is transmitted to the contralateral limb. Hence, the dorsiflexor moment of ground reaction forces around the ankle is reduced significantly. Then, energy stored in the series elastic component causes rapid plantar flexion at the ankle joint and propulsion is provided. Muscle length is shortened during concentric contraction. Besides, intramuscular tension also decreases. There is an inverse relationship between muscle shortening velocity and tension [13].