The Pattern-Related Modifications of Contractile Response of Human Skeletal Muscle

The motor system of vertebrates produces the great variety of movements, but all forms of motor activity are based predominantly on two types of muscular contraction: the sustained maintenance of low tension (postural or tonic mode) and short bursts of activity (phasic mode). The problem of interrelation between two types of activity was discussed in physiology for many years. In this paper we want to discuss not the differences on the level of regulatory mechanisms but only on the level of muscles as actuators. It is known that some invertebrates have separate tonic and phasic muscles, which differ by innervation, structure of myofibrils, contractile proteins etc. Lower vertebrates more often have not tonic muscles but only tonic fibres in mixed muscles. Mammalian muscles consist of phasic (twitch) fibers only, but these fibers are heterogeneous (fast and slow).