The nature of the stabilizing functions of skeletal muscles

The paper defines the term coordination interval (CI), suggesting that it should replace the term electromechanical delay (EMD), if this parameter is determined in vivo for a muscle functioning in a biomechanical system with considerable mobility, but not in a kinematic pair with one degree of rotational freedom (DOF). It was adopted that CI is the time elapsing between the moment of exertion of the resultant muscular force to the surroundings by the ending of the biokinematic chain with many DOFs, and the onset of myopotential in the muscle involved in its actuation. Tested at the same time was the utility efficiency of the system: man--unstable external object with adjustable mobility, as well as the participation and coordination patterns of the upper extremity muscles engaged in stabilizing and motor activities. Twelve adult men, students of physical education, took part in the experiment. The experimental set-up was in the form of a 3.08 m long physical pendulum with the inertial moment of 574 kg m2 and with an exchangeable ending that served as a handle. Three endings were used with zero, one or two DOFs in relation to the pendulum shaft. The subjects of the test were told to push the pendulum ending with maximum force using the upper extremity, so as to impart to it the highest possible kinetic energy, irrespective of its stability type and mobility. The muscles straightening the upper extremity and stabilizing the wrist joint were tested in the EMG measurements. As a result of the experiment, it was found that (1) the utility efficiency of the organism in relation to the force and muscular power exerted on the surroundings decreases to 80 and 60%, respectively, if a driven object requires stabilization, and (2) the patterns of muscular synergism expressed by sequences and frequencies of muscular activation by the nervous system, on the grounds of CI, undergo characteristic changes if a driven object becomes unstable.