Muscular sound and force relationship during isometric contraction in man

The contracting muscle generates a low frequency sound detectable at the belly surface, ranging from 11 to 40 Hz. To study the relationship between the muscular sound and the intensity of the contraction a sound myogram (SMG) was recorded by a contact sensor from the biceps brachii of seven young healthy males performing 4-s isometric contractions from 10% to 100% of the maximal voluntary contraction (MVC), in 10% steps. Simultaneously, the electromyogram (EMG) was recorded as an index of muscle activity. SMG and EMG were integrated by conventional methods (iSMG and iEMG). The relationship between iSMG and iEMG vs MVC% is described by parabolic functions up to 80% and 100% MVC respectively. Beyond 80% MVC the iSMG decreases, being about half of its maximal value at 100% MVC. Our results indicate that the motor unit recruitment and firing rate affect the iSMG and iEMG in the same way up to 80% MVC. From 80% to 100% MVC the high motor units' discharge rate and the muscular stiffness together limit the pressure waves generated by the dimensional changes of the active fibres. The muscular sound seems to reflect the intramuscular visco-elastic characteristics and the motor unit activation pattern of a contracting muscle.     

Recent data on respiratory control during muscular exercise in man

During the last two decades, numerous investigations have been conducted to determine the degree of involvement and mechanism of action of various factors responsible for ventilatory adaptation to exercise in human. The neuromechanical ventilatory system plays a role, exercise ventilation representing a compromise between the necessity of maintaining an adequate level of chemical arterial stimulus and of avoiding mechanical overload and, consequently, respiratory fatigue. The role of arterial chemoreceptors is essential and that of muscular chemoreceptors likely. The limb mechanoreceptors also play a role essentially via small nerve fibers on the other hand. To date, there is no experimental evidence of ventilatory reflexes elicited by pulmonary chemoreceptors. Further, the action of cardiopulmonary baroreceptors was proved only in extraphysiological conditions in animals. It is also possible that a feed-forward mechanism originating in the central nervous system plays an important role. In conclusion, the time course of pulmonary ventilation during exercise could be explained by the action of humoral and neurogenic stimuli.