Electromyographic hyperactivation of skeletal muscles by time-modulated mechanical stimulation

Current skeletal muscle conditioning is typically performed by application of a steady force (load). However, as confirmed by whole-body-vibration applications, the involvement of neuromuscular reflex due to load variations plays an important role in muscular conditioning. Therefore, in this paper we present a prototype for muscular conditioning where the loading force, which is generated by an electromagnetic actuator, is modulated over time during the exercise. The force modulation is controlled by a dedicated control unit implemented on a personal computer. The determination of the transfer function of the mechanical transmission to the user confirms that the force modulation is efficiently transferred up to 40 Hz. A preliminary validation involved the electromyographic analysis of the muscle activation. A 29 Hz sinusoidal force modulation around different baseline loads was applied to both biceps and triceps of 12 subjects. Force modulation resulted in the hyperactivation of the muscle electrical activity, which was estimated as the root mean square of the electromyogram, for all baseline loads ranging from 20 to 100 % of the maximal voluntary contraction. These promising results confirm the important role of reflex for optimal muscle training and motivate further research to realize a dynamic load control based, for instance, on electromyographic features.