Effects of pre-motion electromyographic silent period on dynamic force exertion during a rapid ballistic movement in man

Abstract

The effects of pre-motion silent period (PSP) on dynamic force exertion were studied in ten healthy subjects performing ballistic elbow extensions. The experiments were designed to evaluate the significance of mean differences between the averaged dynamic force curves of two groups: PSP-presence groups and PSP-absence groups. The presence of PSP was judged quantitatively and automatically by means of a newly developed method using statistical analysis. The results indicated that there were two effects of PSP on dynamic force exertion: one was a reducing effect, observed prior to the movement; the other was a reinforcing effect, observed in the first part of the ballistic movement. The duration of the reinforcement was significantly correlated with the duration of the reducing effect of PSP. The findings suggested that the reinforcement of dynamic force may be produced by the pre-stretch of agonistic muscles caused by prior force reduction due to PSP occurrence. The fact that PSP plays an important role in dynamic force exertion suggests that PSP may be incorporated in the central motor control system designed to interrupt the background activity, to stretch the agonist and to reinforce the dynamic force.

Initial conditions influence the characteristics of ballistic contractions in the ankle dorsiflexors

“…The incidence of such silent period varied across subjects (range: 7.4-76.8%) at a rate similar (range: 5-60%) to the silent period in the agonist muscle at the transition of a sustained and ballistic contraction (Mortimer et al 1987;Walter 1988;Aoki et al 1989;Van Cutsem and Duchateau 2005). In the two conditions, the variability of silent period incidence among subjects suggests that it is a learned behavior (Nishizono and Kato 1987;Walter 1989;Moritani 1993;cf., however, Mortimer et al 1987;Zehr et al 1997) rather than an automatic component of a movement program.…”

“…The current results showed that a silent EMG period between the end of the antagonist activity and the onset of the ballistic agonist activity induced by a brief period of relaxation had a similar influence on the rate of torque development than a brief silent period in agonist EMG activity inserted between a sustained and ballistic agonist action. In both conditions, the absence or reduction in EMG activity indicates that most motor neurones innervating both agonist and antagonist muscles are in a resting state before the onset of the ballistic contraction, which enables a briefer agonist activation than during a ballistic contraction superimposed to a sustained submaximal contraction (Conrad et al 1983;Tanii 1984;Mortimer et al 1987;Aoki et al 1989;Van Cutsem and Duchateau 2005). A briefer EMG activity at the onset of a ballistic action is usually accompanied by an increased rate of torque development (Van Cutsem and Duchateau 2005) or speed of movement (Corcos et al 1989).…”

“…However, this decrease in performance is counteracted when a brief silent period is observed at the transition between the sustained and ballistic actions. The silent period, which is characterized by either a complete cessation or a reduction in the agonist EMG activity, has been observed during both shortening and isometric contractions and is associated with an increase in the peak acceleration of the ballistic movement and the rate of torque development (Yabe 1976;Conrad et al 1983;Mortimer et al 1987;Aoki et al 1989;Berardelli et al 1996;Zehr et al 1997;Van Cutsem and Duchateau 2005). The silent period likely enables motor neurones to achieve a non-refractory state leading to a more synchronous recruitment and a higher discharge rate of motor units during the subsequent ballistic action (Tsukahara et al 1995;Van Cutsem and Duchateau 2005).…”

Movement of the upper body and muscle activity patterns following a rapidly applied load: the influence of pre-load alterations

“…The presence of a silent period has been observed in learned tasks with sudden force exertion (Laursen et al 1978;Laursen 1984) andpreceding rapid contractions (Yabe 1976;Aoki et al 1989;Wierzbicka et al 1993). Three possible explanations for this phenomenon can be given.…”

“…Three possible explanations for this phenomenon can be given. One is that the occurrence of a silent period in the EMG signal prior to activation can be considered a means to interrupt the background activity, hereby synchronising the motor units to exert higher and faster muscle forces (Yabe 1976;Aoki et al 1989;Wierzbicka et al 1993). Secondly, the existence of a silent period causes the muscle force to decline, hereby stretching the muscle before shortening.…”