Abstract
During submaximal isometric contraction, there are two different load types: production of a constant force against a rigid restraint (force task), and maintenance of position against a constant load (position task). Previous studies reported that the time to task failure during a fatigue task was twice as long in the force task compared with the position task. Sensory feedback processing may contribute to these differences. The purpose of the current study was to determine the influence of load types during static muscle contraction tasks on the gating effect, i.e., attenuation of somatosensory-evoked potentials (SEPs) and the cortical silent period (cSP). Ten healthy subjects contracted their right first dorsal interosseus muscle by abducting their index finger for 90 s, to produce a constant force against a rigid restraint that was 20% of the maximum voluntary contraction (force task), or to maintain a constant position with 10° abduction of the metacarpophalangeal joint against the same load (position task). Somatosensory evoked potentials (SEPs) were recorded from C3′ by stimulating either the right ulnar or median nerve at the wrist while maintaining contraction. The cortical silent period (cSP) was also elicited by transcranial magnetic stimulation. Reduction of the amplitude of the P45 component of SEPs was significantly larger during the position task than during the force task and under control rest conditions when the ulnar nerve, but not the median nerve, was stimulated. The position task had a significantly shorter cSP duration than the force task. These results suggest the need for more proprioceptive information during the position task than the force task. The shorter duration of the cSP during the position task may be attributable to larger amplitude of heteronymous short latency reflexes. Sensorimotor modulations may differ with load type during constant finger force or position tasks.
Highlights
Muscle activities involving production of a constant force by pulling against a noncompliant restraint or maintenance of a constant limb angle while supporting an equivalent inertial load, seem to involve different neural control mechanisms despite both tasks generating a similar net muscle torque [1,2]
The larger amplitude of the heteronymous short latency reflexes (SLR) and larger gating effect of somatosensory-evoked potentials (SEPs) amplitude (P45) during the position task suggests that maintaining the position of the index finger while supporting a constant load requires more proprioceptive information, which enhances gating of P45
It is entirely fair to say that these two studies have something in common in terms of the need for greater proprioceptive information to induce attenuation of middle SEP components (N33 and P45) during static muscle contraction, as evinced by the fact that attenuation of the P45 component of SEP when the median nerve was stimulated was comparable between the two tasks
Summary
Muscle activities involving production of a constant force by pulling against a noncompliant restraint (force task) or maintenance of a constant limb angle while supporting an equivalent inertial load (position task), seem to involve different neural control mechanisms despite both tasks generating a similar net muscle torque [1,2]. Some studies have reported that position tasks produce an increased amplitude of short latency reflexes (SLR) compared with force tasks [1,15,16]. It has been reported that heteronymous monosynaptic Ia facilitation was greater and homonymous inhibition was depressed during position tasks [14,17,18], and it was thought that the greater reflex amplitude during position tasks was attributable to lower levels of presynaptic inhibition of Ia afferents [14,19]. The rate at which motor units are recruited is greater during position tasks than force tasks [3,9,20] These lower levels of presynaptic inhibition of Ia afferents and more rapid recruitment of the motor unit pool may contribute to the shorter time to failure when a compliant load is supported in a position task [19]
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