Sensorimotor Modulation Differs with Load Type during Constant Finger Force or Position

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.This work was supported by a Grant-in-Aid for Scientific Research (C) No. 08042773 from the Japan Society for the Promotion of Science (JSPS) (http://www.jsps.go.jp/english/e-grants/index.html) and a Research Grant from Niigata University of Health and Welfare (NUHW) (http://www.nuhw.ac.jp/e/). HK received both grants. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Illustration of the experimental setup for the force task (A) and the position task (B).

<p>Each subject was seated upright with the right hand positioned in the custom apparatus, and faced a monitor for visual feedback. The custom-designed device consisted of a wheel connected to a force transducer (a) or inertial load (b) by means of a pulley and nylon line. The index finger was attached to a bar that was connected to the wheel so that the rotational axis of the metacarpophalangeal joint approximated that of the wheel. Subjects were required to match either a target force equal to 20% of their maximal force by pushing up against a rigid bar (force task (A)), or a target position corresponding to 10° abduction of the metacarpophalangeal joint while supporting an equivalent load suspended from the index finger (position task (B)). The abduction angle of the metacarpophalangeal joint during the position task was measured with a wire-type displacement meter (c) attached to the wheel.</p

The EMG from the FDI and APB muscles, and the torque and/or angle waveforms recorded from a representative subject during MVC and isometric contraction when the subject maintained a constant finger force or position (A), and averaged amplitude of the rectified EMG signals (aEMG) from the FDI and APB muscles of the individual subjects for the two tasks (B).

<p>Regression analysis showing the relationship between the aEMG of the force and position tasks. Most of the data fell on the regression line, indicating a very high reproducibility between the force and position tasks.</p

Averaged heteronymous SLR and LLR waveforms recorded from the FDI muscle by stimulating the median nerve during the force (blue line) and position tasks (red line) (A), and the mean amplitude of the SLRs (B) and LLRs (C).

<p>The amplitude of both the SLRs and LLRs during the position task was significantly larger than during the force task (mean ± SEM). (** <i>p</i><0.01).</p

Experimental procedure.

<p>Experimental procedure for reliability evaluation of EMG activity (A), SEP recordings (B), SLR and LLR recordings (C) and MEP and cSP recordings (D). Median or ulnar nerve stimuli were delivered a total of 200 times during each task for SEP recordings. Median nerve stimuli were delivered a total of 150 times during each task for heteronymous SLR and LLR recordings. TMS was delivered 16 times during each task for MEP and cSP recordings.</p

MEP waveforms and cSP (overlay of 16 trials) with TMS over the M1 recorded from a representative subject during the force (blue line) and position tasks (red line) (A), the mean amplitude of MEPs (B) and the duration of cSP (C) during the force and position tasks. The black lines in panel (A) show the average waveform of 16 trials.

<p>The force task resulted in a significantly longer cSP duration than the position task, while the amplitude of MEPs did not differ between the two tasks (mean ± SEM). (** <i>p</i><0.01).</p