Central and peripheral mediation of human force sensation following eccentric or concentric contractions

Abstract

Fatigue was induced in the triceps brachii of the experimental arm by a regimen of either eccentric or concentric muscle actions. Estimates of force were assessed using a contralateral limb-matching procedure, in which target force levels (25 %, 50 % or 75 % of maximum) were defined by the unfatigued control arm. Maximum isometric force-generating capacity was reduced by 31 % immediately following eccentric contractions, and remained depressed at 24 (25 %) and 48 h (13 %) post-exercise. A less marked reduction (8.3 %) was observed immediately following concentric contractions. Those participants who performed prior eccentric contractions, consistently (at all force levels), and persistently (throughout the recovery period), overestimated the level of force applied by the experimental arm. In other words, they believed that they were generating more force than they actually achieved. When the forces applied by the experimental and the control arm, were each expressed as a proportion of the maximum force that could be attained at that time, the estimates matched extremely closely. This outcome is that which would be expected if the estimates of force were based on a sense of effort. Following eccentric exercise, the amplitude of the EMG activity recorded from the experimental arm was substantially greater than that recorded from the control arm. Cortically evoked potentials recorded from the triceps brachii (and extensor carpi radialis) of the experimental arm were also substantially larger than those elicited prior to exercise. The sense of effort was evidently not based upon a corollary of the central motor command. Rather, the relationship between the sense of effort and the motor command appears to have been altered as a result of the fatiguing eccentric contractions. It is proposed that the sense of effort is associated with activity in neural centres upstream of the motor cortex.

Figure 1. The levels of force applied during maximum isometric voluntary contractions (IMVC) prior to and up to 48 h following bouts of either eccentric or concentric exercise

The levels of force applied by the control (unexercised) arm for the group that performed eccentric exercise (○, dotted lines) and the corresponding levels of force applied by the experimental (exercised) arm (□, continuous lines) are shown. The levels of force applied by the control (unexercised) arm for the group that performed concentric exercise (⋄, dashed lines) and the corresponding levels of force applied by the experimental (exercised) arm (▵, long dashed lines) are shown. Instances in which the post-exercise values were distinguished reliably (P < 0.05) from the pre-exercise values are shown as filled symbols. The error bars correspond to 95 % confidence intervals.

Figure 2. The target levels of force applied by the control arm (nominally 25 % (A), 50 % (B) and 75 % (C) IMVC) and the matching levels of force applied by the experimental arm, prior to and 0–2 h, 24 h and 48 h, following bouts of either eccentric (ECC) or concentric (CON) exercise

The forces applied by each arm were normalised with respect to those applied during the respective pre-exercise IMVCs. Instances of a statistically reliable (P < 0.05) difference between the normalised level of force applied by the experimental arm and that applied by the control arm are indicated by *. The error bars correspond to 95 % confidence intervals.

Figure 3. The r.m.s.. EMG recorded from the control arm and the experimental arm, during the application of force (nominally 25 % (A), 50 % (B) and 75 % (C) IMVC), prior to and 0–2 h, 24 h and 48 h, following bouts of either eccentric (ECC) or concentric exercise

The EMG amplitudes for each arm were normalised with respect to those recorded during the respective pre-exercise IMVCs. The error bars correspond to 95 % confidence intervals.

Figure 4. The target levels of force applied by the control arm (nominally 25 % (A), 50 % (B) and 75 % (C) IMVC) and the matching levels of force applied by the experimental arm, prior to and 0–2 h, 24 h and 48 h, following bouts of either eccentric (ECC) or concentric (CON) exercise

The forces applied by each arm were normalised with respect to those applied during the respective contiguous IMVCs (see text). The error bars correspond to 95 % confidence intervals.

Figure 5. Expt 2, 3 and 4

A, the forces applied by the experimental arm in either extension (Expts 2 and 4, mean of 13 participants) or flexion (Expt 3, mean of six participants) have been expressed relative to those applied by the control arm (E/C), during force matching conducted prior to (Pre) and following (Post) bouts of eccentric exercise. The forces are shown both when normalised to the respective pre-exercise IMVCs (%Pre-MVC), and when normalised with respect to the respective contiguous IMVCs (%Contiguous-MVC) (see text). B, the r.m.s. electromyogram recorded from the triceps brachii (Expts 2 and 4, mean of 13 participants) or biceps brachii (Expt 3, mean of six participants) of the experimental arm (Exp) and of the control arm (Control), prior to (Pre-exercise) and following (Post-exercise) eccentric contractions. The EMG is expressed as a percentage of that recorded prior to exercise during an IMVC. C, the mean amplitudes of the maximum m-waves elicited in the triceps brachii (Expt 2, mean of six participants) or biceps brachii (Expt 3, mean of six participants) of the experimental arm, by supramaximal electrical stimulation of the peripheral nerve. The values obtained prior to (Pre) and following (Post) bouts of eccentric exercise are presented. D, the mean amplitude of the potential evoked in triceps brachii and extensor carpi radialis (ECR) by transcranial magnetic stimulation of the contralateral motor cortex prior to (Pre) and following (Post) bouts of eccentric exercise (mean of seven participants). The individual values upon which the means are based were normalised with respect to background EMG (see text). In all panels, the error bars correspond to 95 % confidence intervals. In these experiments the target level of force was 25 % IMVC.