Effects of muscle conditioning on position sense at the human forearm during loading or fatigue of elbow flexors and the role of the sense of effort

Abstract

In a forearm position-matching task in the horizontal plane, when one (reference) arm is conditioned by contraction and length changes, subjects make systematic errors in the placement of their other, indicator arm. Here we describe experiments that demonstrate the importance not just of conditioning the reference arm, but of the indicator arm as well. Total errors from muscle conditioning represented up to a quarter of the angular range available to subjects. The sizes of the observed effects have led us to repeat other, previously reported experiments. In a matching task in the vertical plane, when muscles of both arms were conditioned identically, if the subject supported their arms themselves, or when the arms were loaded by the addition of weights, the loading did not introduce new position errors. To test the effect of exercise, subjects' elbow flexors were exercised eccentrically or concentrically by asking them to lower or raise a set of weights using forearm muscles. The exercise produced 25-30% decreases in maximum voluntary contraction strength of elbow flexors and this led to significant position-matching errors. The directions and magnitudes of the errors were similar after the two forms of exercise and indicated that subjects perceived their exercised muscles to be longer than they actually were. To conclude, the new data from loading the arm are not consistent with the idea that the sense of effort accompanying support of a load, provides positional information in any simple way. Our current working hypothesis is that when muscles are active, position-sense involves operation of a forward internal model. Loading the arm produces predictable changes in motor output and afferent feedback whereas changes after exercise are unpredictable. This difference leads to exercise-dependent errors.

Figure 1. Position-matching errors in the horizontal plane

A, individual values for matching errors for a single subject. The subject's reference arm was placed at 85 deg, included angle, and the subject was asked to match its position with the other, indicator arm. Left-hand values, where the indicator had been extension conditioned (EC Ind). Right-hand values, where the indicator had been flexion conditioned (FC Ind). •, reference arm flexion conditioned (FC Ref); ○, reference arm extension conditioned (EC Ref). Errors were calculated as the difference in matching position between reference and indicator arms. Errors were scored as positive when position of the indicator arm was more extended than the reference arm and negative when the indicator arm was more flexed than the reference arm. Dotted line, zero error. B, mean position errors for each of eight subjects for the four conditions: FC Ref, EC Ref, EC Ind and FC Ind. Symbols as in A. To show the trends in the data, a solid line has been drawn between the FC Ref values and a dashed line joining EC Ref values. Dotted line, zero error. C, pooled means (± s.e.m) for the eight subjects. *Significant differences.

Figure 2. Position-matching errors in the vertical plane from loading the arm

A, individual values for one subject after flexion conditioning of both arms (FC, •) and after extension conditioning of both arms (EC, ○). Matching was carried out under four conditions: (1) with the reference arm supported so that arm muscles remained relaxed; (2) the subject supported the reference arm themselves (unsupported); (3) a weight representing 10% maximum voluntary contraction (MVC) force for elbow flexors was added to the reference arm when it had been placed at the test angle; and (4) the weight was increased to 25% MVC force. Dotted line, zero error. Errors in the direction of extension are positive, and in the direction of flexion are negative. B, Pooled data from 12 subjects. Errors shown as means (± s.e.m). Symbols and their display as in A. Dotted line indicates zero error.

Figure 3. Position-matching error in the vertical plane after eccentric exercise

A, data from a single subject. For all matching trials both arms were flexion conditioned beforehand. The experimenter placed the reference arm at the test angle where the subject held it, unsupported and they moved their unsupported indicator arm to match its position. •, exercised arm as the reference; ○, control arm as the reference. Values on the left, before exercise (Pre-ex); values on the right, after exercise (Post-ex). Dotted line indicates zero error. B, pooled data from nine subjects. Means (± s.e.m) for values before and after exercise. To indicate the trends in the data, values before and after exercise have been joined by lines: solid line when the arm to be exercised acted as the reference, and dashed line, when it acted as the indicator. Dotted line, zero error. *Significant differences, before and after exercise.

Figure 4. Position-matching errors in the vertical plane after concentric exercise

A, data from a single subject. Matching conditions as in Fig. 3. •, exercised arm as the reference; ○, unexercised arm as the reference. Values on the left before exercise (Pre-ex); values on the right after the exercise (Post-ex). B, pooled data from eight subjects. Values shown as means (± s.e.m) before and after exercise. These have been joined by a solid line for values where the arm to be exercised was the reference and a dashed line where it was the indicator. *Significant differences, before and after exercise.