The effect of fatigue from exercise on human limb position sense

Abstract

We have previously shown, in a two-limb position-matching task in human subjects, that exercise of elbow flexors of one arm led the forearm to be perceived as more extended, while exercise of knee extensors of one leg led the lower leg to be perceived as more flexed. These findings led us to propose that exercise disturbs position sense because subjects perceive their exercised muscles as longer than they actually are. In order to obtain further support for this hypothesis, in the first experiment reported here, elbow extensors were exercised, with the prediction that the exercised arm would be perceived as more flexed after exercise. The experiment was carried out under three load conditions, with the exercised arm resting on a support, with it supporting its own weight and with it supporting a load of 10% of its voluntary contraction strength. For each condition, the forearm was perceived as more extended, not more flexed, after exercise. This result was confirmed in a second experiment on elbow flexors. Again, under all three conditions the exercised arm was perceived as more extended. To explore the distribution of the phenomenon, in a third experiment finger flexor muscles were exercised. This had no significant effect on position sense at the elbow. In a fourth experiment, position sense at the knee was measured after knee flexors of one leg were exercised and, as for knee extensors, it led subjects to perceive their exercised leg to be more flexed at the knee than it actually was. Putting all the observations together, it is concluded that while the influences responsible for the effects of exercise may have a peripheral origin, their effect on position sense occurs centrally, perhaps at the level of the sensorimotor cortex.

Figure 1. Position matching at the elbow and knee

A, position matching at the elbow. Blindfolded subjects sat at a table with their forearms strapped to lightweight paddles. The paddles were hinged at one end, the hinges aligned with the subject's elbow joint. Potentiometers attached to the paddle hinges provided a voltage signal proportional to elbow angle. B, exercise used to fatigue elbow extensors. Subjects sat in a chair with the arm to be exercised in the flexed position, resting on a horizontal support. The hand grasped a handle that was attached to a cable which ran through a pulley to a weight. Subjects were asked to slowly extend the arm to raise the weight which was adjusted to be 30% MVC. Once the arm was fully extended, the weight was lowered back to its starting position by the experimenter, ready for the next extension movement. C, position matching at the knee. Subjects were seated in a chair mounted on a steel frame. The height of the chair was adjusted so that the lateral and medial epicondyles of the knee were in line with the pivot point of the position-matching apparatus. This consisted of a pair of lightweight paddles with potentiometers at their hinge points to give a voltage output proportional to knee angle. D, exercising knee flexors with a dynamometer. Subjects were seated with the dynamometer head in line with the lateral epicondyle of the knee. A leg brace was strapped to the lower leg while hips and trunk were strapped to the chair with seatbelts. Subjects were asked to carry out concentric contractions of knee flexors in the manner of a knee curl.

Figure 2. Position errors for elbow extensors and flexors

Upper panel: position errors before and after exercising elbow extensors, matching under different conditions. Mean position errors (±s.e.m.) for the group of subjects, before (open circles) and after (filled circles) exercise of elbow extensors of one arm, under three conditions, with the exercised reference arm lying relaxed on a support (Supported), with the subject supporting the weight of the arm themselves (Unsupported) and with the paddle holding the arm loaded with a counterweight (Loaded). Values before exercise are joined by a dashed line, values after exercise by a continuous line. Dotted line indicates zero error. Asterisks indicate significant differences between errors pre- and post-exercise. Lower panel: position errors before and after exercise of elbow flexors, matching under different conditions. Mean position errors (±s.e.m.) for the group of subjects before (open circles) and after (filled circles) exercise of elbow flexors of one arm. Matching conditions and data display as in upper panel. Asterisks indicate significant differences between errors pre- and post-exercise.

Figure 3. Position matching errors at the elbow and knee before and after exercise

A, position errors before (pre-ex) and after (post-ex) exercise of elbow flexors (filled circles, continuous line) and elbow extensors (open circles, dashed line). B, position errors before and after exercise of knee flexors (filled circles, continuous line) and knee extensors (open circles, dashed line). In both A and B matching errors are for the condition where the subject supported their exercised arm themselves. Dotted line, zero error. Values in A and for flexors in B taken from Experiments 1, 2 and 4. In B, values for extensors taken from Givoni et al. (2007). Asterisks indicate significant differences between errors pre- and post-exercise.