Human forearm position sense after fatigue of elbow flexor muscles

Abstract

After a period of eccentric exercise of elbow flexor muscles of one arm in young, adult human subjects, muscles became fatigued and damaged. Damage indicators were a fall in force, change in resting elbow angle and delayed onset of soreness. After the exercise, subjects were asked to match the forearm angle of one arm, whose position was set by the experimenter, with their other arm. Subjects matched the position of the unsupported reference arm, when this was unexercised, with a significantly more flexed position in their exercised indicator arm. Errors were in the opposite direction when the reference arm was exercised. The size of the errors correlated with the drop in force. Less consistent errors were observed when the reference arm was supported. A similar pattern of errors was seen after concentric exercise, which does not produce muscle damage. The data suggested that subjects were using as a position cue the perceived effort required to maintain a given forearm angle against the force of gravity. The fall in force from fatigue after exercise meant more effort was required to maintain a given position. That led to matching errors between the exercised and unexercised arms. It was concluded that while a role for muscle spindles in kinaesthesia cannot be excluded, detailed information about static limb position can be derived from the effort required to support the limb against the force of gravity.

Figure 1. Changes in MVC and the Error –MVC relation for position errors after eccentric exercise

A, percent change in MVC of elbow flexor muscles over 4 days after eccentric exercise. The pre-exercise value was assigned 100%. Continuous line, exercised arm; dashed line, unexercised arm. Values are means (± s.e.m.) for 6 subjects. An asterisk indicates where values were significantly different from the control. B, relationship between the mean decline in MVC, expressed as a percentage of control values, and mean position matching errors (unsupported matches) between reference and indicator arms, given in degrees of forearm inclination above the horizontal. Matching errors were averaged across 5 trials, 3 angles and 6 subjects. The correlation was significant. The slope of the relation was 0.05 deg per percentage MVC decline.

Figure 2. Position matching errors after eccentric exercise

Forearm position matching errors for one subject with their reference arm unsupported. Forearm angles were expressed in degrees above the horizontal, which was assigned 0 deg. Three test angles were used, 30, 60 and 90 deg. The differences in positions of reference and indicator arms were given in degrees. When matching errors were in the direction of flexion, they were given a positive value, when they were in the direction of extension they were negative. Dashed line and open symbols, reference arm unexercised; continuous line and filled symbols, reference arm exercised. Zero error is indicated by the dotted line. Errors were measured before a period of eccentric exercise, immediately afterwards (0 h) and at 24, 48, 72 and 96 h. All values are means (± s.e.m.) across 5 trials and 3 angles. Asterisks indicate values significantly different from control.

Figure 3. Matching errors for unsupported and supported reference arms

Pooled data for position matching errors for the 3 matching angles for one subject. A, the subject maintained the reference angle voluntarily by contracting elbow flexor muscles (Experiment 1). B, the reference position was maintained by a support and the subjects were asked to fully relax their reference arm (Experiment 2). In each panel, dashed lines and open circles are values when the unexercised arm was the reference; continuous lines and filled circles are when the exercised arm was the reference. The dotted line indicates zero error. All values are means (± s.e.m.) across 5 trials and 3 angles. Asterisks indicate points significantly different from control.

Figure 4. Pooled data for matching errors

Pooled matching errors for 6 subjects, each of which was measured 5 times at each of 3 matching angles before and at various times after a period of eccentric exercise. A, reference arm was unsupported. B, reference arm supported. All values are means (± s.e.m.) across 6 subjects, 5 trials and 3 angles. Continuous line and filled symbols, reference arm exercised; dashed line and open symbols, reference arm unexercised. Dotted line indicates zero error. Asterisks alongside values in A and above or below values in B indicate values significantly different from controls.

Figure 5. Position matching errors after concentric exercise

A, changes in MVC for 6 subjects after a period of concentric exercise. The value measured before the exercise was assigned 100%. Measurements were carried out before the exercise, immediately afterwards and at 2 h and 24 h. Continuous line and filled symbols, MVC values in exercised arm. Dashed line and open symbols, unexercised arm. Asterisks indicate values significantly different from controls. B, mean values for 5 trials, 3 matching angles and 6 subjects of position matching errors, in degrees, between the two forearms after concentric exercise. Reference arm unsupported. Open circles, unexercised arm as reference, filled circles exercised arm as reference. C, plot of relationship between elbow matching errors and fall in MVC. The MVC decline was expressed as the percentage below the control value. Here matching errors were expressed for the exercised arm relative to the unexercised arm, rather than between indicator relative to the reference. It allowed pooling of errors for both arms and calculation of a mean. The four values are from before the exercise, immediately afterwards, at 2 h and at 24 h. The correlation was significant. The slope had a value of 0.053 deg per percentage MVC. In the figure, all values are means (± s.e.m.) across 2 arms, 3 angles, 5 trials and 6 subjects. Asterisks indicate points significantly different from control values.

Figure 6. Biceps EMG, the force of gravity and matching errors for different forearm angles

A, surface EMG from biceps of an unsupported, unexercised arm recorded for 3 different forearm angles, relative to the horizontal (10, 60 and 90 deg). B, estimated changes in the force of gravity with elbow angle over the range 0 to 90 deg. The changes were calculated based on the assumption that the vector acting on the arm was proportional to the cosine of the angle of the forearm to the horizontal. C, mean position matching errors for 30, 60 and 90 deg (dashed line) are shown together with the changes in MVC at these angles (continuous line). The position matching errors are means from 5 trials and 6 subjects and were measured immediately after a period of eccentric exercise. The unexercised arm was the reference and it was unsupported. The length dependence of MVC for elbow flexors used data from eight subjects. Peak MVC was at 90 deg. The 30 deg value was obtained by extrapolation (Weerakkody et al. 2003b; Fig. 6).