Change in contractile properties of human muscle in relationship to the loss of power and slowing of relaxation seen with fatigue

Abstract

Slow relaxation from an isometric contraction is characteristic of acutely fatigued muscle and is associated with a decrease in the maximum velocity of unloaded shortening (V(max)) and both these phenomena might be due to a decreased rate of cross bridge detachment. We have compared the change in relaxation rate with that of various parameters of the force-velocity relationship over the course of an ischaemic series of fatiguing contractions and subsequent recovery using the human adductor pollicis muscle working in vivo at approximately 37 degrees C in nine healthy young subjects. Maximal isometric force (F(0)) decreased from 91.0 +/- 1.9 to 58.3 +/- 3.5 N (mean +/- s.e.m.). Maximum power decreased from 53.6 +/- 4.0 to 17.7 +/- 1.2 (arbitrary units) while relaxation rate declined from -10.3 +/- 0.38 to -2.56 +/- 0.29 s(-1). V(max) showed a smaller relative change from 673 +/- 20 to 560 +/- 46 deg s(-1) and with a time course that differed markedly from that of slowing of relaxation, showing very little change until late in the series of contractions. Curvature of the force-velocity relationship increased (a/F(0) decreasing from 0.22 +/- 0.02 to 0.11 +/- 0.02) with fatigue and with a time course that was similar to that of the loss of power and the slowing of relaxation. It is concluded that for human muscle working at a normal physiological temperature the change in curvature of the force-velocity relationship with fatigue is a major cause of loss of power and may share a common underlying mechanism with the slowing of relaxation from an isometric contraction.

Figure 1. Effects of fatigue on the contractile characteristics of the adductor pollicis

A, original records of an isometric tetanus (80 Hz) in the initial, fresh, state (upper continuous line), after the ninth series of fatiguing contractions (lower continuous line) and following 360 s recovery (dashed line, almost completely superimposed on the fresh trace). B, force–velocity curves for the same subject in the initial (•, continuous line), fatigued (▴, continuous line) and recovered state (□, dashed line). C, the relationship between power (derived from data in Fig. 1B) and velocity for the same subject in the fresh (upper continuous line), fatigued (lower continuous line) and recovered (dashed line) state.

Figure 2. Changes in muscle contractile properties during fatigue and recovery

A, changes in isometric force (80 Hz) during the nine fatiguing series of contractions under ischaemic conditions and the subsequent 6 min of recovery with an intact circulation, beginning at the vertical dotted line. B, changes in maximum power during fatigue and recovery. C, changes in relaxation rate from an isometric tetanus during fatigue and recovery. Data are the mean ± s.e.m. and are expressed as a percentage of the initial values (100% indicated by the horizontal dashed line).

Figure 3. Time course of changes in the force–velocity relationship during fatigue and recovery

A, changes in the maximum velocity of unloaded shortening (V_max) during the nine fatiguing series of contractions under ischaemic conditions and the subsequent recovery with an intact circulation, beginning at the vertical dotted line. B, changes in the optimum velocity to generate power (V_opt) during fatigue and recovery. C, changes in the curvature of the force–velocity relationship (a/F₀) during fatigue and recovery. Data are the mean ± s.e.m. and are expressed as a percentage of the initial values (100% indicated by the horizontal dashed line).

Figure 4. Relaxation rate in relation to changes in V_max and a/F₀

A, mean data taken from Figs 2 and 3 showing the relationship between relaxation rate and V_max and a/F₀ during fatigue and recovery. Vertical arrow indicates the initial values (100%) and the arrowheads indicate the direction of change, first during fatigue and then recovery; note the marked hysteresis of the V_max curve which is much less evident with the relationship between relaxation and a/F₀. B, the relationship between relaxation rate and V_max during fatigue and recovery; individual data points and linear regression are shown. C, the relationship between relaxation rate and a/F₀ during fatigue and recovery; individual data points and linear regression are shown.