Changes in the force-velocity relationship of fatigued muscle: implications for power production and possible causes

Abstract

Slowing of the contractile properties of skeletal muscle is one of the characteristic features of fatigue. First studied as a slowing of relaxation from an isometric contraction, it has become apparent that this slowing is indicative of functional changes in muscle responsible for a major loss of power with all its functional repercussions. There are three factors contributing to the loss of power in mammalian muscle at physiological temperatures, a decrease in isometric force, which mainly indicates a reduction in the number of active cross bridges, a slowing of the maximum velocity of unloaded shortening and an increased curvature of the force-velocity relationship. This latter change is a major cause of loss of power but is poorly understood. It is probably associated with an increase in the proportion of cross bridges in the low force state but there are no clear candidates for the metabolic changes that are responsible for this shift in cross bridge states. The possibility is discussed that the reduction in activating calcium that occurs with metabolically depleted muscle, alters the distribution of cross bridge states, affecting both shortening velocity and curvature.

Figure 1. Relaxation rate, sampled during and after contraction at 50% MVC in one subject

The short breaks for testing t_1/2 (the half-time of the relaxation phase) as fatigue progresses and short (3 s) test contractions during recovery are indicated below. Inset: two records of relaxation to show prolongation with fatigue (from Edwards et al. 1972).

Figure 2. Brief tetanic contractions of the human first dorsal interosseous in the fresh state and after a fatiguing 45 s ischaemic voluntary contraction

Horizontal bar indicates the duration of stimulation at 50 Hz (from Cady et al. 1989).

Figure 3. Records of tension and [Ca²⁺]_i from a single mouse muscle fibre (flexor brevis) during a fatiguing run

A shows the continuous tension record where each tetanus appears as a vertical line. Arrows indicate times when the intervals between tetani were changed. B shows [Ca²⁺]_i and tension of tetani produced at times (a, b, c, d) indicated above the continuous tension record (from Westerblad & Allen, 1993).

Figure 4. Force–velocity relationships of fresh and fatigued human adductor pollicis muscle, together with power

Force is given in absolute units (fresh, square symbols; fatigued, circles) while power is expressed as a percentage of the peak power of the fresh muscle (fresh, dashed line; fatigued continuous line) (from de Ruiter et al. 1999a).

Figure 5. Force–velocity relationships for fresh and fatigued human adductor pollicis muscle both for shortening and lengthening contractions

Fresh muscle, square symbols; fatigued muscle, circles. Data for shortening contractions, open symbols; during lengthening, filled symbols. The vertical dotted line indicates the isometric condition to which the data were normalised (from de Ruiter et al. 2000).

Figure 6. Changes in contractile function during fatigue and recovery

Human adductor pollicis fatigued with a series of ischaemic contractions and then allowed to recover. A, changes in peak power; B, isometric force; C, maximum velocity of unloaded shortening (V_max); D, curvature of the force–velocity relationship (a/P_o). In each case the values are expressed as a percentage of those of the fresh muscle, also indicated by the dotted horizontal line (from Jones et al. 2006).

Figure 7. Changes in muscle metabolites and isometric force with fatigue

Human anterior tibialis muscle fatigued by a series of tetani under ischaemic conditions. A, muscle metabolite concentrations estimated by magnetic resonance spectroscopy. PC, phosphocreatine; P_i, inorganic phosphate; LA, lactate, estimated from changes in pH and likely values of muscle buffering capacity. B, changes in isometric force (from Jones et al. 2009).

Figure 8. ATP turnover and economy during a series of fatiguing contractions

A, ATP turnover derived from the data in Fig. 7A. B, economy of the muscle calculated as turnover in Fig. 8A divided by the force in Fig. 7B. The data have been normalized to the value of the fresh muscle (from Jones et al. 2009).

Figure 9. The relationship between relaxation rate and muscle metabolite concentrations during a series of fatiguing contractions

Data from the experiment shown in Fig. 6. A, hydrogen ion concentration; B, inorganic phosphate (P_i); C, ADP (from Jones et al. 2009).