Word count: 3571
Word count: 3571
Muscle Fatigue and Active Pre-Stretch Contractions
Arnold de Haan
Department of Muscle and Exercise Physiology
Faculty of Human Movement Sciences
Vrije Universiteit
The Netherlands
Muscle fatigue is frequently experienced by every individual during normal daily activities, and often more intensively during exercise and labor. It is not surprising therefore, that many researchers were and are attracted to study the phenomenon of fatigue. Studies have been performed using many different techniques and different preparations, including the intact human and other animals, isolated muscle, intact or skinned fiber preparations, muscle homogenates and isolated proteins. Results from those studies have generated much insight into the effects and mechanisms of fatigue (for a recent review of the field: 9).
Most of the fatiguing exercise used in studies involved isometric contractions, or concentric contractions during which shortening occurred immediately from the start of the contractions or after an isometric pre-phase. But in exercise like walking or running, shortening is often preceded by an active stretch of the muscles. As a result of such an active pre-stretch subsequent power production is increased, as can be seen for instance in the jump height with and without a counter movement. Effects of pre-stretch have therefore been studied extensively by some research groups (for review: 1).
The combination of fatigue and pre-stretch contractions is however poorly addressed. The main purpose of this paper is to review the effects of fatigue and of pre-stretch and to summarize the findings of our study on fatigue of pre-stretch contractions.
Isometric Fatigue
Most experimental work has been based on the measurements of isometric force during contractions lasting up to 60 seconds. In such long-lasting contractions force generating capability decreases. During a maximal voluntary contraction the isometric force decreases gradually to ~50% of the initial force after 60s. Besides this decrease in force, a main feature of fatigue is a reduction in the rate of relaxation. The prolongation of the active state of the muscle consequent upon the slowing of relaxation infers an advantage for the isometrically contracting muscle. In order to get a fused tetanus the muscle needs to be stimulated frequently. As a consequence of the slowing of relaxation a shift occurs in the force-frequency relationship to lower frequencies. Thus less frequent stimulation is needed to maintain a fused tetanus; which may lead to an increase in the economy of the contraction (see below). In maximally voluntary contractions in humans it has been shown that the motor-unit firing frequency to the active muscles decreased soon after the start of the contraction, indicating that changes in muscle characteristics and motoneuron firing rates are closely matched.
There are two possible causes for the slowing of relaxation, i.e. a slower Ca2+-re-uptake in the sarcoplasmic reticulum and a slower cross-bridge cycling rate. Both features seem to contribute to the slowing of relaxation and the importance of each probably depends on the type of exercise that lead to fatigue.
The causes for the reduction in maximal force could be of central origin, that is occurring at the motor neuron level or higher, related to neuromuscular junction or any of the various processes concerned with the generation of force within the muscle fiber. For the purpose of this paper I will limit myself to address only some of the processes within the muscle fibers.
Cellular Processes
During fatigue induced exercise many changes occur in metabolite concentrations. ATP is used as a direct source of energy for the contractile proteins and other energy requiring processes in the cells. Its concentration however remains high during mild exercise owing to fast ATP regenerating processes. In man, at exhaustion following long-duration exercise muscle ATP concentration is still ~70% of the initial concentration but this is usually determined in whole muscle biopsies. In contrast in in-situ muscles we have demonstrated that ATP may decrease to very low levels after high-intensity dynamic exercise. In single fiber preparations, but also in fibers isolated from fatigued muscles, ATP values of near zero have been reported. Taken together these results suggest that ATP concentrations in specific fibers, even in humans performing voluntary but intense exercise, may be very low, although measurements in whole muscle indicate otherwise. Nevertheless, since very low ATP concentrations hardly affected force generation in skinned fiber preparations it is generally believed that changes in ATP cannot explain the reduction in force generating capacity during fatiguing exercise.
Large changes occur in phosphocreatine and lactate concentrations, which will lead to large increases in inorganic phosphate (Pi) and hydrogen ions. Both these parameters have been shown to influence isometric force generating capacity in skinned fiber preparations by action on the cross-bridge cycle (2). Moreover, a decrease of intracellular pH (increase in H+-ions) may affect force generation by a reduction of Ca2+ sensitivity and also indirectly by inhibition of glycolysis.
A decrease in ATP results in small increases in ADP and AMP. Although the absolute changes are rather small, the relatively large increases in the free concentrations may affect force production.
Most of the degraded ATP is deaminated to inosine-monophosphate (IMP). This occurs predominantly when muscles are highly stressed. Since the resting IMP concentration is very low ( ................
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