James, Colin; (1993) The effects of fatigue on the force, power output and rate of relaxation in the human quadriceps muscle. Doctoral thesis (Ph.D.), University College London.

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Abstract

The loss of strength and power with muscle fatigue are common experiences but the causes remain incompletely understood. What is clear is that there are several sites at which failure can occur in isolated muscle preparations, and the situation is even more complicated with human muscle working in situ. Work that has been done on human muscle has concentrated on isometric contractions which are of limited value in interpreting changes in whole body exercise. This work described in this thesis examines the mechanical characteristics of fresh and fatigued human quadriceps muscle during voluntary and stimulated contractions. Force and angle were measured with a modified Cybex II+ isokinetic dynamometer over a range of angular velocities (0-300 degrees/second). Initially several protocols were examined, designed to overcome problems associated with mechanical artefacts on the force records and delay in activating the muscle. The isometric length-tension curves produced by voluntary and femoral stimulated contractions had absolute forces and angle/force relationships which were similar provided measurements of force were made over a limited range of knee extension with the muscle near to the plateau of force. Percutaneous stimulation, which has been extensively used with studies of isometric contractions proved unsatisfactory for dynamic contractions as the proportion of muscle stimulated varied with the length of the muscle. The power measured during a shortening contraction varied according to the protocol. The shapes of the voluntary contraction (VC) and femoral nerve stimulated release contractions (FNR) force-velocity curves were consistent with the classic Hill relationship but that of the voluntary release contraction (VR) displayed a bi-phasic nature. EMG recording showed a silent phase during VR contractions indicating loss of activation at slower velocities probably accounting for the biphasic force-velocity relationship. The greater forces produced by VC over FNR were most likely caused by the addition of force from the hip extensors stretching the rectus femoris. To determine the degree of central fatigue present during 6 minutes of repeated maximum voluntary isokinetic extensions at 90 degrees/second, VC and FNR test contractions were compared after each minute of contractions and over 5 minutes of recovery in 2 subjects. Two protocols were used which might have been expected to produce different degrees of central fatigue. In the first protocol one quadriceps muscle only was exercised and tested, in the other both quadriceps and both hamstrings were exercised. Hean rate, PCO2, PO2, perceived exertion and 5 minute post exercise venous lactates were measured. One subject demonstrated sub-maximal voluntary activation (about 20%) throughout the fatiguing contractions, while the second subject showed no evidence of central fatigue. In neither case was there any greater central fatigue during the harder exercise indicating that central fatigue is not a consequence of the secondary effects of exercise, acidosis, high heart rate or ventilation. Avoiding the complications of central activation by using FNR, the differential effects of fatigue on isometric and isokinetic contractions was examined using a series of three 15 second isometric contractions and testing the muscle with FNR contractions at a range of velocities. Force sustained during the shortening contractions was more severely affected than was the ability to maintain isometric force. The changes in the force-velocity relationship were accompanied by changes in the rate of relaxation from an isometric tetanus, suggesting that the two phenomena share a common mechanism. It is suggested that the mechanism is a slowing of cross bridge detachment in fatigued muscle. Finally, a comparison was made between the fatiguing effects of intermittent dynamic and isometric contractions. Dynamic exercise resulted in greater fatigue in one subject but the effect was not so clearly seen in a larger group of subjects. The results presented in this thesis stress the importance of measuring the performance of dynamic contractions rather than relying on isometric contractions in the study of fatigue. The work takes the experimental investigation of fatigue one step closer to the goal of understanding the complexities of whole body exercise.

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