The development of muscular fatigue during exercise is a common phenomenon, and several forms depend on the precise type of exercise performed. The causes are still not clearly established, although the involvement of electrical and metabolic factors have been demonstrated. Several techniques which allow for the analysis of muscle function in terms of electrical activation and energy metabolism are (a) a needle biopsy of muscle for histochemical and metabolic studies, (b) magnetic resonance spectroscopy for the non-invasive study of muscle energy metabolism and pH, (c) electromyographic analysis of the electrical characteristics of muscle, and (d) percutaneous electrical stimulation of muscle for the force-frequency and relaxation characteristics of muscle. Endurance training increases the capacity to sustain exercise possibly by altering muscle energy metabolism and contractile properties. Fatigue is a self-protective mechanism against the damage of contractile machinery of muscle as, for example, with the development of rigor, which occurs if the energy stores are depleted. To illustrate the roles of energy supply and electrical properties in muscle in fatigue, the 'catastrophe theory' used in engineering has been applied. This may explain abrupt changes of function of individual muscle cells, while for the muscle as a whole, fatigue may be manifested as a more gradual loss of force.
To exanine the role of delay in recovery of peripheral muscle function following exercise in the fatigue experienced by patients with the chronic fatigue syndrome (CFS) and to examine the influence of effort perception in limiting exercise performance in these patients, a study was carried out on a group of twelve patients with chronic fatigue syndrome and 12 sex and age-matched sedentary control subjects. Symptom limited incremental cycle exercise tests including measurements of perceived exertion were performed followed by examination of the contractile properties of the quadriceps muscle group for up to 48 hours. Muscle function was assessed by percutaneous electrical stimulation and maximum voluntary contractions. Muscle function at rest and during recovery was normal in CFS patients as assessed by maximum isometric voluntary contraction, 20:50 Hz tetanic force ratio and maximum relaxation rate. Exercise duration and the relationship between heart rate and work rate during exercise were similar in both groups. CFS patients had higher perceived exertion scores in relation to heart rate during exercise representing a reduced effort sensation threshold of 3-2 units on an unmodified Borg scale in CFS patients. Patients with chronic fatigue syndrome show normal muscle physiology before and after exercise. Raised perceived exertion scores during exercise suggest that central factors are limiting exercise capacity in these patients. (7 Neurol Neurosurg Psychiatry 1993;56:993-998)
SUMMARY1. Human adductor pollicis was fatigued using intermittent trains of programmed stimulation at 1, 10, 20, 50, 100 and 1 Hz, during activity with and without circulatory occlusion, to investigate the relationships between force generation, excitation and maximal relaxation rate (MRR).2. The relationship between force generation and excitation was markedly dependent on stimulation frequency. Force loss was greatest at low frequencies, with little reduction in excitation, but as frequency increased force was well maintained despite marked loss of excitation.3. Changes in MRR during activity and recovery were independent of stimulation frequency.4. Marked increases of force at 1 Hz (pre-tetanic) and 10 Hz occurred, with little reduction in excitation, during activity with and without circulatory occlusion. This may be due to post-tetanic potentiation in-addition to slowing of relaxation (MRR).5. At high frequency a 'safety factor' may thus operate to maintain force, despite obvious loss of excitation, while at low frequencies there may be marked potentiation of force, despite unchanged excitation. These mechanisms could permit resistance to fatigue with muscle function remaining optimal over a range of conditions.
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