The effect of temperature, ischaemia and contractile activity on the relaxation rate of human muscle.

The relaxation rate from electrically stimulated isometric contractions of human adductor pollicis has been measured during rewarming following cooling, during ischaemia and during ischaemic contractile activity. The Q10 for relaxation rate (25-37 degrees C intramuscular temperature) was estimated as 2 . 3. Relaxation rate was found to decline at a rate of 1 . 5% initial value per minute of ischaemia. Relaxation rate declined more rapidly during ischaemic contractile activity than during ischaemia alone. The rate of decline was more closely related to the force X time performed than the number of excitatory impulses. During a supramaximal 20 Hz tetanus, relaxation rate declined markedly at a time when only slight force fatigue had occurred; hence the decline could not be explained by selective fast muscle fibre fatigue. No recovery occurred during ischaemic rest following ischaemic activity but, following restoration of the circulation, recovery occurred with a half time of one minute being virtually complete at 5-7 min. Changes in relaxation rate during fatiguing contractions and recovery from them follow different time courses from muscle excitability and force production. Change in relaxation rate cannot be simply related to changes in the concentration of major energy metabolites. It is proposed that relaxation rate is related to the rate of energy turnover in the contracting muscle.