Human muscle weakness and fatigue : the effects of disuse, age and exercise

• Main Author: White, Michael J.
• Published: University of Nottingham 1987
• Subjects: 612; WE Muscoskeletal system
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.525800

Weakness and fatigue in the human triceps surae have been assessed objectively by the measurement of absolute force evoked using supramaximal stimulation. The effects of disuse, age and exercise were systematically investigated. Under control conditions the triceps surae of young men were found to generate high maximal tetanic forces, have a mean twitch time to peak tension of 107 msec and did not fatigue readily. This was indicative of a large muscle mass with a predominance of type I (slow twitch) fibres. Muscle temperature manipulation over the range 29.5 to 39.1°C did not affect maximal force generation but had a profound effect on the force and time course of twitch and unfused tetanic responses. The triceps surae of 70 year old men were found to be slower contracting weaker and yet, paradoxically more fatiguable than, those of young men. These changes may be explained by a slowing of the Ca 2+ kinetics in the remaining muscle fibres of the elderly and restricted blood supply during intermittent exercise. Long term immobilisation due to injury caused a substantial reduction in the force generating capacity of the triceps surae and a change in twitch time course which could be explained by selective type I fibre atropy. In contrast voluntary immobilisation for 2 weeks caused a reduction of maximal voluntary force and a prolongation of the twitch response which could not be accounted for by loss of contractile machinery. Voluntary dynamic exercise involving concentric contraction of the triceps surae produced small short lasting force decrements. Eccentric contractions caused large long lasting decreases in force particularly at low stimulus frequencies, which were explained by uncoupling of excitation and contraction. Responses to submaximal stimulation were found to be voltage dependent and did not accurately reflect the response of the whole muscle. The need for supramaximal stimulation in the assessment of weakness and fatigue in the human triceps surae was highlighted.