1. Muscle strength of the adductor pollicis (AP) was studied throughout the menstrual cycle to determine whether any variation in force is similar to the known cyclical changes in ovarian hormones. Three groups of young women were studied: trained regularly menstruating athletes (trained), untrained regularly menstruating (untrained) and trained oral contraceptive pill users (OCU). In addition a group of untrained young men was studied as controls. 2. Maximum voluntary force (MVF) of AP was measured over a maximum period of 6 months.Ovulation was detected by luteinizing hormone measurements or change in basal body temperature. There was a significant increase in MVF (about 10%) during the follicular phase of the menstrual cycle when oestrogen levels are rising, in both the trained and untrained groups. This was followed by a similar drop in MVF around the time of ovulation. Neither the OCU nor the male subjects showed cyclical changes in MVF.We have shown previously that the maximum voluntary force (MVF) which can be exerted by the adductor pollicis muscle (AP) relative to its cross-sectional area (CSA) is 28 % lower in old than in young people (Phillips, Rook, Siddle, Bruce & Woledge, 1993b). In women this decline in MVF/CSA occurs at the time of the menopause, i.e. at the time when ovarian failure leads to a permanent decline in sex hormone secretion. In postmenopausal women using hormone replacement therapy MVF/CSA is greater than in age-matched controls and not less than that of young women (Phillips et al. 1993b). These facts suggest that oestrogen may have a muscle-strengthening action. If this is correct, and if the action is exerted within a few days, then we could expect to see changes in MVF during the menstrual cycle. It is well recognized that, during the follicular phase of the menstrual cycle, oestrogen levels rise to a peak and then fall during the day or two before ovulation, while during the luteal phase the oestrogen levels remain relatively stable but at a higher level than that at the start of the cycle. In contrast, progesterone levels are negligible during the follicular phase but, after ovulation, rise to a peak during the luteal phase (Moghissi, Syner & Evans, 1972). Therefore, it would be predicted from the hypothesis of oestrogen increasing muscle strength, that a rise in MVF would be seen during the follicular phase of the cycle followed by a fall near the time of ovulation. Moreover, if the action of oestrogen was not opposed by that of progesterone, MVF would remain higher during the luteal phase than at the start of the cycle. This paper reports the results of experiments carried out to test these predictions. We compared highly trained athletes, during their training season, with non-training subjects for two reasons: (1) the highly physically active subjects may have developed oligomenorrhoea or amenorrhoea giving anovulatory cycles with little change in cyclical hormones or force; and (2) physical activity itself may saturate any force effect caused by changes in the levels of s...

Results gleaned from use of temperature as a probe to study skeletal muscle performance and mechanisms of activation and contraction are reviewed. Steady-state and non-steady-state responses to changes in temperature are considered. Temperature sensitivities, Q10 values, of mechanical and energetic parameters range from nearly 1 to greater than 5 in frog skeletal muscle. Factors that are less temperature sensitive (Q10 less than or equal to 1.5) are peak tetanic force, instantaneous stiffness, curvature of force-velocity relation, magnitude of labile heat, and mechanical efficiency. Rates with intermediate temperature sensitivities (Q10 greater than 2 but less than 3) include rate of isometric force development, maximum shortening velocity, and relaxation from a brief tetanus. Rates with high temperature sensitivities (Q10 greater than 3) include cross-bridge turnover during an isometric tetanus, isometric economy, maximum power output, Ca2+ sequestration by sarcoplasmic reticulum, relaxation from a prolonged tetanus, and recovery metabolism. The observation that the Q10 for relaxation rate depends on tetanic duration can be explained in terms of the possible role of parvalbumin as a soluble relaxing factor.

SUMMARYMuscle weakness accompanies ageing but its causes are still uncertain. We report maximum voluntary force measurements of adductor pollicis muscle, normalized for cross-sectional area, in twenty-three elderly subjects. Normalized force was lower in the elderly compared with a group of fifty-five young adult subjects by 27±4 % (S.E.M.). This shows that atrophy alone is not the cause of the weakness of old age.

Myosin crossbridges in muscle convert chemical energy into mechanical energy. Reported values for crossbridge efficiency in human muscles are high compared to values measured in vitro using muscles of other mammalian species. Most in vitro muscle experiments have been performed at temperatures lower than mammalian physiological temperature, raising the possibility that human efficiency values are higher than those of isolated preparations because efficiency is temperature dependent. The aim of this study was to determine the effect of temperature on the efficiency of isolated mammalian (mouse) muscle. Measurements were made of the power output and heat production of bundles of muscle fibres from the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles during isovelocity shortening. Mechanical efficiency was defined as the ratio of power output to rate of enthalpy output, where rate of enthalpy output was the sum of the power output and rate of heat output. Experiments were performed at 20, 25 and 30• C. Maximum efficiency of EDL muscles was independent of temperature; the highest value was 0.31 ± 0.01 (n = 5) at 30• C. Maximum efficiency of soleus preparations was slightly but significantly higher at 25 and 30• C than at 20 • C; the maximum mean value was 0.48 ± 0.02 (n = 7) at 25• C. It was concluded that maximum mechanical efficiency of isolated mouse muscle was little affected by temperature between 20 and 30• C and that it is unlikely that differences in temperature account for the relatively high efficiency of human muscle in vivo compared to isolated mammalian muscles.

The efficiency of energy transduction is defined as the ratio of the work done by a muscle to the free energy change of the chemical processes driving contraction. Two examples of the experimental measurement of muscle efficiency are: (1) the classical method of Hill which measures the value during a steady state of shortening, (2) measuring the overall efficiency during a complete cycle of a sinusoidal process, which comes closer to the situation during natural locomotion. The reasons why fatigue might lower efficiency are the following. (1) The reduction in PCr concentration and increase in Pi and Cr concentration which are characteristic of fatigued muscle, reduce the free energy of PCr splitting. This will reduce the efficiency of the recovery process. It is not known whether the efficiency of the initial process is increased to compensate. (2) There is a general conflict between efficiency and power output when motor units are chosen for a task or when the timing of activation is decided. During fatigue more powerful units have to be used to achieve a task which is no longer within the scope of less powerful units. (3) The slowing of relaxation that is sometimes found with fatigue may make it impossible to achieve the short periods of activity required for optimum efficiency during rapid cyclical movements. A reason why fatigue might increase efficiency is that muscles are thought to be more efficient energy converters when not fully activated than when fully active. Full activation is often not achieved in muscle which is considerably fatigued. Available observations do not allow us to find where the balance between these factors lies. The conclusion is thus that experiments of both the types discussed here should be performed.