When body water loss has occurred, various effects on neuromuscular function and short-term power have been reported. Muscle strength during a muscle contraction is determined by the ability of the nervous system to recruit motor units in concert with the number of muscle contractile units in cross-section. Therefore, it is of interest in this section to consider whether a reduction in muscle water has the potential to alter force generation capability or energy production when maximally stimulated.
The majority of published studies indicate that dehydration, up to a loss of 7% of body mass, can largely be tolerated without a reduction in measured maximal isometric or isotonic muscle contractions. When muscle strength reductions have been noted with dehydration, the upper body muscles appear to be affected to a greater extent than the lower body muscles. Also, the published evidence suggests that there appears to be a greater likelihood of strength reduction if dehydration is induced as a result of prolonged food and fluid restriction and questions have been raised as to the role that factors other than dehydration per se may have on the findings reported. When maximal aerobic power has been investigated with regard to hydration status, the findings suggest that when hypohydration of less than 3% of body mass is present there is no subsequent effect on maximal aerobic power, but when the dehydration increases to 3–5% of body mass power reductions have been recorded. Therefore, the available evidence suggests that body water loss equivalent to 3% of body mass may be the critical level when aerobic power is being considered.
Also, as seems to be the case with endurance exercise performance, decrements in maximal aerobic power appear to exist with slightly lower levels of dehydration when the environmental conditions are hot.