The CMJ test appears a suitable athlete-monitoring method for NM-fatigue detection. However, the current approach (ie, CMJ-TYP) may overlook a number of key fatigue-related changes, and so practitioners are advised to also adopt variables that reflect the NM strategy used.

It has been suggested that a critically high body core temperature may impair central neuromuscular activation and cause fatigue. We investigated the effects of passive hyperthermia on maximal isometric force production (MVC) and voluntary activation (VA) to determine the relative roles of skin (T(sk)) and body core temperature ( T(c)) on these factors. Twenty-two males [VO(2max)=64.2 (8.9) ml x kg(-1) min(-1), body fat=8.2 (3.9)%] were seated in a knee-extension myograph, then passively heated from 37.4 to 39.4 degrees C rectal temperature (T(re)) and then cooled back to 37.4(o)C using a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5 degrees C intervals during both heating and cooling. Passive heating to a T(c) of 39.4(o)C reduced VA by 11 (11)% and MVC by 13 (18)% (P<0.05), but rapid skin cooling, with a concomitant reduction in cardiovascular strain [percentage heart rate reserve decreased from 64 (11)% to 29 (11)%] and psychophysical strain did not restore either of these measures to baseline. Only when cooling lowered T(c) back to normal did VA and MVC return to baseline (P<0.05). We conclude that an elevated T(c) reduces VA during isometric MVC, and neither T(sk) nor cardiovascular or psychophysical strain modulates this response. Results are given as mean (SD) unless otherwise stated.

Fatigue during hyperthermia may be due in part to a failure of the central nervous system to fully activate the working muscles. We investigated the effects of passive hyperthermia on maximal plantar flexor isometric torque (maximal isometric voluntary contraction) and voluntary activation to determine the roles of local skin temperature, core temperature, and peripheral muscle temperature in fatigue. Nine healthy subjects were passively heated from 37.2 to 39.5 degrees C (core temperature) and then cooled back down to 37.9 degrees C using a liquid-conditioning garment, with the right leg kept at a thermoneutral temperature throughout the protocol, whereas the left leg was allowed to heat and cool. Passive heating resulted in significant decreases in torque from [mean (SD)] 172 N x m (SD 39) to 160 N x m (SD 44) and in voluntary activation from 96% (SD 2) to 91% (SD 5) in the heated leg, and maximal isometric voluntary contraction decreased similarly from 178 N xm (SD 37) to 165 N x m (SD 38) and voluntary activation from 97% (SD 2) to 94% (SD 5) in the thermoneutral leg. The initiation of cooling, which produced a rapid decrease in skin temperature and cardiovascular strain [heart rate reserve decreased from 58% (SD 12) to 31% (SD 12)], did not immediately restore either torque or voluntary activation. However, when core temperature was lowered back to normal, torque and voluntary activation were restored to baseline values. It was concluded that an increase in core temperature is a factor responsible for reducing voluntary activation during brief voluntary isometric contractions and that temperature-induced changes in the contractile properties of muscle and local thermal afferent input from the skin do not contribute significantly to the decrement in torque.

This study investigated the relationship between sprint start performance (5-m time) and strength and power variables. Thirty male athletes [height: 183.8 (6.8) cm, and mass: 90.6 (9.3) kg; mean (SD)] each completed six 10-m sprints from a standing start. Sprint times were recorded using a tethered running system and the force-time characteristics of the first ground contact were recorded using a recessed force plate. Three to six days later subjects completed three concentric jump squats, using a traditional and split technique, at a range of external loads from 30-70% of one repetition maximum (1RM). Mean (SD) braking impulse during acceleration was negligible [0.009 (0.007) N/s/kg) and showed no relationship with 5 m time; however, propulsive impulse was substantial [0.928 (0.102) N/s/kg] and significantly related to 5-m time ( r=-0.64, P<0.001). Average and peak power were similar during the split squat [7.32 (1.34) and 17.10 (3.15) W/kg] and the traditional squat [7.07 (1.25) and 17.58 (2.85) W/kg], and both were significantly related to 5-m time ( r=-0.64 to -0.68, P<0.001). Average power was maximal at all loads between 30% and 60% of 1RM for both squats. Split squat peak power was also maximal between 30% and 60% of 1RM; however, traditional squat peak power was maximal between 50% and 70% of 1RM. Concentric force development is critical to sprint start performance and accordingly maximal concentric jump power is related to sprint acceleration.