Neuromuscular fatigue in males and females during strenuous heavy resistance loading.

To examine effects of fatiguing heavy resistance loading on voluntary neural activation and force production ten male and nine female athletes loaded their leg extensor muscles by performing 10 sets in the squat-life exercise by performing 10 repetitions in each set with the maximal load possible (about 70% of 1 RM in each set; 10*10*70%). A recovery time of 3 minutes was allowed between each set. Maximal voluntary neural activation (integrated EMG), maximal isometric force, force-time and relaxation time curves of the leg extensor muscles were measured before, during and immediately after the loading session. The maximal force was recorded also after rest for 1 hour, 2 hours, 1 day and 2 days. The relative decrease of 47.1 +/- 10.5% (p < 0.001) recorded in maximal force in males was significantly (p < 0.01) greater than the decrease of 29.4 +/- 13.8% (p < 0.001) recorded for females. Significant decreases were also observed in the maximal IEMGs of the exercised muscles both in males (p < 0.01) and females (p < 0.05). The exercise loading led also to great shifts in the shape of the force-time curve, on average, in males by 49.3 +/- 11.5% (p < 0.001) but less (p < 0.05) in females by 32.4 +/- 16.2% (p < 0.001). In males significant (p < 0.01) decreases took place also in the voluntary rapid neural activation of the exercised muscles in the IEMG-time curve. The time of force relaxation lengthened (p < 0.05) in both groups. The mean blood lactate concentration increased in males up to 15.0 +/- 4.0 mmol x 1(-1) (p < 0.001) and less (p < 0.001) in females up to 6.0 +/- 1.8 mmol x 1(-1) (p < 0.001). Maximal force recovered during the first two hours and the first day of rest less in males than in females. The present findings suggest that strenuous heavy resistance loading may result in remarkable acute fatigue in the neuromuscular system observable not only in the contractile characteristics of the exercised muscles as the decrease in force production but also by the decrease in the voluntary neural activation of the exercised muscles. In males neuromuscular fatigue may be greater and recovery from fatigue slower than in females.