Effect of fatigue on neuromuscular adaptations in endurance-trained and recreationally active males.

Baseline maximal voluntary isometric contraction torque of the knee extensors did not differ between endurance-trained and recreationally active groups.

• 11 endurance-trained (ET) and 11 recreationally active (RA) male participants were tested
• Both groups performed maximal voluntary isometric contractions (MVIC) of the knee extensors prior to the fatiguing task
• No statistically significant difference in baseline MVIC was observed between groups

The endurance-trained group showed a significantly lower rate of increase in muscle fibre conduction velocity (MFCV) during the ascending phase of the trapezoidal contraction compared to the recreationally active group.

• The ascending phase involved ramping up to 60% MVIC
• The MFCV-torque relationship during the ascending phase was assessed using a mixed linear model
• The ET group showed a significantly lower rate of increase in MFCV (p < 0.001) compared to the RA group
• The ET group also showed lower absolute MFCV at 60% MVIC (p < 0.001) compared to the RA group

Both groups reached task failure at similar times and exhibited comparable reductions in MVIC of approximately 25% following the sustained fatiguing task.

• The fatiguing task was a sustained isometric contraction at 30% MVIC held to task failure
• MVIC reductions were approximately 25% in both groups post-fatigue
• An additional MVIC was completed immediately post-fatigue task to quantify force loss
• Similar task failure times suggest comparable overall fatigue levels between groups

During the sustained fatiguing task, the recreationally active group exhibited significant declines in both median frequency (MDF) and MFCV, whereas these parameters remained unchanged in the endurance-trained group.

• The RA group showed significant decline in MDF (p < 0.001) during the fatiguing contraction
• The RA group showed significant decline in MFCV (p = 0.04) during the fatiguing contraction
• MDF and MFCV remained statistically unchanged in the ET group during the fatiguing contraction
• High-density surface electromyography (HDsEMG) was recorded from the vastus lateralis to estimate RMS, MDF, and MFCV

The authors concluded that endurance training is associated with altered motor unit recruitment and/or muscle membrane properties, likely linked to differences in muscle fibre characteristics.

• Lower MFCV in the ET group at matched force levels may reflect preferential use of slower, smaller motor units
• Unchanged MFCV and MDF during fatigue in the ET group may indicate greater fatigue resistance of recruited muscle fibres
• The authors attributed differences to likely distinctions in muscle fibre type composition between endurance-trained and recreationally active individuals
• These findings suggest that neuromuscular fatigue mechanisms differ between training backgrounds despite similar overall performance decrements