Monitoring Muscle Fatigue Progression during Dynamic Exercise.

PURPOSE: To develop and validate a novel model for assessment of peripheral fatigue progression during dynamic exercise.

METHODS: Seven males and four females (24 ± 3 yr) completed one-legged knee-extensor exercise at 40% of peak power output (PPO) for 12 min (40% PPO). Additionally, an exhaustive bout lasting approximately 6 min (84% ± 2% PPO; 59 ± 10 W) was contrasted to two bouts completed at ±5% of the target workload. Trials were completed in randomized order. Percutaneous electrical stimulation of m. vastus lateralis during the passive knee-flexion phase allowed quantification of maximal twitch force every 30 s in parallel with RPE.

RESULTS: Elicited twitch force remained unchanged during the 40% PPO trial. During the three exhaustive bouts, exercise differed in duration (561 ± 154 s, 366 ± 64 s, 245 ± 61 s; P < 0.001) and amplitude of elicited twitch force showed a curvilinear decline across time. Elicited twitch force at exhaustion was approximately 60% reduced and similar between the exhaustive trials (intraclass correlation coefficient, 0.76; 95% confidence interval, 0.48-0.92). The increase in RPE during exercise was strongly correlated to the gradually reduced evoked twitch force (repeated-measures correlation, 0.89; 95% confidence interval, 0.62-0.97).

CONCLUSIONS: The developed model permits quantification of muscle fatigue progression during continuous dynamic one-legged knee-extension and a biphasic fatigue pattern is demonstrated during intense exercise. The model is sensitive to small changes in intensity, and it provides a novel approach for studying muscular mechanisms and their temporal relation to fatigue progression in vivo.