Intramuscular pressure and tissue oxygenation during low-force static contraction do not underlie muscle fatigue

Aim: To test the hypothesis that time-wise increase in intramuscular pressure (IMP) and subsequent decrease in muscle tissue oxygenation (TO2) results in muscle fatigue development during a non-exhaustive, low-force contraction evidenced by changes in electromyogram (EMG) and particular mechanomyogram (MMG). Methods: Seven subjects performed static elbow flexion at 10% maximal voluntary contraction (MVC) for 10 min (10% MVC10 min). Surface EMG, MMG, IMP and TO2 measured by near-infrared spectroscopy was recorded from m. biceps brachii during 10% MVC10 min and during 5% MVC test contractions of 1 min duration performed before 10% MVC10 min, 10 and 30 min post-exercise. EMG and MMG were analysed for root mean square (rms) and mean power frequency (mpf). Results: During 10% MVC10 min MMGrms increased from initial level of 0.04 +/- 0.01 to 0.11 +/- 0.07 m s(-2) in the last minute and MMGmpf and EMGmpf decreased from 34.9 +/- 8.2 to 21.3 +/- 3.8 Hz and from 71.7 +/- 10.9 to 61.7 +/- 10.0 Hz respectively. Similar changes were present in 5% MVC test contractions 30 min post-exercise. Initially, TO2 decreased by 6.9 +/- 6.5% of resting level but returned to rest within 1 min. IMP remained constant during the contraction after an initial fourfold increase from resting level of 12.2 +/- 10.4 mmHg. Conclusion: IMP was anticipated to increase with time of contraction due to e.g. increased muscle water content; but this was not confirmed. Consequently, muscle blood flow was unlikely to be impeded with contraction time, which may account for the maintenance of TO2. Thus, decreased TO2 did not underlie either acute or long-term muscle fatigue development evidenced by changes in EMG and particular MMG variables.