Intramuscular pressure and electromyographic responses of the vastus lateralis muscle during repeated maximal isokinetic knee extensions

The purpose of the study was to investigate changes in intramuscular pressu re (IMP) (maximal during contraction - peak IMP, and between contraction, r elaxation IMP - RxIMP) and surface electromyographic activity (EMG) paramet ers [mean frequency of the power spectrum (fmean), and signal amplitude, ro ot mean square (RMS)] throughout 100 repetitive isokinetic contractions for six healthy subjects. Parameters were recorded simultaneously from the vas tus lateralis muscle during maximal knee extension. Regression analyses rev ealed significant decreases for peak IMP and fmean, and an increase in RxIM P; RMS, however, did not change. All parameters demonstrated trends of chan ge throughout the contractions that were non-linear. Details and inter rela tions for RxIMP and fmean were highlighted to express intramuscular fluid a ccumulation and fatigue development, respectively. Individual regression an alyses for RxIMP revealed significant positive correlations for all subject s (range of r=0.62 to 0.89). At group level, mean RxIMP increased from 6.0 mmHg for the 1st contraction to 14.0 mmHg for the 100th contraction. For fm ean, individual regressions were significantly negative for all subjects (r =-0.75 to -0.89). Fmean decreased from 89.2 Hz for the 1st contraction to 6 3.3 Hz for the 100th contraction. When the data were delineated between the fatigue (contractions 1-40) and endurance phases (41-100), the slopes of i ncrease for RxIMP, and of decrease for fmean were significantly greater dur ing the fatigue phase. RxIMP throughout the 100 contractions correlated neg atively with fmean for each subject (r=-0.54 to -0.78); when delineated, th e correlation between parameters was significantly greater for the fatigue as compared with the endurance phase. Relaxation IMP trends are mainly attr ibuted to intramuscular water accumulations during repetitive contractions. In spite of consistent correlations between RxIMP and fmean a causal assoc iation could not be established. It may be suggested that a common factor o ccurring during the fatiguing process governs changes in RxIMP and fmean.