Early phase adaptations of muscle use and strength to isokinetic training

The purpose of this study was to investigate the effect of short periods of isokinetic resistance training on muscle use and strength. Seven men train ed the right quadriceps femoris muscles (QF) 9 d for 2 wk using 10 sets of 5 knee extensions each day. Isometric and isokinetic torques of QF were mea sured at six angular velocities. Cross-sectional areas (CSA) of QF were det ermined from axial images using magnetic resonance imaging (MRI). Transvers e relaxation time (T2) and activated area of QF, which represented the area greater than the mean resting T2 + 1SD in MRI pixels, were calculated at r est and immediately after repetitive isokinetic knee extensions based on T2 -weighted MR images. Muscle fiber types, fiber area, and phosphofructokinas e (PFK) activities were determined from biopsies of the vastus lateralis mu scle. No changes were found in CSA of QF, muscle fiber types, fiber area, a nd PFK activities after the training. Isometric and isokinetic peak torques at 60-240 degrees.s(-1) and relative area of QF activated by knee extensio ns increased significantly after the training. These results suggest that m uscle strength increases after short periods of isokinetic resistance train ing without muscle hypertrophy would be due to increased muscle contractile activity.