Akim, H., Takahashi, H., Kuno, S., Masuda, K., Masuda, T., Shimojo, H., Anno, I., Itai, Y., & Kaaatsuta, S. (1999). Early phase adaptations of muscle use and strength to isokinetic training. Medicine and Science in Sports and Exercise, 30, 588-594.

This study investigated the effect of short periods of isokinetic resistance training on muscle use and strength. Males (N = 7) trained the right quadriceps femoris (QF) muscles for nine days, within a two week period, using 10 sets of 5 knee extensions each day. Isometric and isokinetic torques of QF were measured at six angular velocities. Cross-sectional areas of QF were determined from axial images using magnetic resonance imaging (MRI). Transverse 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 rest and immediately after repetitive isokinetic knee extensions based on T2-weighted MR images. Muscle fiber types, fiber area, and phosphofructokinase (PFK) activities were determined from biopsies of the vastus lateralis muscle.

No changes were found in the cross-sectional area of the quadriceps femoris muscles, muscle fiber types, fiber area, and PFK activities after the training. Isometric and isokinetic peak torques at 60-240 degrees/second and relative area of QF activated by knee extensions increased significantly after the training.

Muscle strength increases after short periods of isokinetic resistance training occur without muscle hypertrophy and are possibly due to increased muscle contractile activity.

Implication. Early strength improvements following isokinetic training are most likely due to muscle contractile activity and not hypertrophy.

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