2016 Volume 37 Pages 138-146
The purpose of the present study was to examine the factors influencing previously reported exercise-dependence in muscle hypertrophy of the biarticular thigh muscles （i.e., single-joint training induces sizable hypertrophic response while multi-joint training does not）, through measurement of muscle activation patterns during a multi-joint exercise. Twelve healthy men performed leg press at a load corresponding to 80% of one repetition maximum （1RM） until exhaustion. Surface electromyographic （EMG） signals were obtained from the quadriceps femoris （vastus lateralis, vastus medialis, rectus femoris） and hamstrings （biceps femoris, semitendinosus, semimembranosus）, and root mean square values of the EMG signals （RMS-EMG） during each repetition were determined （experiment 1）． In the experiment 2, five healthy men conducted leg press at a 40% of 1RM （3 sets with 16 repetitions） and an 80% of 1RM （6 sets with 4 repetitions） load on separate days （separated by one week）． In addition, transverse relaxation time （T2）-weighted magnetic resonance images of the thigh were obtained before and immediately after the leg press. The T2 of each muscle at mid-thigh was calculated. In the experiment 1, significant increases in the activation of the vastus lateralis and medialis were found following fatiguing leg press, whereas those of the biarticular muscles did not change except for the biceps femoris. In the experiment 2, the leg press exercise induced an increase in T2 of the monoarticular muscles, but not of the biarticular muscles. The relative increase in T2 of each muscle was similar between the two exercise intensities. These results suggest that activation patterns of the thigh muscles differ between monoarticular and biarticular muscles. These differences can explain the lack of hypertrophic response of the biarticular muscles following multi-joint training found in previous studies.
