Responses of vastus lateralis muscle fibers to cycling training with different loading parameters in human

Responses of single muscle fibers of human vastus lateralis muscle to 8-week training with different loading parameters were studied in 22 male subjects, using a specially designed training machine with compulsory rotation of pedals induced by electrical engine. Three types of cycling were compared: 1) forward (concentric) with high velocity and 2) rearward (eccentric) compulsory rotation of pedals with high or 3) low velocity. Pedaling/rotation rate was 70 per min for high velocity groups and 35 rotations per min for low velocity group. Loads for high and low velocity groups were approximately 60% and 90% of the maximal alactic power accordingly. The repetitive exercise consisting of 1-min work periods divided by long (10 min) resting periods was used. Exercise bouts were repeated 5-7 times with 10-min resting periods. The training was performed twice a week for 8 weeks. Biopsy samples were taken from the vastus lateralis muscle before and after the training. Single muscle fibers were mechanically isolated and fiber cross-sectional area (CSA), myonuclear number, myonuclear CSA, and myonuclear domain were analyzed using a confocal microscope. Myonuclear CSA and myonuclear number per mm of fiber length did not change significantly after training in both regimes, although fiber CSA was increased significantly after eccentric training at either low or high intensity. Myonuclear domain size was also increased significantly by concentric and eccentric exercise. It was suggested that eccentric training was more effective for induction of muscle fiber hypertrophy. However, this phenomenon was not directly related to the increase of number and/or size of myonuclei.