Strength and muscle architecture changes after isokinetic training

A muscle’s architecture (e.g. pennation or fascicle angle, fascicle length, cross-sectional area, etc.) is purported to have significant effects on its functional properties. While isotonic resistance training (RT) in humans has been shown to elicit specific architectural changes in muscle, it is as yet unclear how muscle adapts to training of prescribed velocities, modes and ranges of motion; isokinetic training would allow the assessment of architectural changes in response to such training. The purpose of the present study was twofold: 1. to examine the magnitude of strength and architecture changes in subjects performing maximal concentric/eccentric isokinetic training to determine its viability for future studies investigating muscle architecture changes in humans, and 2. to determine the time-course of architecture adaptations and to determine whether they are related to the early strength changes observed with RT. Seven men and 8 women completed 5 wk of unilateral (right leg), concentric/eccentric, isokinetic knee extension RT at 1.05 rad.s-1 (60deg.s-1) while 7 men and 7 women served as non-training controls. No subjects had performed RT in the preceding year. Training was performed three times a week with four (weeks 1 – 2.5) or five (weeks 2.5 – 5) sets of 6 maximal repetitions being performed with 1 min rest. Concentric and eccentric knee extensor torque of the right leg at 1.05 and 3.14 rad.s-1 was tested before and after 2.5 and 5 weeks of training. Fascicle angle (FA), aponeurosis angle (AA), cross-sectional muscle thickness (MT) and physiological muscle thickness (PT; MT calculated perpendicular to line of muscle fascicles) at three regions of the vastus lateralis muscle was also measured using ultrasonography; the variables were calculated using Peak Motus software after the necessary muscle landmarks were digitised from ultrasound images. Fascicle length (FL) was then estimated from FA, AA and MT. Isokinetic torque changes for all subjects are shown in Figure 1. Concentric knee extensor torque at 1.05 rad.s-1 increased significantly after 5 wk in men and there was a near-significant increase in women after 2.5 wk (p\u3c0.06, ES \u3e 1.0). There were also significant increases in eccentric torque after 2.5 and 5 wk in both men and women. There were no significant changes in knee extensor torque at 3.14 rad.s-1 in any group and no strength changes in the control subjects. Despite significant strength increases after training, there were no changes in MT, PT, FL or FA in either control or training groups, although there were differences in MT and FA along the length of the vastus lateralis (PT and FL were only calculated at mid-length). After training, concentric and eccentric torque correlated moderately with MT and PT (r = 0.48 – 0.61; no correlation before training), however there was no correlation between the subjects’ strength changes and their architectural changes over the training period. The results suggest that changes in knee extension torque are rapid (2.5 wk) in previously non-strength trained subjects. Given that both concentric and eccentric phases were performed maximally in training, the relatively greater increases in eccentric strength compared to concentric strength suggests that a greater window of adaptation exists in the eccentric phase. These changes were not mirrored by changes in muscle architecture. As such, significant increases in isokinetic strength seem to occur without architecture adaptation; this implicates neural mechanisms in the strength changes. The finding in the present study that muscle size was related to strength after, but not before, training is consistent with this hypothesis since subjects with larger muscles produced more force only after the training period. It is unclear if further strength increases would be associated with architectural adaptation