| It has been suggested that velocity specific adaptations to strength training may be related to how fast a muscle group contracts (internal velocity) regardless of external resistance (external velocity). This study was designed to investigate the effects of 8 weeks of high velocity isokinetic concentric and isometric ballistic strength training on velocity-specific adaptations. In addition, modifications in muscle fiber type and cross sectional area (CSA), myosin heavy chain (MHC) isoforms, and neuromuscular adaptations were also assessed. Twenty adult male subjects were randomly divided into two training groups: isokinetic concentric at 300°·sec-1 (IC, n = 9) and isometric ballistic at 60° knee angle (IB, n = 11). Both groups performed maximum knee extension despite the external resistance. All subjects were pre- and posttested for peak torque and peak power at 60, 120, 180, 240, and, 300°·sec-1, maximum isometric progressive contraction (MIPC) and maximum isometric ballistic contraction (MIBC). Electromyography signals (EMG) were recorded from the vastus lateralis (VL) and vastus medialis (VM) muscles. Electromechanical delay (EMD), peak and average rate of force development (RFDpeak and RFDaverage ), and time to peak torque (TPT) were calculated. Muscle biopsy samples were extracted from the VL and analyzed for changes in CSA, muscle fiber type, and MHC isoform composition. The results showed that muscle fiber type and CSA, EMD, TPT, and EMG did not change significantly in either the IC or IB group. Myosin heavy chain types I and Ha percentage changed from 32.8% to 41.9% and from 52.4 to 42.5%, respectively, in the IB group (P < 0.05). Maximum isometric progressive contraction, MIBC, and RFDpeak improved significantly in both groups. Significant increases (P < 0.05) in peak torque and peak power were observed only in the IC group. The greatest change in torque occurred at 60°·sec-1 (21.7%) followed by 180 (17.5%), 240 (17.3%), 120 (13.5%), and 300·sec-1 (12%). Power increased 12% to 18% at all velocities with exception of 60°·sec-1. We conclude that training involving external velocity increased both isokinetic concentric and isometric torque whereas training with internal velocity improved only isometric torque production. |
