Influence of length-restricted strength training on athlete's power-load curves of knee extensors and flexors.

This study investigated whether different length-restricted strength training regimens affect voluntary explosive concentric power-load curves of the quadriceps femoris (QF) and hamstring (HAM) muscles. Thirty-two athletes were divided into 3 different training groups (G1-G3): G1 performed isometric training at knee joint angles corresponding to long muscle-tendon unit (MTU) length for QF and HAM; G2 conducted concentric-eccentric contraction cycles that were restricted to a knee joint range of motion corresponding to predominantly long MTU length for QF and HAM; and G3 combined the protocols of G1 and G2. Knee joint angle-dependent power-load curves during maximal voluntary explosive concentric knee extensions and flexions were measured for loads corresponding to 40, 60, and 80% of individual 1 repetition maximum at 5 different occasions: 2 times before, after 5 and 8 weeks of training, and 4 weeks post training. Power values of each subject were normalized to the largest value produced at any knee joint position (percent maximum). Obtained by curve fitting, the optimal knee joint angle for power production of QF and HAM remained unaltered throughout the course of the study for all testing loads and training groups. Therefore, different strength training regimens with a common restriction to long MTU lengths failed to induce length-dependent alterations in athlete's voluntary concentric power-load curves of knee extensors and flexors. The approach to develop strength training programs that induce systematic shifts in length-dependent power production of QF and HAM is of direct practical relevance for athletic activities such as cycling, ice skating, and skiing. However, restricting the muscle excursion range during loading seems to be an inappropriate trigger to cause length-dependent alterations in athlete's voluntary concentric power-load curves.