Conventionally, the hamstring:quadriceps strength ratio is calculated by dividing the maximal knee flexor (hamstring) moment by the maximal knee extensor (quadriceps) moment measured at identical angular velocity and contraction mode. The agonist-antagonist strength relationship for knee extension and flexion may, however, be better described by the more functional ratios of eccentric hamstring to concentric quadriceps moments (extension), and concentric hamstring to eccentric quadriceps moments (flexion). We compared functional and conventional isokinetic hamstring: quadriceps strength ratios and examined their relation to knee joint angle and joint angular velocity. Peak and angle-specific (50°, 40°, and 30° of knee flexion) moments were determined during maximal concentric and eccentric muscle contractions (10° to 90° of motion; 30 and 240 deg/sec). Across movement speeds and contraction modes the functional ratios for different moments varied between 0.3 and 1.0 (peak and 50°), 0.4 and 1.1 (40°), and 0.4 and 1.4 (30°). In contrast, conventional hamstring:quadriceps ratios were 0.5 to 0.6 based on peak and 50° moments, 0.6 to 0.7 based on 40° moment, and 0.6 to 0.8 based on 30° moment. The functional hamstring:quadriceps ratio for fast knee extension yielded a 1:1 relationship, which increased with extended knee joint position, indicating a significant capacity of the hamstring muscles to provide dynamic knee joint stability in these conditions. The evaluation of knee joint function by use of isokinetic dynamometry should comprise data on functional and conventional hamstring:quadriceps ratios as well as data on absolute muscle strength.
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