Quadriceps and hamstring isokinetic strength and electromyographic activity measured at different ranges of motion: A reproducibility study

Isokinetic strength measurements of the quadriceps and hamstring that are commonly conducted using a 90° range of motion (RoM) may involve some risk to specific knee patient groups. Testing these muscles at a much shorter RoM may reduce the risk but in order to render this method clinically acceptable the reproducibility of the derived test findings has to be established. Therefore the main objective of this study was to assess the reproducibility of isokinetic peak torque and normalized EMG scores of these muscles based on 90° (0–90° flexion, LR) and three successive short RoMs: 0–30° (SR1), 30–60° (SR2) and 60–90° (SR3). Eight healthy subjects were tested three times with a 2 week between-session interval. All tests were performed on the dominant limb and consisted of maximal concentric and eccentric exertions. The velocities applied were 90°/s for LR and 30°/s for each of the SRs. Findings indicated no between-session improvement in strength. Based on the coefficient of variation the measurement error for all isokinetic strength scores remained stable throughout the testing sessions ranging 0.6–13.9% with the absolute majority of instances less than 10%. The reproducibility of the EMG scores was poorer ranging 1.5–25% and 0.5–19% for the quadriceps and hamstring, respectively. It is concluded that testing of knee muscles at short (30°) RoMs does not compromise the reproducibility of the strength or EMG scores derived from the commonly used RoM of 90°. However, whereas strength was reproducible to within the accepted clinical standards, the corresponding EMG scores were characterized by a wider error band.