Explosive Strength of the Knee Extensors: The Influence of Criterion Trial Detection Methodology on Measurement Reproducibility

Abstract

The present study was conducted to assess test-retest reproducibility of explosive strength measurements during single-joint isometric knee extension using the IsoMed 2000 dynamometer. Thirty-one physically active male subjects (mean age: 23.7 years) were measured on two occasions separated by 48–72 h. The intraclass correlation coefficient (ICC 2,1) and the coefficient of variation (CV) were calculated for (i) maximum torque (MVC), (ii) the peak rate of torque development (RTDpeak) as well as for (iii) the average rate of torque development (RTD) and the impulse taken at several predefined time intervals (0–30 to 0–300 ms); thereby explosive strength variables were derived in two conceptually different versions: on the one hand from the MVC-trial (version I), on the other hand from the trial showing the RTDpeak (version II). High ICC-values (0.80–0.99) and acceptable CV-values (1.9–8.7%) could be found for MVC as well as for the RTD and the impulse taken at time intervals of ≥100 ms, regardless of whether version I or II was used. In contrast, measurements of the RTDpeak as well as the RTD and the impulse taken during the very early contraction phase (i.e. RTD/impulse0–30ms and RTD/impulse0–50ms) showed clearly weaker reproducibility results (ICC: 0.53–0.84; CV: 7.3–16.4%) and gave rise to considerable doubts as to clinical usefulness, especially when derived using version I. However, if there is a need to measure explosive strength for earlier time intervals in practice, it is, in view of stronger reproducibility results, recommended to concentrate on measures derived from version II, which is based on the RTDpeak-trial.

Device and method matter: A critical evaluation of eccentric hamstring muscle strength assessments

“…The conflicting finding compared to the results in van Dyk et al and the low eccentric torque (~110 Nm) on the IKD compared to similar observations from individuals exposed to exercises on an isokinetic dynamometer (~170‐180 Nm), is presumably predominantly elucidated by the hip‐dependent effects on the length‐tension relationship of the hamstring muscle. Measures in a position of hip flexion — a position that is still common in isokinetic measurement — cause greater hamstring muscle length during IKD movements and can increase the obtained eccentric hamstring torque by a factor of ~1.5 . We have not conducted functional tests that might help to predict some transfer effects to athletes’ performance, but knowing that competitions in various sports or daily activities hardly require deep hip flexion, it seems rational that a measure with an extended hip has been considered the most appropriate method in relation to the physiological muscle length‐tension relationship .…”

“…Participants were fastened in a supine position and secured to the IKD via adjustable straps and pads across the shoulders, chest, pelvis, and thigh . Their hip joint angle was set at 0° (0° = full extension), and the knee joint center was carefully aligned with the dynamometer axis of rotation (Figure A).…”

Specificity of eccentric hamstring training and the lack of consistency between strength assessments using conventional test devices

“…In IKD testing, participants lay in a supine position, with the hip joint angle set at 0° (0° = full extension) and non-tested body segments firmly fixed at anatomical positions 42 . The dynamometer axis of rotation and the knee joint centre were carefully aligned, and participants were instructed to pull the lever arm as hard and fast as possible toward the buttocks.…”

Familiarization and Reliability of the Isometric Knee Extension Test for Rapid Force Production Assessment

“…• Rate of force development (RFD): the contractile RFD was obtained from the slope of the force-time curve (ΔForce/Δtime) expressed in N•s− 1 ; thus, the instantaneous RFD peak (RFDmax) was the highest slope of the curve [17]. Average RFD was calculated for three overlapping periods in milliseconds to collect measures in three different phases of the contraction: 0-50 ms (RFD0-50), 0-150 ms (RFD0-150) and 0-250 ms (RFD0-250) [9,27]. • Impulse: the impulse was calculated through integration of force over time (i.e., cumulated area under the force-time curve) expressed in N•s [17].…”