Reliability and validity of a new device to measure isometric strength in polyarticular exercises

Background, The purpose of this study was to assess the validity of a new; device to evaluate isometric strength during multi-joint exercise such as t he squat or bench press. methods, The device used an electric motor-driven bar to simultaneously gen erate and measure forces during weight lifting exercises. This new device a nd a force platform measured the forces generated by either the motor on a fixed telescopic steel girder (passive condition) or a subject pushing vert ically against the bar from three squat positions (active condition). In th e passive condition, 252 measurements were made, with 3 trials for 6 bar he ights and 14 bar loads. In the active condition, 8 young physically active students (age, height and body mass were 25.1+/-2.6 years, 179.3+/-7.2 cm a nd 82.0+/-9.9 kg, respectively) performed 3 maximal isometric strength (MIS ) trials in each of the 3 squat exercise positions (parallel, half and quar ter squat), and 3 additional MTS trials in one position randomly assigned t wo weeks later to test inter-day reliability, Results. In the passive condition, no differences were observed between the forces measured by the force platform and the new device. The coefficient of linear regression (r) and the coefficient of variation (CV) were 1 and b elow 0.23 %, respectively, In the active condition, the peak MIS measured w as 2828 N and the values oft and CV were above 0.982 and below 5.96%, respe ctively, The assessment of inter-day reliability showed an r value of 0.984 and a CV of 3.98 %. Conclusions. This study demonstrated that the new electric motor-driven exe rciser provides valid and reliable data when used to generate forces and me asure isometric strength throughout the load and motion ranges commonly use d in squat exercise.