Within- and between-day test-retest reliability and agreement of isometric and isokinetic, multi-joint, upper- and lower-body strength testing

The aim of this study was to assess the within- and between-day test-retest agreement and reliability of upper- (chest press; CP and seated row; SR) and lower-body (leg press; LP) multi-joint isometric and isokinetic dynamometry using Exerbotics devices. Fourteen recreationally active adults (male = 11, female = 3) completed three testing sessions over a three-week period. On each day, participants performed two isometric testing trials followed by one isokinetic testing trial. Analyses were conducted within a Bayesian estimation framework using multivariate models summarising posterior distributions by their mean point estimate and 95% quantile intervals (QI) for both limits of agreement and variance decomposition ratios (comparable to intraclass correlation coefficients). For isometric testing, the within-day limits of agreement with the mean for the CP were 28.82 [95%QI: 23.12, 35.88], for the LP were 38.76 [95%QI: 30.87, 49.22], and for the SR were 16.65 [95%QI: 13.3, 21.18], and between-day limits of agreement with the mean for the CP were 34.06 [95%QI: 28.46, 41.4], for the LP were 50.46 [95%QI: 41.57, 62.43], and for the SR were 23.2 [95%QI: 19.04, 28.98]. Variance decomposition ratios were 0.939 [95%QI: 0.895, 0.964] and 0.937 [95%QI: 0.893, 0.963] for CP, 0.968 [95%QI: 0.944, 0.981] and 0.967 [95%QI: 0.942, 0.981] for LP, and 0.970 [95%QI: 0.947, 0.983] and 0.969 [95%QI: 0.946, 0.982] for the SR, for within- and between-day, respectively. For isokinetic testing, between day limits of agreement for concentric muscle actions for the CP were 25.73 [95%QI: 17.94, 37.67], for the LP were 74.24 [95%QI: 55.11, 100.1], and for the SR were 17.87 [95%QI: 12.08, 26.91], and for eccentric muscle actions for the CP were 37.42 [95%QI: 25.82, 55.54], for the LP were 119.25 [95%QI: 85.73, 168.24], and for the SR were 28.55 [95%QI: 19.88, 41.77]. Variance decomposition ratios were 0.954 [95%QI: 0.89, 0.984] and 0.931 [95%QI: 0.828, 0.977] for CP, 0.912 [95%QI: 0.797, 0.968] and 0.855 [95%QI: 0.652, 0.95] for LP, and 0.971 [95%QI: 0.927, 0.991] and 0.937 [95%QI: 0.851, 0.978] for the SR, for concentric and eccentric muscle actions respectively. In summary, these data suggest good agreement and good to excellent reliability for multi-joint isometric and isokinetic force measurement.