Reliability and measurement error of a maximal voluntary toe plantarflexion measurement process

Despite the growing interest, information regarding the psychometric properties of maximal voluntary isometric toe plantarflexion force and rate of force development (RFD) is lacking. Hence, we investigate the test-retest reliability and measurement error of these outcome measurement instruments measured with a custom-built dynamometer. Twenty-six healthy adults participated in a crossed design with four sessions separated by 5-7 days. RFD was quantified using manual onset and calculating the impulse and the slope in the following time windows: 0-50 ms, 0-100 ms, 0-150 ms, 0-200 ms, 0-250 ms. We estimated the systematic bias of the mean, the intraclass correlation coefficient (ICC) and standard error of measurement (SEM) from the agreement and consistency models. The ICC and the SEM agreement for maximal voluntary isometric toe plantarflexion force along the perpendicular axis were respectively 0.87 (95%CI: 0.76, 0.93) and 27 N (22, 32), while along the resultant of the perpendicular and anterior posterior axis they were 0.85 (0.73, 0.92) and 29 N (23, 35). The results of the consistency model were similar as the estimated variance for session was closer to zero. A systematic bias of the mean between session 1 and 3 was found. For the RFD variables, the ICC agreement ranged from 0.35 to 0.65. The measurement process was found to be reliable to assess maximal voluntary isometric toe plantarflexion force but not RFD. However, a familiarization session is mandatory and these results need to be confirmed in less coordinated (e.g. aging population) individuals.