Test-retest reliability of gastrocnemius medialis fascicle force-length relationship

Fascicle force-length relationship is one major basic mechanical property of skeletal muscle, subsequently influencing movement mechanics. While force-length properties are increasingly described through ultrafast ultrasound imaging, their test-retest reliability remains unknown. Using ultrafast ultrasound, and electrically evoked contractions at various ankle angles, gastrocnemius medialis fascicle force-length relationship was assessed twice, few days apart, in sixteen participants. The test-retest reliability of the resulting fascicle force-length relationship key parameters - i.e., maximal force (F_max), and optimal fascicle length (L₀) - was evaluated considering (i) all the trials obtained at each ankle joint and (ii) the mean of the two trials obtained at each tested angle. Considering all trials, L₀ indicated a 'high' test-retest reliability, with intra-class correlation coefficients (ICC) of 0.89 and F_max a 'moderate' reliability (ICC = 0.71), while when averaging the two trials L₀ reliability was 'very-high' (ICC = 0.91), and F_max reliability 'moderate' (ICC = 0.73). All values of coefficient of variation and standard error of measurement were low, i.e., ≤7.7 % and ≤0.35 cm for L₀ and ≤3.4 N for F_max, respectively. Higher absolute reliability was reported for L₀ than F_max, with better reliability when averaging the two trials at each angle. All these parameters, in accordance with the limit of agreement, demonstrated that L₀ and F_max test-retest reliability is acceptable, particularly when averaging multiple points obtained at a given angle. Interestingly, the shape of the fascicle force-length relationship is more variable. Therefore, L₀ and F_max can be used to compare between days-effects following an intervention, while a comparison of fascicle operating lengths may require more precautions.