Experimental approach
Participants layed in a prone position on an isokinetic dynamometer (IsoMed 2000, D. & R. Fersti GmbH, Hemau, Germany). Based on the neutral position of the ankle joint, the ankle from the randomly chosen leg was set in 25° plantarflexion to obtain a standardized starting point of passive motion. It was then moved passively from the starting position to the maximal achievable dorsal extension using the continuous passive function of the dynamometer. After a warm-up of three plantar flexion–extension cyclesAll participants underwent the assessment procedure twice with a wash out period of 24 h: in one session, the knee was fixed in extension (0° knee angle) while in the other condition, the knee was fixed in 60° flexion. The order of the two conditions was determined using balanced randomization. The 60° angle in the flexed position was chosen, because the gastrocnemius has been shown to be slack at this point and hence, no or only small magnitudes of force transmission can be expectedSchematic visualization of the experimental approach. Participants underwent two conditions: in randomized order, in prone position, the ankle was moved between plantar flexion and dorsal extension by use of a dynamometer (arrows) with the knee in extended (Tissue displacement during passively induced ankle motion was visualized using video recordings with two high-resolution ultrasound devices (Siemens Acuson X300 PREMIUM EDITION, Siemens Medical Solutions USA, Inc., Mountain View, CA and Siemens Acuson Redwood, Siemens Healthcare GmbH, Erlangen, Germany). Linear array transducers (VF10-5 linear array, 5.0–10.0 MHz, Siemens Medical Solutions USA, Inc., Mountain View, CA and 10L4 linear array, 3.3–11.4 MHz, Siemens Healthcare GmbH, Erlangen, Germany) were placed over (1) the GM and (2) the SM muscle. In detail, the transducers were positioned on the proximal third of the muscle belly of the GM and on the distal third of the SM. US transducer locations as well as electrode locations were marked on the skin with a permanent marker to ensure identical measurement sites between the two conditions. Participants were instructed to renew the markings if necessary. A schematic illustration of the experimental approach is shown in Fig. Exemplary measurement with the knee 60° flexed. In prone position, the ankle was moved three times between plantar flexion and dorsal extension, using the continuous passive function of the dynamometer. Ultrasound recordings of the gastrocnemius medialis and the semimembranosus were made to estimate tissue displacement induced by ankle motion. Electromyography was used to confirm muscle inactivity.