Stiffness measurements.

Passive and reflex mediated components of stiffness around the ankle joint were obtained in the participants with CP by biomechanical and electrophysiological evaluation according to Lorentzen et al. (2010) and Willerslev-Olsen et al. (2013). Briefly, the CP participants were seated in a reclining armchair with the examined foot attached to a foot plate. The foot plate could be rotated by a motor (model 26; CEM), which was driven by a DC power amplifier (model 2708; Brüel & Kjaer) and could deliver maintained torques up to 80 Nm and peak torques up to 120 Nm. An electrogoniometer connected to the foot plate measured the angle position, and a torque meter measured the torque exerted on the foot plate before and during the stretch perturbations. The hip joint was positioned in 100° flexion, the knee in 55° flexion, and the ankle joint in 20° plantar flexion. The plantar flexors were stretched by perturbations consisted of ramp and hold dorsiflexions with an amplitude of 6° at 17 different velocities between 5° and 220°/s and with a hold time of 460 ms. These stretches were delivered every second in a random order until 10 trials per velocity were collected. The interval between stretches was 1 s.

Passive stiffness was calculated as a mean of torque responses at slow velocities (10°/s) that did not elicited stretch responses in any participants. Reflex-mediated stiffness was calculated as a mean of the torque responses to the fastest velocities (210°/s) minus the passive stiffness (Lorentzen et al. 2010). The fastest velocity was chosen because this elicited the maximal reflex response. To be confident that this was the case, the other 15 velocities were included, although they are not used in the present study. For more detailed description we kindly refer to Lorentzen et al. (2016). Stiffness measurements from the dorsiflexor muscles were not included, because unpublished work from our laboratory (Lorentzen J, personal observation) shows that dorsiflexor stiffness is overruled by the much larger muscle group of the triceps surae during plantar flexor activation.