Experimental Design

Participants visited the laboratory for a familiarization session that included measurement of muscle strength and body mass to facilitate group allocation, as well as practice isometric ramp contractions. Thereafter, two duplicate laboratory measurement sessions were conducted both pre (sessions 7–10 days apart prior to the first training session) and post (2–3 and 4–6 days after the last training session). MRI scans of the thigh and knee were conducted pre (5 days prior to the start of the first training session) and post (2–3 days after the final training session) to measure knee extensor tissue size (quadriceps muscle volume, vastus lateralis aponeurosis area, patellar tendon CSA) and patellar tendon moment arm. All measurement and training sessions were performed with the same isometric apparatus and the same joint angle configuration [knee and hip angles of 115° and 126° (180° = full extension)]. Training for ECT and SCT group’s involved unilateral isometric contractions of both legs three times a week for 12 weeks (36 sessions in total), whereas CON participants attended only the measurement sessions and maintained their habitual activity. All participants were instructed to maintain their habitual physical activity and diet throughout the study, which was verified by informal questioning during post measurement. Measurement sessions involved a series of contractions of the dominant (preferred kicking) leg in the following order: maximum voluntary contraction [MVCs to establish maximum voluntary torque (MVT)]; ramp voluntary contractions of the knee extensors to establish tendinous tissue properties, and knee flexor MVCs. Knee joint torque was recorded throughout contractions. Knee flexor surface electromyography was recorded during knee flexor MVCs, as well as during knee extensor ramp contractions to account for antagonist co-activation in the estimate of tendon force in knee extensor ramp contractions. Ultrasound images of the vastus lateralis muscle and patellar tendon were recorded to assess tissue elongation during the ramp contractions in order to derive force–elongation relationships (to determine stiffness) of the distal tendon–aponeurosis complex and patellar tendon, as well as stress–strain relationships for the patellar tendon (to determine Young’s modulus). Measurement sessions were at a consistent time of day and started between 12:00 h and 19:00 h.