The force producing capacity of a muscle is linked among other things to its length. During tasks such as squatting, lower limb joint rotations produce continuous changes in muscle length which requires the central nervous system to not only accurately monitor muscle length, but provide the appropriate timing and level of activation. Previous studies of triceps surae recruitment during a combined task of active isometric plantar flexion with passive knee flexion have shown a switch in recruitment from medial gastrocnemius (MG) to soleus (SOL) as changes in MG and SOL fascicle length were decoupled from each other (1,2). Whether the same pattern of MG deactivation and SOL activation occur during a similar but more functional exercise task like squatting is currently unknown. Therefore, the aim of this study was to investigate the relationship between MG and SOL muscle lengths and their respective recruitment and de-recruitment patterns when performing a squatting exercise. We hypothesised that muscles at longer lengths would be preferentially recruited to produce the required plantar flexion torque. Eight healthy subjects stood on two force platforms (one for each leg) and performed a squatting exercise with an additional mass of 20% bodyweight across their shoulders. Squats were performed from upright standing to 60° of knee flexion. Only the middle 40° of downward knee motion motion was analysed. Intramuscular electromyography was recorded from MG, lateral gastrocnemius (LG) and SOL. 3D motion analysis (200Hz) provided relevant joint kinematics and ultrasound images were simultaneously recorded at 80 Hz to measure muscle fascicle lengths of MG, LG, SOL. The results showed that MG activity decreased by 31 ± 20% over 40o of knee flexion which was accompanied by a shortening of its fascicles by 1.3 mm. SOL activity also decreased by a similar amount (31 ± 17%) over the same range of motion, however its fascicles lengthened by 1.7 mm. LG activity remained unchanged over the 40o range of measurement. The changes in MG and SOL activation were accompanied by a 40 ± 5% decrease in ankle torque and a 45 ± 4% increase in knee torque. The increase in fascicle length of SOL likely resulted in an advantageous shift in its length tension relationship allowing SOL to compensate for the decreased contribution of MG despite its lower level of activation. The current result further highlight the complex neural control of mono and bi-articualr muscles in everyday tasks.
© Copyright 2014 19th Annual Congress of the European College of Sport Science (ECSS), Amsterdam, 2. - 5. July 2014. Published by VU University Amsterdam. All rights reserved.