Force–Length Relationship Modelling of Wrist and Finger Flexor Muscles

Introduction Because the hand joints possess a broad range of motion, the muscle length can vary importantly which might result in significant variations of the muscle force-generating capacities. However, facing the complexity of this musculoskeletal system, no study has examined the effect of hand muscles length change on muscle force. This study aimed to characterize the force-length relationship of muscles involved in wrist and metacarpophalangeal (MCP) flexion. Methods Eleven participants performed two sessions: i) one for the wrist flexor muscles, and ii) one for the finger flexor muscles. For each session, the participants performed two maximal voluntary contractions (MVC) and then two progressive isometric ramps from 0 to 100% of their maximal force capacity at five different wrist/MCP angles. Torque, kinematic and electromyographic data were recorded. An ultrasound scanner was used to measure the myotendinous junction displacement of flexor carpi radialis (FCR) and flexor digitorum superficialis (FDS) during isometric contractions. A 3D relationship between muscle length, force and activation level was modeled using optimization procedure. Results Globally, the FCR was stronger and shorter compared to FDS. The results showed that the 3D relationships fitted well the experimental data (mean R2 = 0.92 ± 0.07 and 0.87 ± 0.11 for FCR and FDS, respectively). Using joint angle and EMG data, this approach allows to estimate the muscle force with low estimation errors (

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