Short communicationIncorporating the length-dependent passive-force generating muscle properties of the extrinsic finger muscles into a wrist and finger biomechanical musculoskeletal model
Introduction
The forces produced by soft tissue structures that surround a joint (including passive muscles) play a crucial role in the control and stabilization of dynamic movements of low mass and inertia systems. Experimental work on biomechanical systems ranging from insect legs to human wrists (Charles and Hogan, 2012, Hooper et al., 2009, Souza et al., 2009, Wu et al., 2012) has demonstrated that passive viscoelastic forces, and the joint torques that result, influence dynamic movement trajectories of low mass systems more than the momentum and inertia of the segments.
Comprised of 27 bones with masses ranging between 0.002 and 0.04 kg (Le Minor and Rapp, 2001, McFadden and Bracht, 2003, Mirakhorlo et al., 2016, Saul et al., 2015), the hand is a small mass and inertia system. As a result, the inclusion of passive viscoelastic forces are critical for the simulation of controlled dynamic movements of the hand and fingers (Esteki and Mansour, 1997, Kamper et al., 2002). Passive viscoelastic forces in the hand are produced by soft tissue structures, either those that act within the hand (e.g., ligaments, joint capsules, skin, and intrinsic finger muscles) or the extrinsic finger muscles, which originate proximally, cross the wrist, and attach distally on the fingers (Knutson et al., 2000, Kuo and Deshpande, 2012). Because the force a muscle produces depends on length, it varies as a function of the posture of every joint the muscle crosses. Thus, forces produced by the passive extrinsic finger muscles are a complex, multi-dimensional function of joint postures of the distal upper limb (Bhardwaj et al., 2011, Knutson et al., 2000, O'Driscoll et al., 1992, Richards et al., 1996).
Biomechanical models that are implemented for dynamic simulations of finger movements include passive torques about each finger joint; however, most commonly these models exclude the wrist and distal upper limb (e.g. Babikian et al., 2016, Brook et al., 1995, Esteki and Mansour, 1997, Goislard de Monsabert et al., 2012, Kamper et al., 2002, Li and Zhang, 2009, Sancho-Bru et al., 2003, Sancho-Bru et al., 2001). While previous simulation work integrating the wrist and hand included active muscle properties that varied with proximal joint posture, the passive viscoelastic torques about each finger joint were defined as a function of a single joint, independent of other joint postures (Adamczyk and Crago, 2000). Here, we incorporate the length-dependent passive forces of the extrinsic index finger muscles into a biomechanical model of the hand and demonstrate their influence on combined passive movements of the wrist and hand.
Section snippets
Dynamic biomechanical model
A dynamic biomechanical model was developed in OpenSim v3.2 (Delp et al., 2007) by adapting an existing dynamic model of the upper extremity (Saul et al., 2015). The original model included the kinematics of the shoulder, elbow, and wrist, without additional degrees of freedom distal to the wrist. As described previously (Blana et al., 2016), the kinematics of the original model were augmented to include degrees of freedom for digits 1 (thumb) through 5 (pinky finger) (Fig. 1). Mass and inertia
Results and discussion
Simulation of length-dependent passive force-generating properties of extrinsic finger muscles yielded coupled movements between the wrist and index finger during dynamic forward simulations (Fig. 3). With the forearm pronated, prescribed wrist flexion produced coordinated MCP extension (initial position: 83° flexion, final position: 21.8° extension) and PIP extension (11.1–1.7° flexion; Fig. 3b–d), mimicking tenodesis (Johanson and Murray, 2002, Su et al., 2005). With the forearm supinated,
Conclusion
Passive torques are critical to achieve controlled and stabilized dynamic free movements of the wrist and fingers (Babikian et al., 2016, Blana et al., 2016, Charles and Hogan, 2012, Kamper et al., 2002). Additionally, passive coupling of the fingers and wrist is a fundamental component of hand function in the severely disabled hand, such as following tetraplegia (Johanson and Murray, 2002, Su et al., 2005). The methods implemented in this study are novel in that they enable incorporation of
Conflict of interest
None.
Acknowledgements
This work was funded by the National Institutes of Health under the Award Numbers R01EB011615, R01HD084009, and T32EB009406. We acknowledge the contributions of Christa Nelson in completing an initial set of simulations that we then expanded to complete the analysis included in the Appendix.
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