The sensitivity of endpoint forces produced by the extrinsic muscles of the thumb to posture

Abstract

This study utilizes a biomechanical model of the thumb to estimate the force produced at the thumb-tip by each of the four extrinsic muscles. We used the principle of virtual work to relate joint torques produced by a given muscle force to the resulting endpoint force and compared the results to two separate cadaveric studies. When we calculated thumb-tip forces using the muscle forces and thumb postures described in the experimental studies, we observed large errors. When relatively small deviations from experimentally reported thumb joint angles were allowed, errors in force direction decreased substantially. For example, when thumb posture was constrained to fall within ±15° of reported joint angles, simulated force directions fell within experimental variability in the proximal-palmar plane for all four muscles. Increasing the solution space from ±1° to an unbounded space produced a sigmoidal decrease in error in force direction. Changes in thumb posture remained consistent with a lateral pinch posture, and were generally consistent with each muscle’s function. Altering thumb posture alters both the components of the Jacobian and muscle moment arms in a nonlinear fashion, yielding a nonlinear change in thumb-tip force relative to muscle force. These results explain experimental data that suggest endpoint force is a nonlinear function of muscle force for the thumb, support the continued use of methods that implement linear transformations between muscle force and thumb-tip force for a specific posture, and suggest the feasibility of accurate prediction of lateral pinch force in situations where joint angles can be measured accurately.

A Probabilistic Approach to Quantify the Impact of Uncertainty Propagation in Musculoskeletal Simulations

“…23 Recent musculoskeletal studies have used repeated sampling methods to quantify output variability and sensitivity of inverse dynamics and muscle force prediction to variability in model parameters. 1,22,27,34,39,44 Although these studies provide insight into factors that affect a particular model at a single stage in the simulation, the current study introduces new methodology to musculoskeletal simulation practices that characterizes the impact and interaction of multiple sources of uncertainty, and quantifies the propagation of uncertainty through each stage of the musculoskeletal simulation process.…”

Connecting the wrist to the hand: A simulation study exploring changes in thumb-tip endpoint force following wrist surgery

“…Instead, these studies have elucidated the biomechanics of force production at the index finger (Valero-Cuevas et al 1998; Wu et al 2008; Lee and Kamper 2009), the thumb (Valero-Cuevas et al. 2003; Goehler and Murray 2010; Wohlman and Murray 2013), and the end-points of multiple digits to produce coordinated grip forces (Esteki and Mansour 1997; Sancho-Bru et al 2003) in isolation from the proximal upper limb.…”

“…To test this hypothesis, we leverage available biomechanical data characterizing the wrist surgeries (Blankenhorn et al. 2007; Nichols et al 2015; Nichols et al 2016; Nichols et al 2017) and computational advances in biomechanical simulation of force development at the thumb (Valero-Cuevas et al 2003; Goehler and Murray 2010; Wohlman and Murray 2013) to develop dynamic musculoskeletal simulations of lateral pinch force that integrate the wrist and thumb. We then utilize these simulations to specifically examine how having a nonimpaired versus a surgically-altered wrist influences joint torques, muscle force transmission, and muscle control strategies during lateral pinch.…”

Computational development of Jacobian matrices for complex spatial manipulators

“…The second example considers a musculoskeletal model of the hand that adds the contributions of wrist movement (flexion and deviation) to an existing 4-DOF model of the thumb [10]. This augmentation actually adds four separate rotations about four non-intersecting, non-orthogonal axes of rotation resulting in a 3×8 manipulator Jacobian matrix.…”