NEUROMECHANICAL ACTIVITY OF THE WRIST MUSCLES DURING STABILIZATION TASKS

Ellenor J. Brown

School of Biological Sciences, Georgia Institute of Technology

July 12, 2017, 11am, Room 1253, 555 14th St.

Committee Members: Jun Ueda PhD (Advisor), Minoru Shinohara PhD (Co-Advisor), Boris Prilutsky PhD, Lena Ting  PhD, Thomas Burkholder PhD

Abstract:

Wrist joint stability is vital to hand function and overall upper limb function.  A strategy for adjusting the stability of a joint is co-contraction of antagonist muscles.  The overarching goals of this dissertation are to 1) understand the neural mechanisms of wrist stability, given the unique neural mechanisms involved, and 2) quantify the amplitudes and oscillations of wrist muscle neural and mechanical activity during wrist stabilization.

At a hinge joint, such as the humeroulnar joint in the elbow, movement is controlled about one axis and groups of muscles work as antagonists to generate opposing torques about this axis.  However, the wrist is a condyloid joint that has two defined axes of motion : flexion/extension and radial/ulnar deviation.  To achieve these movements, pairs of muscles acting about the wrist can be controlled as antagonists or synergists depending on the movement.  This flexibility in the muscles’ functional relationships is reflected in the involved spinal mechanisms, which deviate from those seen between “true antagonists” about a hinge joint.  Aims 1 and 2 focus on the functional consequences of these unique spinal mechanisms during voluntary agonist contractions and co-contractions at the wrist in humans.

The neural mechanisms controlling motor output cause coordinated muscle activation and contractions.  The most common way to measure muscle activation is through electromyography (EMG). However, both surface and intramuscular EMG have limitations that make it difficult to accurately measure the neural activity of wrist muscles.  Surface EMG is susceptible to “crosstalk” from surrounding muscles, particularly over limb segments with small, closely-packed muscles with redundant function (e.g. the muscles of the forearm).  Intramuscular EMG reduces crosstalk but is invasive and strong muscle contractions can dislodge the electrodes.  For these reasons, alternative methods are needed to measure wrist muscle activity non-invasively with adequate spatial resolution.  One such method is ultrasound elastography.  The ultrasound image provides spatial resolution while the elastography provides mechanical data that correlates linearly with muscle force.  In Aim 3, an automated image processing method for analyzing muscle ultrasound shear-wave elastography is presented with application to data from a wrist muscle during stabilization of the wrist and proximal joints against complex loading.