Simulated effects of the peripheral musculoskeletal system on hand tremor

Although tremor is the most common movement disorder, the role of the peripheral musculoskeletal system in shaping tremor is not fully understood. Elucidating how tremorogenic muscle activity propagates through the peripheral musculoskeletal system on its way to becoming tremor at the hand is important for understanding the effects of (and improving) tremor-suppression strategies. We present the first model of tremor propagation throughout the upper limb from tremorogenic muscle activity to hand tremor. Using this linear, time-invariant multi-input multi-output model, we simulated the hand tremor caused by all 50 upper-limb muscles (excluding intrinsic hand muscles) individually and collectively, across seven postures representative of daily activities and clinical assessments. To ensure robustness, Monte Carlo simulations were repeated at many different input and model parameter values. Modeling revealed that to the extent that the musculoskeletal system of the upper limb can be approximated as linear and time-invariant during postural tremor, single-frequency tremorogenic drive to any number of muscles causes the hand to trace an elliptical path once steady state is reached. Each muscle is associated with a tremor ellipse, characterized in terms of direction and magnitude. Muscles acting on the same degrees of freedom (whether synergists or antagonists) tended to produce tremor in similar directions. Tremor direction varied significantly with posture, but taken together, the individual tremor ellipses formed a relatively flat ellipsoid whose dominant plane remained nearly perpendicular to the long axis of the forearm and hand. According to our simulations across a variety of postures, the peripheral musculoskeletal system generally amplified tremorogenic input to distal muscles (particularly wrist muscles) more than other muscles. These results highlight the critical role of peripheral biomechanics in shaping hand tremor. Tremor ellipses offer a useful framework for understanding the first-order effects of the musculoskeletal system on tremor and for estimating the potential of individual muscles to contribute to hand tremor.