Potential of Individual Upper-Limb Muscles to Contribute to Postural Tremor: Simulations From Neural Drive to Joint Rotation.

BACKGROUND

It is unclear which muscles contribute most to tremor and should therefore be targeted by tremor suppression methods. Previous studies used mathematical models to investigate how upper-limb biomechanics affect muscles' potential to generate tremor. These investigations yielded principles, but the models included at most only 15 muscles. Here we expand previous models to include 50 upper-limb muscles, simulate tremor propagation, and test the validity of the previously postulated principles.

METHODS

Tremor propagation was characterized using the gains between tremorogenic neural drive to the 50 muscles (inputs) and tremulous joint rotations in the 7 joint degrees-of-freedom (DOF) from shoulder to wrist (outputs). Each gain can be interpreted as the potential of a muscle to generate tremor in a DOF. Robustness and sensitivity analyses were performed to assess the effects of model parameter variability on gains.

RESULTS

Simulations of postural tremor using the expanded model confirmed the previously postulated principles and revealed new insights, including: 1) most of the muscles with the largest gains were among the 15 muscles in the original model; 2) some gains depended strongly on posture; 3) averaged across the postures included in this study, the largest gains belonged to input-output pairs involving biceps/forearm/wrist muscles and forearm/wrist DOF, 4) although some shoulder and extrinsic hand muscles also exhibited large gains, especially in select postures.

DISCUSSION: These observations suggest that in the absence of additional information (such as tremorogenic neural drive to muscles), peripheral tremor suppression efforts should start by targeting biceps/forearm/wrist muscles or forearm/wrist DOF.