Three-dimensional assessment of postural tremor during goal-directed aiming.

The performance of fine motor tasks which require a degree of precision can be negatively affected by physiological tremor. This study examined the effect of different aiming positions on anterior-posterior (AP), medial-lateral (ML) and vertical (VT) postural tremor. Participants were required to aim a mock handgun at a target located in front of them at eye level. Changes in AP, ML and VT tremor from the forearm and gun barrel were assessed as a function of limb (i.e., whether one or both arms were used) and upper arm position (elbow bent or extended). Tremor was recorded using triaxial accelerometers. Results showed that, across all tasks, the ML and VT tremor for any point was characterized by two frequency peaks (between 1-4 and 8-12 Hz) with amplitude increasing from proximal (forearm) to distal (gun barrel). Interestingly, irrespective of the posture adopted, ML accelerations were of greater amplitude than VT oscillations. AP oscillations were markedly smaller compared to VT and ML tremor, did not display consistent frequency peaks, and were not altered by the arm conditions. Altering the aiming posture resulted in changes in VT and ML tremor amplitude, with oscillations being greater when using a single arm as compared to when two arms were used together. Similarly, tremor amplitude was reduced when the task was performed with the elbow bent compared to the straight arm condition. Overall, these results highlight that ML oscillations make as significant a contribution to the overall tremor dynamics as those observed in the VT direction. However, the origin of ML tremor is not simply the product of voluntary adjustments to maintain aim on the target, but also exhibits features similar to the neural generated 8-12-Hz tremor seen under postural conditions.

Bracing of the trunk and neck has a differential effect on head control during gait.

During gait, the trunk and neck are believed to play an important role in dissipating the transmission of forces from the ground to the head. This attenuation process is important to ensure head control is maintained. The aim of the present study was to assess the impact of externally restricting the motion of the trunk and/or neck segments on acceleration patterns of the upper body and head and related trunk muscle activity. Twelve healthy adults performed three walking trials on a flat, straight 65-m walkway, under four different bracing conditions: 1) control-no brace; 2) neck-braced; 3) trunk-braced; and 4) neck-trunk braced. Three-dimensional acceleration from the head, neck (C7) and lower trunk (L3) were collected, as was muscle activity from trunk. Results revealed that, when the neck and/or trunk were singularly braced, an overall decrease in the ability of the trunk to attenuate gait-related oscillations was observed, which led to increases in the amplitude of vertical acceleration for all segments. However, when the trunk and neck were braced together, acceleration amplitude across all segments decreased in line with increased attenuation from the neck to the head. Bracing was also reflected by increased activity in erector spinae, decreased abdominal muscle activity and lower trunk muscle coactivation. Overall, it would appear that the neuromuscular system of young, healthy individuals was able to maintain a consistent pattern of head acceleration, irrespective of the level of bracing, and that priority was placed over the control of vertical head accelerations during these gait tasks.