Visually guided targeting enhances bilateral force variability in healthy older adults.

This study observed the effect of visual feedback on between-limb force variability relationships in young and older adults. Abduction force was examined in healthy young (n = 15, 25 ± 4 years) and older adults (n = 18, 71 ± 6 years) during simultaneous isometric contractions of both index fingers. Target forces ranged from 5% to 30% maximum voluntary contraction (MVC), where force variability and first dorsal interosseus activity were measured while (1) subjects viewed visual targets for both index fingers, (2) a visual target was provided for the dominant index finger only, and (3) visual targets were removed for both index fingers during bilateral isometric contractions. When subjects were provided with bilateral visual feedback during simultaneous contractions at low forces (5% and 10% MVC), older adults produced greater force variability than younger subjects (p = 0.002). However, when bilateral visual feedback was removed, age-related differences in variability were no longer present. Between-limb force variability differences existed at higher force outputs (20% and 30% MVC) when visual feedback was removed for the nondominant limb during bilateral isometric index finger abduction (p = 0.002). The control of bilateral force variability is compromised in older adults when visuomotor processes are engaged. However, age-related differences in force variability are abolished when no task-related visual feedback is available, and isometric contractions are based on internally guided feedback.

Removing visual feedback for a single limb alters between-limb force tremor relationships during isometric bilateral contractions.

This study examined how force tremor and muscle activity are altered between limbs when a visual target is removed for one limb during bilateral index finger abduction. Isometric index finger abduction force was examined in healthy adults (23 ± 4 years) when both index fingers abducted simultaneously. Abduction forces ranged from 5 to 20% maximum voluntary contraction, and these target forces were displayed on a PC monitor in front of the subject. Force tremor and first dorsal interosseous (FDI) activity were first collected while subjects viewed visual targets for both index fingers and then when the visual target was removed for the non-dominant index finger. Subjects successfully matched the force amplitudes generated for both limbs regardless of visual condition. When the visual target was removed for one limb, force tremor increased in this limb (p < 0.01). Different power spectral profiles were evident for each FDI EMG when targets were available for both limbs (p < 0.05); however, when one target was removed, the pattern of FDI EMG for the limb without a visual target closely reflected FDI EMG for the limb which had the visual target. The CNS actively modulates muscle activity in each limb to perform visually guided isometric contractions. Given that the goal was to match force output with both limbs, the requirements of the task must be established from the limb that had a visual target, and a copy of those motor commands appears to have been sent to the FDI of the limb without a visual target.

The effect of isometric contraction on the regulation of force tremor in the contralateral limb.

This study examined how regulating force tremor in a single limb is altered when the opposite limb is actively engaged in a force generating task. Index finger abduction force and first dorsal interosseous (FDI) activity were assessed in thirteen healthy subjects, at target forces from 5% to 60% MVC for the non-dominant limb (unilateral task), and again when the dominant limb simultaneously generated a submaximal abduction force (bilateral task). When the non-dominant limb generated force at 20% MVC, tremor was greater during the bilateral task compared with the unilateral task; a finding reflected in the amplitude of peak power of force. Bilateral responses were also examined during a prolonged 60% MVC unilateral contraction. Force tremor and muscle activity amplitude increased while the frequency of activity decreased for the contracting limb. Additionally, force tremor significantly decreased towards the end of the prolonged contraction in the contralateral limb. Overall, it appears that the process of performing isometric contractions invokes tremor-related changes in the opposite limb at selective force targets, and performing prolonged unilateral contractions invokes tremor-related changes in the opposite limb when it is at rest.