The effect of inertial loading on wrist kinetic tremor and rhythmic muscle activity in individuals with essential tremor

ObjectivesDetermine the effect of concentric and eccentric movement and contraction intensity on the strength of rhythmic muscle activity in individuals with essential tremor (ET).Methods21 ET subjects and 22 healthy controls produced wrist flexion–extension movements while supporting sub-maximal loads (no-load, 5%, 15% and 25% 1-repetition maximum). Kinetic tremor and wrist extensor neuromuscular activity were recorded using an angular displacement sensor and electromyography (EMG).ResultsRhythmic muscle activity was twice as big during movement compared to previous results involving postural or isometric tasks. ET subjects with greater rhythmic muscle activity had (1) larger overall kinetic tremor amplitude, (2) greater tremor spectral power during eccentric compared to concentric movement and (3) a reduction in overall kinetic tremor amplitude and the percentage of EMG spectral power accounted for by the tremor spectral peak in the presence of inertial loading.ConclusionsGreater than normal kinetic tremor amplitude appears to be limited to ET subjects with higher levels of rhythmic muscle activity. Furthermore, rhythmic muscle activity is much greater during movement compared to during postural or closed-kinetic tasks.SignificanceThe strength of rhythmic muscle activity in ET is influenced by the type of contraction (i.e., static vs. dynamic) being performed. Clinicians and researchers should include measures of simple kinetic tremor as part of their assessments.