The effect of contraction intensity on force fluctuations and motor unit entrainment in individuals with essential tremor.
OBJECTIVE
Quantify the effect of increasing contraction intensity on the amplitude of force fluctuations and neuromuscular and force tremor spectral power.


METHODS
Twenty-one subjects with essential tremor (ET) and 22 healthy controls applied isometric wrist extension contractions. Various sub-maximal contraction intensities were evaluated (5%-, 10%-, 20%- and 30%-MVC). Force fluctuations and wrist extensor neuromuscular activity were recorded using a load cell and electromyography (EMG).


RESULTS
Higher contraction intensities were associated with larger amplitude force fluctuations and greater neuromuscular activation. However, spectral power associated with tremor peaks remained relatively constant (EMG) or decreased (force) with increasing contraction intensity.


CONCLUSIONS
Motor unit entrainment associated with centrally generated oscillatory inputs does not increase with greater levels of muscle activation.


CONCLUSIONS
Rather than influencing a constant proportion of active motor units, abnormal oscillatory drive influences a relative constant number of total motor units. When combined with the findings from our previous study on postural tremor, the present results provide preliminary evidence that abnormal stretch reflex activity may contribute to this motor unit entrainment.
Héroux
M E
ME
School of Human Kinetic, University of British Columbia, 210 - 6081 University Boulevard, BC, Canada. mheroux@interchange.ubc.ca
Pari
G
G
Norman
K E
KE
D000328:Adult; D000368:Aged; D001683:Biological Clocks / Q000502:physiology; D001696:Biomechanical Phenomena; D004185:Disability Evaluation; D004576:Electromyography; D020329:Essential Tremor / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000503:physiopathology*; D014953:Wrist / Q000503:physiopathology