EMG Activity During Stable And Unstable Standing And Sitting

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Comparison Of EMG Activity During Stable And Unstable Standing And Sitting In Patients With Primary Orthostatic Tremor And Controls
Ha, A.D., Chen, S., Mahant, N., Fung, V.S.C.

Objective:
To compare and contrast electromyographic (EMG) activity of patients with primary orthostatic tremor (OT) and controls during isometric contraction in the standing and sitting positions, with accompanying manoeuvres to reduce or elicit subjective unsteadiness, and to correlate the findings with subjective measures of unsteadiness.

Background:
OT is characterised by an intense sense of unsteadiness and a high frequency (14-16 Hz) leg and truncal tremor when standing. There are conflicting data as to whether the aetiology is specifically related to the control of stance as opposed to isometric muscle contraction.

Methods:
Surface EMG was recorded over tibialis anterior, gastrocnemius, quadriceps and biceps femoris, T1, T6, and T12 paraspinals in 9 patients with OT and 8 controls in the following positions: quiet standing, unsteady standing, supported standing (leaning against a wall), supported sitting, unsupported sitting (back braced against a wall, feet on the ground and hips and knees flexed at 90 degrees but without a seat). A handheld potentiometer was used to indicate levels of subjective unsteadiness.

Results:
During quiet standing, OT patients developed 14-16 Hz EMG bursting in leg and truncal muscles, whereas controls did not. During unsteady standing, all OT patients developed 14-16Hz EMG bursting. Only 1 control demonstrated 14-16Hz EMG bursting which was limited to the lower leg muscles. With supported standing, the tremor amplitude reduced, but the frequency remained constant. Subjective unsteadiness was also reduced. During isometric contraction in the seated position, irregular, non-16Hz bursts were seen in OT patients but not controls. During unsupported seating with isometric contraction, OT patients demonstrated irregular, non-16Hz bursts. In the supported and unsupported sitting conditions, no tremor activity was seen in controls, despite a subjective feeling of unsteadiness during the latter condition.

Conclusions:
The results of our study suggest that OT is primarily a task specific tremor related to the act of standing and maintenance of balance, and not simply isometric contraction when upright.


Ha, A.D., Chen, S., Mahant, N., Fung, V.S.C.; Comparison of EMG activity during stable and unstable standing and sitting in patients with primary orthostatic tremor and controls [abstract]. Movement Disorders 2012;27 Suppl 1 :1247