There is evidence that instability in the control of a human muscle may occur under appropriate conditions of inertial and spring loading (Joyce & Rack, 1974; Matthews & Muir, 1980; Matthews, 1997). This is believed to depend on the stretch reflex and causes an oscillation in the 8-12 Hz range. However, the extent to which this mechanism contributes to normal and exaggerated physiological tremor is uncertain. Here we analyse the tremor in three muscles, known to have different stretch reflex characteristics: strong in biceps brachii, weak in first dorsal interosseous of the hand (FDI) and absent in anterior digastric.

The nine subects taking part (aged 22-73 years) had no history of neurological disorder, but four of them commented that their tremor was more noticeable than they thought normal (exaggerated physiological tremor). The study was approved by the local Ethical Committee and subjects gave written, informed consent. The surface electromyogram (EMG) recorded from each muscle was amplified (bandwidth 50 Hz – 3 kHz), rectified and filtered (0.5-60 Hz). Force of muscle contraction was recorded through a strain gauge via a rigid connection (isometric) or via a spring (anisometric: compliance 0.31 mm N^-1) and the signal also filtered as for the rectified EMG. Both signals were sampled at 500 Hz. Subjects were asked to maintain a known proportion of their maximum voluntary contraction (MVC) for 20 seconds for three periods separated by a min of rest. For high levels of force, the maintained period was reduced to 10 s and repeated 6 times to avoid fatigue. Recordings during contraction were analysed to compute auto-and cross-spectra.

For normal subjects, no peak in the 8-12 Hz range was observed for either the EMG or force spectra for any muscle in the isometric condition. In anisometric conditions, the spectra of biceps were both dominated by peaks in this region, particularly at 30-50 % MVC. In FDI, peaks were either absent or small, even when the finger was inertially loaded to bring its natural frequency to be similar to that for the forearm. In digastric, no peaks were observed under any conditions. These observations are consistent with the instability being due to the stretch reflex.

For subjects with exaggerated tremor, the clearest difference was the presence of 8-12 Hz oscillations in biceps and FDI in both isometric and anisometric conditions. Signs of instability were never seen in digastric. It appears that a cause of exaggerated physiological tremor may be an enhanced stretch reflex.