Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Randomized Controlled Trial
Dissociation of nociceptive modulation of a human jaw reflex from the influence of stress.
In human beings, inhibitory jaw reflexes can be depressed by painful stimulation of remote parts of the body. Since similar effects can be produced by the stress of anticipating pain, we wished to investigate whether the effects of remote painful stimuli are dependent on stress. EMG recordings were made from a masseter muscle while subjects maintained activity in the muscle at approximately 12.5% of maximum using visual feedback. ⋯ A second series of experiments suggested that these lesser effects during the random sequences were not substantially due to any loss of temporal summation of the conditioning mechanisms. The evidence for this was that application of pairs of conditioning stimuli did not produce a significantly greater effect than single conditioning stimuli within a random sequence (39.9 +/- 9.6% as opposed to 32.7 +/- 9.1% reductions in the reflex, P = 0.117, paired t-test). Therefore since any stress in the random sequences would not have been "tied" to the conditioned responses alone, the effects of remote painful stimuli on this inhibitory jaw reflex cannot be entirely secondary to stress.
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This study compared fatigue and the spatial distribution of upper trapezius electromyographic (EMG) amplitude during a 6-min constant force shoulder elevation task at 20% of the maximal voluntary contraction force (MVC) (constant force) and during the same task interrupted by brief (2 s) periodic increases in force to 25% MVC every 30 s (variable force). Surface EMG signals were recorded with a 13 x 5 grid of electrodes from the upper trapezius muscle of nine healthy subjects. The centroid (center of activity) of the EMG root mean square map was computed to assess changes over time in the spatial distribution of EMG amplitude. ⋯ The centroid of EMG amplitude shifted in the cranial direction across the duration of the variable force contraction (P < 0.05) but not during the constant force contraction (shift of 2.9 +/- 2.3 mm and 1.4 +/- 1.1 mm, respectively). The results demonstrate that periodic increases in force during a sustained contraction enhance the modifications in spatial distribution of upper trapezius EMG amplitude and reduce fatigue compared to a constant force contraction performed at a lower average load. The change in spatial distribution of EMG amplitude over time during a sustained contraction may reflect a mechanism to counteract fatigue during prolonged muscle activity.