Long-term registration of daily jaw muscle activity in juvenile rabbits

Understanding control of muscles during various tasks and their adaptive changes requires information on all motor behavior used throughout the day. The total duration of muscle activity depends on the magnitude of its activation and can change during maturation. Therefore, the purpose of this study was to examine the duration of muscle activity (i.e. duty time) exceeding various activity levels in maturing jaw muscles. A telemetric device was implanted into nine juvenile male New Zealand White rabbits to continuously record muscle activity during maturation weeks 9-14. Electrodes were inserted into digastric, superficial and deep masseter, medial pterygoid, and temporalis muscles. Duty times (expressed as a percentage of time) were calculated for activation exceeding different levels (5-90%) of EMG peak activity per 24-h period. At 10 weeks of age, for activation exceeding the 5% level, the duty time of the temporalis (20.0+/-5.2%) was statistically significantly higher than that of the medial pterygoid (11.2+/-1.5%), digastric (11.0+/-5.1%), superficial (12.6+/-5.6%), and deep masseter (8.6+/-5.5%). Duty times declined with increasing activity level. For activation exceeding the 40% level the duty times of the superficial masseter and medial pterygoid were significantly higher than those of the other muscles. During maturation none of the muscles showed a significant change in duty time. However, for activation exceeding the 5% level, the inter-individual variation in duty time decreased significantly for the digastric, and superficial and deep masseter.