The active length-tension relation was determined for the left digastric muscle of seven New Zealand White rabbits anesthetized with pentobarbital. Measurements of muscle length and fiber architecture were made from photographs of resting and actively contracting muscle. There was a marked difference between length-tension curves based upon resting as compared to active muscle length. The active length-tension relation had a longer descending limb than ascending limb, whereas the length-tension relation based on passive muscle length tended to be symmetrical around optimum length. On the average, muscle fibers lengthened 0.77 mm for each 1 mm of extension of the muscle belly. Since the rabbit digastric muscle is unipinnate, this suggests that pinnation serves to enhance the range of muscle excursion in this muscle.

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The consistency of occurrence and also the timing of TMJ sounds during jaw opening and closing were studied by means of an audio-visual sound recording system in an attempt to address the possible causes of temporomandibular joint (TMJ) sounds. From a group of 347 orthodontic patients, 104 were found to have medium- or high-amplitude TMJ sounds during jaw opening or closing. Most patients (53%) had reciprocal clicking--that is, a single sound on opening and on closing; another 12% had multiple sounds on opening or closing; 22% had a single closing sound; and 13% had a single opening sound. Sounds occurred at all degrees of jaw opening throughout this sample, but in most patients opening sounds tended to be closer to maximum opening, whereas closing sounds tended to occur in the middle of the closing movement. No statistically significant association was found between the timing of the opening and closing sounds. In 42.3% of patients, the sound was inconsistent in its occurrence on successive opening and closing cycles. Twenty-three percent of patients reported pain, jaw locking, or limitation of movement, but these were not associated with the timing of the opening sound. The findings suggest that the reciprocal click, widely associated with anterior disc displacement with reduction, was relatively common, but that other explanations for the joint sounds should also be considered. Conversely, a large variation may exist in the timing and the occurrence of sounds in patients with anterior disc displacement in the absence of pain and limitation of movement.