Meeting Abstract
For feeding muscles, our understanding of the operating length-ranges with respect to optimal length (Lo, where isometric peak force is generated) are based on twitch contraction studies of whole jaw muscles. These studies suggest that jaw muscles operate at short lengths on their length-tension (LT) curve. However, muscles are rarely twitch recruited in vivo, and in pennate muscle, fiber contractile behavior may differ from whole muscle behavior. Recent limb muscle data reveal variations in Lo with recruitment level, but the effect of recruitment on in vivo operating length of feeding muscle remains unknown. We measured fiber length change, muscle activation, and muscle force from rat deep masseter, a key jaw elevator, during food processing. An in situ approach was then used to construct LT curves for twitch (minimal), sub-maximal (intermediate), and tetanic (maximum) stimulation conditions. This combined approach revealed how in vivo fiber operating lengths are shaped by recruitment level and LT effects. Based on recent limb muscle data, we predicted Lo to be shortest for tetanic, intermediate for sub-maximal, and longest for twitch contractions. In line with our prediction, tetanic Lo was approx. 17% shorter than twitch Lo. The LT curve for skeletal muscle describes a trade-off between weak but stable contractions at short lengths, peak force production at intermediate lengths, and weak as well as unstable contractions at long lengths. In contrast to twitch data from earlier jaw muscle studies, our data suggest that chewing at wide gape on hard food may render jaw muscles vulnerable to sarcomere instabilities as they are forced to operate at long, weak, and unstable lengths. These results have implications for determining potential causes of oral physiological dysfunctions.
