Length-tension relationships of masseter and digastric muscles of miniature swine during ontogeny.

At incremental whole muscle lengths, active isometric and passive elastic forces were recorded from the masseter and digastric muscles of anaesthetized miniature pigs (Hanford) weighing 2.0-20.0 kg. Wet muscle mass and maximum tetanic tension values for masseter exceed those for digastric and increase more rapidly with body mass (age). At any body mass, masseter exceeds digastric in the ratio of optimum length (that length at which maximum tetanic tension is produced) to in situ muscle length (that length which corresponds to the jaw in a closed position) and the proportion of passive tension comprising total (passive plus active) tension. Passive elastic tension begins to rise in masseter at lengths as short as 87% of optimum (in younger pigs). In digastric, passive tension is absent until the muscle is stretched to a length slightly longer than optimum in younger pigs but occurs at shorter lengths in older pigs. Contractile properties explain functional differences between masseter and digastric more clearly than they explain ontogenetic changes in either muscle. The behavioural transition from infant suckling to adult mastication of solid food is best characterized by a disproportionate increase in mass (and force) of the masseter, relative to digastric, and increased reliance upon active (rather than passive) tension.