The force with which a muscle can contract depends on its initial length. The so-called length–tension curve of muscle characterizes this dependence and demonstrates that muscles intrinsically have some optimal length for generating force, and longer or shorter lengths lead to decreased force production. Despite our understanding of this general relationship, we remain blissfully unaware of actual muscle operating lengths during most behaviors, and thus don't know where on their length–tension curves muscles are working. Recent work on a fish pharyngeal jaw adductor muscle by Nick Gidmark, Beth Brainerd and colleagues at Brown University highlights how a better understanding of operating lengths and force–length characteristics can reveal constraints on behavior and performance.
Many fish have two sets of jaws, one oral like the rest of us, and one farther back in the throat known as the pharyngeal jaws. Black carp (Mylopharyngodon piceus) use their pharyngeal jaws to crush snails of varying size for food. Prey size of course determines how wide apart the jaws must be at the start of a bite, which in turn stretches and influences the length of the jaw-closing muscles before they begin to contract to generate bite forces. Gidmark and colleagues took advantage of this and used prey size to manipulate the operating range of a jaw adductor muscle and ask whether force–length properties alter bite performance. The researchers first created artificial prey to feed three fish and assess how bite force was affected by prey size. They then used X-ray video to record multiple feeding trials from each fish feeding on prey of varying size. These videos were later used to measure the operating lengths of the jaw-closing levator arcus branchialis muscle during each feeding event. Finally, fish were killed and a preparation of a single pharyngeal jaw with its closing muscle attached was developed to allow stimulation and force measurement as the muscle's initial length was systematically varied so as to develop an in situ force–length curve for the muscle.
X-ray data combined with the in situ force–length results revealed that prey size indeed altered the operating length of the jaw-closing muscle and thus its potential for developing force. The larger the prey item, the further the jaw adductor was stretched and moved onto regions of the force–length curve where force-generating capacity was reduced. Indeed, all three fish had trouble crushing large prey, even when they were of comparable strength to smaller prey that could be crushed readily. In short, it pays to be big if you're a snail living with black carp. Such results not only help us better appreciate how prey size can impact bite performance in this species, but also remind us more generally of how intrinsic muscle properties can constrain function in behaviors where muscles operate over different length ranges.
