Ultimately, muscle powers all flight and swimming in the animal world. Muscular contraction is driven by myosin heads that consume ATP and slide actin filaments along myosin filaments. While much is known about functional specialisation in vertebrate muscle, the approaches that invertebrates have adopted to modulate muscle performance are less well known. William Kier and Frederick Schachat (p. 164) discuss some unusual adaptations found in squid. Squid mantle,fin and arm muscles are obliquely striated (with Z disks linking actin filaments inclined at an angle) and tend to be characterised by long myosin and actin filaments, which are capable of a large contraction range while generating high tensile stresses. However, the tentacle extensor muscle is more similar to cross striated vertebrate skeletal muscle, but with much shorter myofilaments. The result is that the tentacle extensor muscle can contract very rapidly, but generates small tensile stresses. According to Kier and Schachat, the molecular components of both muscle types are virtually indistinguishable, and it is mainly the different architecture that gives rise to the muscles' different properties.
Where as squid tentacles have to act fast to grab passing prey, the major muscles involved in flapping bird flight must generate great force while contracting repeatedly as birds take off and remain aloft. Yet, according to Bret Tobalske and Andy Biewener, little is known about the mechanical properties of the second largest flight muscle in birds, the supracoracoideus muscle (p. 170), which was thought to be responsible for flipping the wing upwards at the beginning of an upstroke (supination) during flight. Measuring muscle activity, and bone and muscle strain, in freely flying pigeons, Tobalske and Biewener confirmed that the muscle does indeed flip the wing at the beginning of the upstroke. Also the muscle's elastic tendon stores up to 60% of the muscle's work,presumably releasing the energy later to reduce the metabolic cost of flight. However, two of the team's predictions were not born out; strain in the supracoracoideus muscle was much greater and the power generated by the muscle was less than the team had predicted. Tobalske and Biewener say `it is sobering that our predictions of function from anatomy were only partially correct,' but van Leeuwen describes the team's findings in freely flying birds as `quite an experimental achievement'.
Concluding the discussion of muscle function in locomotion, Bertrand Tanner, Michael Regnier and Tom Daniel describe their theoretical work on a three dimensional molecular model of muscle. By modelling the kinetics and molecular structure of half a sarcomere, the team successfully predicts muscle function for Manduca sexta(p. 180). Knowing that the major flight muscle only operates at 40% maximal power and that extra power required for manoeuvres during flight was probably generated by`shifting activation phase [timing] to produce higher mechanical output', the team were also able to model how the timing of activation influences the energetics of contraction. Calculating ATP consumption as well as mechanical work done, power and efficiency, Daniel and colleagues found that the mechanical work generated and ATP consumption vary depending on the timing of myosin activation. Daniel says `to our knowledge, no prior study has shown that myosin cross-bridge ATPase rates vary with phase of activation'.
