Muscle Function in vivo: A Comparison of Muscles used for Elastic Energy Savings versus Muscles Used to Generate Mechanical Power1

SYNOPSIS. The function of muscles used to generate force economicallyand facilitate elastic energy savings in their tendons is comparedwith muscles that function to produce mechanical power. Theunderlying architectural design of the muscle and its tendon(if present) dictate much of their functional capacity and rolein animal locomotion. Using methods that allow direct recordingsof muscle force and fiber length change, the functional designof muscle-tendon systems can now be investigated in vivo. Thesestudies reveal that, in the case of wallaby hindleg muscles,the fibers can maintain sufficient stiffness during tendon stretchand recoil to ensure useful elastic energy recovery and savingsof metabolic energy. In the case of the pectoralis muscle ofpigeons, although isometric or active lengthening of the muscle'sfibers may occur late in the upstroke of the wing beat cycleto enhance force development, the fibers shorten extensivelyduring the downstroke (up to 35% of their resting length) toproduce mechanical power for aerodynamic lift and thrust. Oscillatorylength change, with force enhancement during active lengtheningmay be a general feature of muscles that power aerial and aquaticlocomotion. Similarly, force enhancement by active lengtheningis likely to be important to the design and function of musclesthat primarily generate force to minimize energy expenditure/unitforce generated, as well as for elastic energy savings withina long tendon. Architectural features of muscle-tendon unitsfor effective elastic energy savings, however, are likely toconstrain locomotor performance when mechanical work is required,as when an animal accelerates, either limiting performance orrequiring the recruitment of functional agonists with greatermechanical power generating capability (i.e., longer fibers)