Concord Field Station

Muscle recruitment and function in relation to gait and varying locomotor conditions

(collaboration with Gary B. Gillis, Mt. Holyoke college)

A. Changes in muscle function during terrestrial locomotion occur in relation to speed, gait, and incline.
In a studies of of rat locomotion [G. Gillis], we have explored how two primary hip and knee extensors, the biceps femoris (BF) and vastus lateralis (VL), change their contractile function with changes of the speed, gait, and incline. In general, we found ( Gillis & Biewener, JEB 2001) that the function of these two muscles differs during steady level locomotion: the BF undergoes net shortening to perform mechanical work, whereas the VL is actively lengthened absorbing energy. These two roles are generally maintained across speed and gait; however, changes do occur for the asymmetrical timing and use of the limbs in galloping. When running up an incline or down a slope a general shift in the relative of amount of positive (incline) or negative (decline) work must be performed by limb muscles. How do different muscle groups within the limb shift their mechanical behavior to accommodate the change in mechanical work? Are certain muscles preferentially recruited to shift their contractile behavior compared with others? In our studies of the rat BF and VL, we found that these muscles shift their shortening behavior as expected, with the BF shortening more and the VL remaining more isometric, absorbing less energy. Interestingly, when moving downhill the BF turns off and is passively shortened, reflecting its diminished role and need for positive work production by the limb.

B. Muscle shortening and activation patterns in hindlimb muscles of toads (B. americanus) during swimming vs hopping and jumping [G. Gillis].
This study seeks to understand how shifts in muscle function and recruitment pattern are achieved that allow certain species to undergo large changes in locomotor conditions, as when they move through different media. In the case of toads, the function and activation of key hindlimb muscles were studied during aquatic propulsion and compared with their function during hopping and jumping. We asked whether certain muscles modulated more than others and whether certain muscles less plastic in response to a major locomotor shift due to their fiber-tendon architecture? Alternatively, is there an overall shift in the pattern of muscle recruitment that facilitates the change in locomotor function? We used electromyography, sonomicrometry, and limb kinematics to address these questions. We found that the large cruralis muscle (knee extensor) is the primary muscle that is modulated in its contractile behavior, showing strong activation and shortening during jumping and less activation, but earlier contraction during swimming.

C. In vivo performance of the mallard (A. platyrynchos) during swimming vs terrestrial gait.
This represents a second study examining the plasticity of muscle function across a major shift in locomotor requirements. This muscle is an important ankle extensor during both surface s In this study, we whether this muscle exhibits more isometric behavior (doing less mechanical work) during terrestrial gait compared with its function during swimming; and shift in contractile function An advantage of this species is that the lateral and medial heads of the gastrocnemius allow us to record the force-length behavior independently from each muscle belly. This provides us with an opportunity to study how recruitment within a functional agonist may be modulated with respect to changes in locomotor requirement. Our work to date has shown that the mallard gastrocnemius, unlike the gastrocnemius of wild turkeys, does not exhibit a major shift in its contractile behavior, but does considerable mechanical work during both modes, likely reflecting the overriding importance for fluid propulsion during swimming.

D. Muscle-tendon design and function in relation to the tradeoff between force economy and elastic energy savings vs power output in hindleg muscles of wallabies (T. eugenii) hopping on a level vs an incline.
This is a third study that examines how shifts in locomotor condition are accomodated, or may be constrained, by the functional design of limb muscles. Our previous work of level hopping shows that two key leg muscles, the plantaris and lateral gastrocnemius, undergo very little length change when developing force to support the animal's weight during stance. As a result, the muscles peform little work, but are designed well for generating a high level of force with reduced energy expenditure. This facilitates the role of these muscles' long tendons for elastic energy storage and recovery. Over a range of hopping speeds, the bouncing, spring-like gait of these animals enables their leg tendons to recover as much as 25% of the work that their muscles would otherwise have to perform, potentially reducing their metabolic cost of transport by as much as 50%. We are now interested in what happens when these animals hop up an incline, which requires increased positive muscle work. Will these muscles contribute usefully to the required increase in muscle work by shortening more? Or, will they be limited by their architecture to operate isometrically, necessitating a shift in muscle power to either hip or knee extensors for the additional work required?

Effects of limb mechanical advantage (EMA) on the mechanics and energetics of terrestrial locomotion.

A. How does incline affect limb mechanical advantage (ratio of ground reaction force to muscle force) in relation to muscle force generation and metabolic energy cost?
Using our runway facility we hope to investigate how changes in incline affect the amount of muscle force required to support the body in relation to locomotor limb posture. Does an incline lead to an overall increase in the amount of force that the muscles must generate or only to an increase in muscle shortening? How does this relate to the change in metabolic cost that occurs with incline? These experiments may be carried out on different sized bipeds and quadrupeds.

B. Change in limb mechanical advantage in relation to acceleration and deceleration: similar studies seek to evaluate how shifts in ground reaction force direction relative to limb position may affect muscle loading when an individual accelerates or decelerates.