Abstract
Here we investigate the interplay between intrinsic mechanical and neural factors in muscle contractile performance during running, which has been less studied than during walking. We report in vivo recordings of the gastrocnemius muscle of the guinea fowl (Numida meleagris), during the response and recovery from an unexpected drop in terrain. Previous studies on leg and joint mechanics following this perturbation suggested that distal leg extensor muscles play a key role in stabilisation. Here, we test this through direct recordings of gastrocnemius fascicle length (using sonomicrometry), muscle-tendon force (using buckle transducers), and activity (using indwelling EMG). Muscle recordings were analysed from the stride just before to the second stride following the perturbation. The gastrocnemius exhibits altered force and work output in the perturbed and first recovery strides. Muscle work correlates strongly with leg posture at the time of ground contact. When the leg is more extended in the drop step, net gastrocnemius work decreases (-5.2 J kg(-1) versus control), and when the leg is more flexed in the step back up, it increases (+9.8 J kg(-1) versus control). The muscle's work output is inherently stabilising because it pushes the body back toward its pre-perturbation (level running) speed and leg posture. Gastrocnemius length and force return to level running means by the second stride following the perturbation. EMG intensity differs significantly from level running only in the first recovery stride following the perturbation, not within the perturbed stride. The findings suggest that intrinsic mechanical factors contribute substantially to the initial changes in muscle force and work. The statistical results suggest that a history-dependent effect, shortening deactivation, may be an important factor in the intrinsic mechanical changes, in addition to instantaneous force-velocity and force-length effects. This finding suggests the potential need to incorporate history-dependent muscle properties into neuromechanical simulations of running, particularly if high muscle strains are involved and stability characteristics are important. Future work should test whether a Hill or modified Hill type model provides adequate prediction in such conditions. Interpreted in light of previous studies on walking, the findings support the concept of speed-dependent roles of reflex feedback.
Highlights
IntroductionAppropriate integration of musculoskeletal structure and feedforward activation of muscles may allow relatively simple intrinsic mechanical responses to stabilise locomotion, minimising the need for a rapid reflex response (Kubow & Full, 1999; Brown & Loeb, 2000; Daley & Biewener, 2006; Sponberg & Full, 2008)
Others have failed to find this trend, but noted that the reflex threshold differs between walking and running, so that reflexes contribute less to muscle activity in running (Ferris et al 2001). These findings suggest that sensorimotor reflexes and higher brain centres are likely to play a larger role in slow locomotion, such as walking, whereas intrinsic mechanical factors are likely to play a larger role in the control of rapid locomotion, such as running
The specific variables measured were: peak force before stance (F pk,prior) and during stance (F pk,stance); muscle fascicle strain and velocity at peak stance phase force (LpkF, V pkF); fascicle strain at 50% stance peak force (LFt50); mean velocity between toe down (TD) and 50% stance peak force (V Ft50); the fractional fascicle length change from force onset to the beginning of stance ( Lprior); total EMG intensity before stance, during stance and over the entire stride (Eprior + Estance = EMG intensity over a given time period (Etot)); and net work before stance, during stance and over the stride (W prior + W stance = W tot)
Summary
Appropriate integration of musculoskeletal structure and feedforward activation of muscles may allow relatively simple intrinsic mechanical responses to stabilise locomotion, minimising the need for a rapid reflex response (Kubow & Full, 1999; Brown & Loeb, 2000; Daley & Biewener, 2006; Sponberg & Full, 2008). Others have failed to find this trend, but noted that the reflex threshold differs between walking and running, so that reflexes contribute less to muscle activity in running (Ferris et al 2001) Overall, these findings suggest that sensorimotor reflexes and higher brain centres are likely to play a larger role in slow locomotion, such as walking, whereas intrinsic mechanical factors are likely to play a larger role in the control of rapid locomotion, such as running
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