Data from: Linking in vivo muscle dynamics to in situ force-length and force-velocity reveals that guinea fowl lateral gastrocnemius operates at shorter than optimal lengths

Force-length (F-L) and force-velocity (F-V) properties characterize
skeletal muscle’s intrinsic properties under controlled conditions, and it
is thought that these properties can inform and predict in vivo muscle
function. Here, we map dynamic in vivo operating range and mechanical
function during walking and running, to the measured in situ F-L and F-V
characteristics of guinea fowl (Numida meleagris) lateral gastrocnemius
(LG), a primary ankle extensor. We use in vivo patterns of muscle (tendon)
force, fascicle length, and activation to test the hypothesis that muscle
fascicles operate at optimal lengths and velocities to maximize force or
power production during walking and running. Our findings only partly
support our hypothesis: in vivo LG velocities are consistent with
optimizing power during work production, and economy of force at higher
loads. However, LG does not operate at lengths on the force plateau (±5%
Fmax) during force production. LG length was near L0 at the time of EMG
onset but shortened rapidly such that force development during stance
occurred almost entirely on the ascending limb of the F-L curve, at
shorter than optimal lengths. These data suggest that muscle fascicles
shorten across optimal lengths in late swing, to optimize the potential
for rapid force development near the swing-stance transition. This may
provide resistance against unexpected perturbations that require rapid
force development at foot contact. We also found evidence of passive force
rise (in absence of EMG activity) in late swing, at lengths where passive
force is zero in situ, suggesting that history dependent and viscoelastic
effects may contribute to in vivo force development. Direct comparison of
in vivo work loops and physiological operating ranges to traditional
measures of F-L and F-V properties suggests the need for new approaches to
characterize dynamic muscle properties in controlled conditions that more
closely resemble in vivo dynamics.