Differential segmental strain during active lengthening in a large biarticular thigh muscle during running

Abstract

SUMMARYThe iliotibialis lateralis pars postacetabularis (ILPO) is the largest muscle in the hindlimb of the guinea fowl and is thought to play an important role during the stance phase of running, both absorbing and producing work. Using sonomicrometry and electromyography, we examined whether the ILPO experiences differential strain between proximal, central and distal portions of the posterior fascicles. When the ILPO is being lengthened while active, the distal portion was found to lengthen significantly more than either the proximal or central portions of the muscle. Our data support the hypothesis that the distal segment lengthened farther and faster because it began activity at shorter sarcomere lengths on the ascending limb of the length-tension curve. Probably because of the self-stabilizing effects of operating on the ascending limb of the length-tension curve, all segments reached the end of lengthening and started shortening at the same sarcomere length. During shortening, this similarity in sarcomere length among the segments was maintained, as predicted from force-velocity effects, and shortening strain was similar in all segments. The differential active strain during active lengthening is thus ultimately determined by differences in strain during the passive portion of the cycle. The sarcomere lengths of all segments of the fascicles were similar at the end of active shortening, but after the passive portion of the cycle the distal segment was shorter. Differential strain in the segments during the passive portion of the cycle may be caused by differential joint excursions at the knee and hip acting on the ends of the muscle and being transmitted differentially by the passive visco-elastic properties of the muscle. Alternatively, the differential passive strain could be due to the action of active or passive muscles in the thigh that transmit force to the IPLO in shear. Based on basic sarcomere dynamics we predict that differential strain is more likely to occur in muscles undergoing active lengthening at the beginning of contraction than those undergoing only shortening.

Mechanisms producing coordinated function across the breadth of a large biarticular thigh muscle

“…More detailed descriptions of sonomicrometry and electromyography surgical techniques, sarcomere measurements, and calculations of length and velocity are given by Carr et al (Carr et al, 2011a).…”

“…The mean rectified value was calculated both for duration of the burst and the duration of the stride. As described by Carr et al (Carr et al, 2011a), we standardized the mean amplitude values by dividing the mean amplitude measured for each electrode by the mean amplitude measured when the animals were running uphill at 2.0ms -1 (Carr et al, 2011b). In vivo segment lengths measured by sonomicrometry (L so ) were converted to sarcomere lengths using reference sonomicrometer lengths (L so,ref ) and sarcomere lengths (L sc,ref ) measured in rigor.…”

“…The sarcomere length was then converted to fractions of optimal length (L 0 ) by dividing by 2.36, which is the sarcomere length expected in the center of the plateau of the length-tension curve determined according to the filament lengths (Carr et al, 2011a). Fascicle strains are similarly given as ⌬L sc divided by 2.36.…”

“…The possibility of regional differences in function both along the axis of long muscle fascicles and in parallel fascicles across the breadth of a muscle can limit the extent to which the function of large architecturally complex muscles can be inferred from localized measurements or modeled predictions. For example, in a companion study to the work presented here (Carr et al, 2011a), strain in-series along the posterior fascicles in the iliotibialis lateralis pars postacetabularis (ILPO) of guinea fowl was examined. During active lengthening, the distal portion of the posterior fascicles experiences the most strain.…”

Function of a large biarticular hip and knee extensor during walking and running in guinea fowl (Numida meleagris)

“…The present study combined data on regional strain and muscle activation from two companion studies of level running (Carr et al, 2011a;Carr et al, 2011b) with data on uphill running collected here, and muscle energy use from previous studies. We used these data to test hypotheses related to alterations in mechanical function induced by speed and slope.…”