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

Abstract

SUMMARYWe examined the hypothesis that structural features of the iliotibialis lateralis pars postacetabularis (ILPO) in guinea fowl allow this large muscle to maintain equivalent function along its anterior-posterior axis. The ILPO, the largest muscle in the hindlimb of the guinea fowl, is a hip and knee extensor. The fascicles of the ILPO originate across a broad region of the ilium and ischium posterior to the hip. Its long posterior fascicles span the length of the thigh and insert directly on the patellar tendon complex. However, its anterior fascicles are shorter and insert on a narrow aponeurosis that forms a tendinous band along the anterior edge of the muscle and is connected distally to the patellar tendon. The biarticular ILPO is actively lengthened and then actively shortened during stance. The moment arm of the fascicles at the hip increases along the anterior to posterior axis, whereas the moment arm at the knee is constant for all fascicles. Using electromyography and sonomicrometry, we examined the activity and strain of posterior and anterior fascicles of the ILPO. The activation was not significantly different in the anterior and posterior fascicles. Although we found significant differences in active lengthening and shortening strain between the anterior and posterior fascicles, the differences were small. The majority of shortening strain is caused by hip extension and the inverse relationship between hip moment arm and fascicle length along the anterior-posterior axis was found to have a major role in ensuring similar shortening strain. However, because the knee moment arm is the same for all fascicles, knee flexion in early stance was predicted to produce much larger lengthening strains in the short anterior fascicles than our measured values at this location. We propose that active lengthening of the anterior fascicles was lower than predicted because the aponeurotic tendon of insertion of the anterior fascicles was stretched and only a portion of the lengthening had to be accommodated by the active muscle fascicles.

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

“…By inference it first absorbs work and then produces work while active. The lengthening in early stance is related to knee flexion (Carr et al, 2011b), and shortening corresponds to a period of both knee extension and hip extension (Carr et al, 2011b). The overall strain cycle of the active ILPO found in the present study, taking into consideration regional strain, was qualitatively similar to previous studies of this muscle in guinea fowl and turkeys using a single segment length (Buchanan, 1999;McGowan et al, 2006;Roberts et al, 2007), and quantitative differences can probably be explained by differences in regional strain (Carr et al, 2011a).…”

“…The sarcomere lengths were 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 based on filament lengths (Carr et al, 2011a Small but statistically significant differences in strain and velocity were found between the anterior and posterior fascicles of the ILPO (Carr et al, 2011b). Therefore, we calculated a mass-weighted average of the values from the anterior and posterior regions to arrive at the values presented here.…”

“…No significant regional differences were found in measures of EMG amplitude or timing among the various electrode sites (Carr et al, 2011a;Carr et al, 2011b). Therefore, we calculated the average values for the whole muscles by taking the mean of the values at the different electrode sites.…”

“…During swing, the ILPO showed periods of passive lengthening and shortening (Carr et al, 2011a;Carr et al, 2011b), but these passive length changes will not be considered here. Active strain during lengthening and shortening was significantly influenced by speed and slope (Table1; Fig.3).…”

“…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.…”