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1 Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, United States
* To whom correspondence should be addressed. E-mail: cmcgowan{at}oeb.harvard.edu.
The goal of this study was to test whether the contractile patterns of two major hindlimb extensors of guinea fowl are altered by load carrying exercise. We hypothesized that changes in contractile pattern, specifically a decrease in muscle shortening velocity or enhanced stretch activation, would result in a reduction in locomotor energy cost relative to the load carried. We also anticipated that changes in kinematics would reflect underlying changes in muscle strain. Oxygen consumption, muscle activation intensity and fascicle strain rate were measured over a range of speeds while animals ran unloaded versus when they carried a trunk load equal to 22% of their body mass. Our results showed that loading produced no significant (p>0.05) changes in kinematic patterns at any speed. In vivo muscle contractile strain patterns in the iliotibialis lateralis pars postacetabularis (ILPO) and the medial head of the gastrocnemius (MG) showed a significant increase in active stretch early in stance (p< 0.01), but muscle fascicle shortening velocity was not significantly affected by load carrying. Oxygen consumption rate increased by 17% (p< 0.01) during loaded conditions, equivalent to 77% of the relative increase in mass. Additionally, relative increases in EMG intensity (quantified as mean spike amplitude) indicated less than proportional recruitment, consistent with force enhancement via stretch activation, in the proximal ILPO; however, a greater than proportional increase in the MG was observed. As a result, when averaged for the two muscles, EMG intensity increased in direct proportion to the fractional increase in load carried.
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