Regional heterogeneity in muscle fiber strain: the role of fiber architecture.

E Azizi,Amber R Deslauriers

doi:10.3389/fphys.2014.00303

Abstract

The force, mechanical work and power produced by muscle fibers are profoundly affected by the length changes they undergo during a contraction. These length changes are in turn affected by the spatial orientation of muscle fibers within a muscle (fiber architecture). Therefore any heterogeneity in fiber architecture within a single muscle has the potential to cause spatial variation in fiber strain. Here we examine how the architectural variation within a pennate muscle and within a fusiform muscle can result in regional fiber strain heterogeneity. We combine simple geometric models with empirical measures of fiber strain to better understand the effect of architecture on fiber strain heterogeneity. We show that variation in pennation angle throughout a muscle can result in differences in fiber strain with higher strains being observed at lower angles of pennation. We also show that in fusiform muscles, the outer/superficial fibers of the muscle experience lower strains than central fibers. These results show that regional variation in mechanical output of muscle fibers can arise solely from architectural features of the muscle without the presence of any spatial variation in motor recruitment.

Highlights

The mechanical output of a muscle is strongly affected by the length changes and shortening velocity of muscle fibers
The well-described force-length and force-velocity properties of muscles have long been considered important constraints on muscle performance (Hill, 1938; Gordon et al, 1966). These familiar relationships define how the force, work and mechanical power output of a muscle are affected by the length of muscle fibers as they shorten
Even in the absence of differential motor recruitment, the architectural features of a muscle can cause regional variation in fiber strain. It is well-established that the spatial orientation of muscle fibers and fascicles within a muscle can influence the relationship between the length changes of a fiber and that of a whole muscle (Brainerd and Azizi, 2005)

Summary

INTRODUCTION

The mechanical output of a muscle is strongly affected by the length changes and shortening velocity of muscle fibers. The well-described force-length and force-velocity properties of muscles have long been considered important constraints on muscle performance (Hill, 1938; Gordon et al, 1966) These familiar relationships define how the force, work and mechanical power output of a muscle are affected by the length of muscle fibers as they shorten. Even in the absence of differential motor recruitment, the architectural features of a muscle can cause regional variation in fiber strain It is well-established that the spatial orientation of muscle fibers and fascicles within a muscle can influence the relationship between the length changes of a fiber and that of a whole muscle (Brainerd and Azizi, 2005). The amplification of fiber strain arises from the fact that fibers in a pennate muscle shorten along their length, they change orientation (pennation angle) during the contraction www.frontiersin.org

Azizi and Deslauriers

PENNATE MUSCLES

FUSIFORM MUSCLE

RESULTS AND DISCUSSION