The sarcomere force-length relationship in an intact muscle-tendon unit

The periodic striation pattern in skeletal muscle reflects the length of the basic contractile unit: the sarcomere. More than half a century ago, Gordon, Huxley and Julian provided strong support for the ‘sliding filament’ theory that allowed prediction of the active isometric force as a function of sarcomere length (SL). Although the theory was established using single muscle fibers, the sarcomere force-length (FL) relationship has been extrapolated to whole muscles in an attempt to unravel in-vivo muscle function. However, these extrapolations were frequently associated with non-trivial assumptions, such as muscle length changes corresponding linearly to SL changes. However, the sarcomere force-length relationship in whole muscles may not behave as theoretically predicted due to the structural complexity of muscles, the force transmission, pressures and shear forces developed inside muscles, and the great SL non-uniformity recently observed in whole muscles. Here, we determined the in-situ sarcomere FL relationship in a whole muscle preparation by simultaneously measuring muscle force and individual SLs in an intact muscle-tendon unit (MTU) using state-of-the-art multi-photon excitation microscopy. We found that despite great SL non-uniformity, the mean value of SLs measured from a minute volume of the mid-belly, equivalent to about 5 x 10−6 % of the total muscle volume, agrees well with the theoretically predicted FL relationship, but only if the precise contractile filament lengths are known, and if passive forces from parallel elastic components and activation-associated sarcomere shortening are considered properly. As SLs are not uniformly distributed across the whole muscle and changes in SLs with muscle length are location-dependent, our results may not be valid for the proximal or distal parts of the muscle. The approach described here, and our findings, may encourage future studies to determine the role of SL non-uniformity in influencing sarcomere FL properties in different muscles and for different locations within single muscles.