Abstract
Sarcomere lengths have been a crucial outcome measure for understanding and explaining basic muscle properties and muscle function. Sarcomere lengths for a given muscle are typically measured at a single spot, often in the mid-belly of the muscle, and at a given muscle length. It is then assumed implicitly that the sarcomere length measured at this single spot represents the sarcomere lengths at other locations within the muscle, and force-length, force-velocity, and power-velocity properties of muscles are often implied based on these single sarcomere length measurements. Although, intuitively appealing, this assumption is yet to be supported by systematic evidence. The objective of this study was to measure sarcomere lengths at defined locations along and across an intact muscle, at different muscle lengths. Using second harmonic generation (SHG) imaging technique, sarcomere patterns in passive mouse tibialis anterior (TA) were imaged in a non-contact manner at five selected locations (“proximal,” “distal,” “middle,” “medial,” and “lateral” TA sites) and at three different lengths encompassing the anatomical range of motion of the TA. We showed that sarcomere lengths varied substantially within small regions of the muscle and also for different sites across the entire TA. Also, sarcomere elongations with muscle lengthening were non-uniform across the muscle, with the highest sarcomere stretches occurring near the myotendinous junction. We conclude that muscle mechanics derived from sarcomere length measured from a small region of a muscle may not well-represent the sarcomere length and associated functional properties of the entire muscle.
Highlights
Sarcomeres are the basic force producing units of muscles
This approach requires the insertion of the micro-endoscope into the muscle and it remains unclear what effect such a mechanical perturbation might have on the resulting sarcomere length
At full dorsiflexion, mean sarcomere lengths ranged from 2.1 to 2.3 μm, with sarcomeres at the “medial” site being significantly longer than sarcomeres at the “proximal” site (Figure 3)
Summary
Sarcomeres are the basic force producing units of muscles. Under a light microscope, muscle fibers have alternating black and white bands due to the contractile filaments myosin (anisotropic band, A-band) and actin (isotropic band, I-band) that make up sarcomeres. Sarcomere length can be measured readily from muscle micrographs generated by light microscopy (Huxley and Peachey, 1961; Telley et al, 2006; Infantolino et al, 2010) and electron microscopy (Goulding et al, 1997), or from the diffraction pattern resulting from shining a laser beam through a muscle (ter Keurs et al, 1978). Sarcomeres were successfully imaged in vivo using a micrometer-sized endoscope (Llewellyn et al, 2008; Cromie et al, 2013) This approach requires the insertion of the micro-endoscope into the muscle and it remains unclear what effect such a mechanical perturbation might have on the resulting sarcomere length. An improvement to existing techniques involves the examination of sarcomere lengths that involves no contact with the muscle
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