In Vivo Sarcomere Lengths Become More Non-uniform upon Activation in Intact Whole Muscle

Abstract

The sarcomere force-length relationship has been extensively used to predict muscle force potential. The common practice is to measure the mean sarcomere length (SL) in a relaxed muscle at a single location and at a given length, and this mean SL is assumed to represent the SLs at other locations across the muscle. However, in a previous study, we found that SLs are highly non-uniform across an intact passive muscle. Moreover, SL non-uniformity increases during activation in single myofibril experiments. Myofibrils lack some structural proteins that comprise an intact muscle, and therefore, the increased SL dispersion upon activation seen in myofibrils may not occur in intact whole muscle. The objectives of the current study were (i) to measure the distribution of SLs in an activated intact muscle; and (ii) to assess the feasibility of using the mean SL measured at a specific location of the muscle to predict muscle force. Using state-of-the-art multi-photon microscopy and a miniature tendon force transducer, in vivo sarcomeres in the mouse tibialis anterior were imaged simultaneously with muscle force during isometric tetanic contractions. We found that in vivo SL dispersion increased substantially during activation and reached average differences of ~1.0 μm. These differences in SL are associated with theoretical force differences of 70–100% of the maximal isometric force. Furthermore, SLs measured at a single location in the passive muscle were poor predictors of active force potential. Although mean SLs in the activated muscle were better predictors of force potential, predicted forces still differed by as much as 35% from the experimentally measured maximal isometric forces.

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

“…We also measured great SL non-uniformity in our small target muscle volumes (Fig. 3B), as previously described in myofibrillar (Johnston et al, 2016;Johnston et al, 2019;Leonard and Herzog, 2010;Telley et al, 2006) and whole-muscle preparations (Lichtwark et al, 2018;Llewellyn et al, 2008;Moo and Herzog, 2018;Moo et al, 2017a).…”

“…titin) interact to affect force production in neighbouring sarcomeres with vast SL non-uniformity, either in a single myofibril or in a fibre or a muscle, as tested here. Furthermore, the SL non-uniformity observed in a tiny region of the muscle is even greater when sarcomeres from other sites of the muscle are included, thereby further exaggerating the nonuniformity (Moo and Herzog, 2018;Moo et al, 2017a). It is currently unclear why SL non-uniformity exists across all structural levels of muscle, and how its presence affects the mechanical properties of muscles.…”

“…We recently developed imaging techniques based on multi-photon excitation microscopy that allows for the investigation of sarcomere contraction dynamics in actively contracting muscle (Moo and Herzog, 2018;Moo et al, 2017a). In the present study, we built on this imaging technique in an attempt to compare the sarcomere FL relationship of an intact muscle-tendon unit (MTU) with the theoretically predicted sarcomere FL relationship using the mean SL obtained from a small portion of the mid-belly of the contracting muscle.…”

Optical prediction of single muscle fiber force production using a combined biomechatronics and second harmonic generation imaging approach

“…Early snap-freezing and subsequent EM studies in glycerinated fiber bundles from rabbit psoas muscle showed no major changes in the filament lengths of actin or myosin within sarcomeres under rigor, isometric contraction or relaxing conditions 46 . Using an E-shaped tendon force transducer clamped to a tibialis anterior muscle of living mice and applying SHG imaging to an electrically stimulated whole muscle in vivo, a vast increase in the sarcomere length variability was seen as a broadened SL distribution, in particular at short muscle lengths (90° ankle) over long muscle lengths (180° ankle) 47 . That very recent study focused on planar images from the top 100 µm and collecting backscattered SHG signals from whole muscle without further details on the 3D aspect of myofibrillar structure under contractile activation; however, it may provide a basis to explain our observed increase in angular disorder of myofibrils if one assumes that lateral forces resulting from different sarcomere lengths of adjacent myofibrils connected by extrasarcomeric proteins, for example desmin, would induce tilts and twists to myofibrils.…”

Time Course and Association of Functional and Biochemical Markers in Severe Semitendinosus Damage Following Intensive Eccentric Leg Curls: Differences between and within Subjects

“…Moreover, the chronic exposure to eccentric-biased exercise (i.e., sprint accelerations in athletics or football) might have induced hamstring muscles architectural changes in the moderate responders. Because sarcomere lengthening is non-uniform (Moo et al, 2016 , 2017 ) and fascicles may have a regional response (i.e., more pronounced mechanical stretch might have been applied to single sarcomeres and thus fascicles) (Franchi et al, 2017 ), increases in fascicle length might be expected as an adaptive response to eccentric exercise. Those changes in fascicle length theoretically imply the addition of sarcomeres in series (Franchi et al, 2017 ), which may had a positive effect on eccentric hamstring strength at longer muscle lengths (Brockett et al, 2001 ) and produced a protective effect on the hamstring muscles (Timmins et al, 2016 ).…”