Single sarcomere contraction dynamics in a whole muscle

Abstract

The instantaneous sarcomere length (SL) is regarded as an important indicator of the functional properties of striated muscle. Previously, we found greater sarcomere elongations at the distal end compared to the mid-portion in the mouse tibialis anterior (TA) when the muscle was stretched passively. Here, we wanted to see if SL dispersions increase with activation, as has been observed in single myofibrils, and if SL dispersions differ for different locations in a muscle. Sarcomere lengths were measured at a mid- and a distal location of the TA in live mice using second harmonic generation imaging. Muscle force was measured using a tendon force transducer. We found that SL dispersions increased substantially from the passive to the active state, and were the same for the mid- and distal portions of TA. Sarcomere length non-uniformities within a segment of ~30 serial sarcomeres were up to 1.0 µm. We conclude from these findings that passive, mean SLs obtained from a single location are not necessarily representative of the distribution of SL in active muscle, and thus may be misinterpreted when deriving muscle mechanical properties, such as the force-length relationship. In view of these findings, it seems crucial to determine how SL distributions within a muscle relate to the most fundamental properties of muscle, such as the maximal isometric force.

Repercussions on sarcomeres of the myotendinous junction and the myofibrillar type adaptations in response to different trainings on vertical ladder

“…We also reveal the effect of NLT on MTJ region, with longer ones in proximal and distal sarcomeres; this was repercussion of the transmission of force by MTJ and may be associated with formation variability and sarcomeric disposition in response to chronic activities performed by skeletal muscles, as presented by Moo and Herzog (2018).…”

“…The myofibrillar types establish the skeletal muscle functional properties and consequently its adaptive responses, even more when associated with muscles with differentiated fiber type predominance and its action aspects required for different exercise modalities (Qaisar et al, 2016). During muscle contraction, the training influences the sarcomeric arrangement (Moo & Herzog, 2018) and can influence an important related force transmission region of the myotendinous interface. Due to the different muscle functional aspects required for load training, the adaptations at MTJ can provide benefits in performance and directly act in the prevention of sports muscle injuries.…”

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.…”