Differential serial sarcomere number adaptations in knee extensor muscles of rats is contraction type dependent

Abstract

Sarcomerogenesis, or the addition of sarcomeres in series within a fiber, has a profound impact on the performance of a muscle by increasing its contractile velocity and power. Sarcomerogenesis may provide a beneficial adaptation to prevent injury when a muscle consistently works at long lengths, accounting for the repeated-bout effect. The association between eccentric exercise, sarcomerogenesis and the repeated-bout effect has been proposed to depend on damage, where regeneration allows sarcomeres to work at shorter lengths for a given muscle-tendon unit length. To gain additional insight into this phenomenon, we measured fiber dynamics directly in the vastus lateralis (VL) muscle of rats during uphill and downhill walking, and we measured serial sarcomere number in the VL and vastus intermedius (VI) after chronic training on either a decline or incline grade. We found that the knee extensor muscles of uphill walking rats undergo repeated active concentric contractions, and therefore they suffer no contraction-induced injury. Conversely, the knee extensor muscles during downhill walking undergo repeated active eccentric contractions. Serial sarcomere numbers change differently for the uphill and downhill exercise groups, and for the VL and VI muscles. Short muscle lengths for uphill concentric-biased contractions result in a loss of serial sarcomeres, and long muscle lengths for downhill eccentric-biased contractions result in a gain of serial sarcomeres.

Avaliação de parâmetros histomorfométricos em sóleos de ratos submetidos à remobilização por salto em meio aquático

“…Butterfield and Herzog 30 highlighted that tension in the muscle-tendon unit is not the primary source of sarcomerogenesis but it may occur in some cases of eccentric exercise 28 like contractions and disregarded in the present study.…”

“…According to Butterfield et al 28 the addition of sarcomeres in series have a great impact on muscular performance because they enhance the contracting speed and potency. As immobilization promotes reduction of sarcomeres 29 , in this study we tested the hypothesis of jump as a way to recover longitudinal morphological characteristics of soleus muscle in rats.…”

Mechanotransduction in skeletal muscle

“…While there are clear morphological specializations at the myotendinous junction that suggest it plays an important role in transmitting that force to the extracellular matrix (ECM) and tendon (32,33), it is also clear that transmission of force by shear, often referred to as "lateral force transmission," contributes to whole muscle force transmission (19,34). The presence of elastic tissue, whether tendon, aponeurosis, or perimysium, in series with the contractile tissue means that active force generation will result in myofiber shortening during fixed endpoint activations (35,36), and potentially during lengthening activations (37,38). This means that nominally isometric activations are associated with negative, or compressive, strain relative to the passive condition and highlights the care required in deciphering force and stretch in muscle.…”

Does Stretching Have Long-Term Effects on Muscle Performance? A Clinical Commentary

“…Sarcomerogenesis is the addition of sarcomeres in series in a myofibril [29]. It is speculated that regular stretching may induce sarcomerogenesis [25], which potentiates the force-length relationship improving performance during static contractions [14].…”