Dilson E. Rassier

Residual force enhancement in skeletal muscle

Residual force enhancement has been observed consistently in skeletal muscles following active stretching. However, its underlying mechanism(s) remain elusive, and it cannot be explained readily within the framework of the cross‐bridge theory. Traditionally, residual force enhancement has been attributed to the development of sarcomere length non‐uniformities. However, recent evidence suggests that this might not be the case. Rather, it appears that residual force enhancement has an active and a passive component. The active component is tentatively associated with changes in the cross‐bridge kinetics that might be reflected in decreased detachment rates following active muscle stretching, while the passive component possibly originates from a structural protein, such as titin, whose stiffness might be regulated by calcium.

Stretch-induced, steady-state force enhancement in single skeletal muscle fibers exceeds the isometric force at optimum fiber length.

Stretch-induced force enhancement has been observed in a variety of muscle preparations and on structural levels ranging from single fibers to in vivo human muscles. It is a well-accepted property of skeletal muscle. However, the mechanism causing force enhancement has not been elucidated, although the sarcomere-length non-uniformity theory has received wide support. The purpose of this paper was to re-investigate stretch-induced force enhancement in frog single fibers by testing specific hypotheses arising from the sarcomere-length non-uniformity theory. Single fibers dissected from frog tibialis anterior (TA) and lumbricals (n=12 and 22, respectively) were mounted in an experimental chamber with physiological Ringers solution (pH=7.5) between a force transducer and a servomotor length controller. The tetantic force-length relationship was determined. Isometric reference forces were determined at optimum length (corresponding to the maximal, active, isometric force), and at the initial and final lengths of the stretch experiments. Stretch experiments were performed on the descending limb of the force-length relationship after maximal tetanic force was reached. Stretches of 2.5-10% (TA) and 5-15% lumbricals of fiber length were performed at 0.1-1.5 fiber lengths/s. The stretch-induced, steady-state, active isometric force was always equal or greater than the purely isometric force at the muscle length from which the stretch was initiated. Moreover, for stretches of 5% fiber length or greater, and initiated near the optimum length of the fiber, the stretch-enhanced active force always exceeded the maximal active isometric force at optimum length. Finally, we observed a stretch-induced enhancement of passive force. We conclude from these results that the sarcomere length non-uniformity theory alone cannot explain the observed force enhancement, and that part of the force enhancement is associated with a passive force that is substantially greater after active compared to passive muscle stretch.

Dynamics of individual sarcomeres during and after stretch in activated single myofibrils

It is generally assumed that sarcomere lengths (SLs) change in isometric fibres following activation and following stretch on the descending limb of the force–length relationship, because of an inherent instability. Although this assumption has never been tested directly, instability and SL non–uniformity have been associated with several mechanical properties, such as ‘creep’ and force enhancement. The aim of this study was to test directly the hypothesis that sarcomeres are unstable on the descending limb of the force–length relationship. We used single myofibrils, isolated from rabbit psoas, that were attached to glass needles that allowed for controlled stretching of myofibrils. Images of the sarcomere striation pattern were projected onto a linear photodiode array, which was scanned at 20 Hz to produce dark–light patterns corresponding to the A– and I–bands, respectively. Starting from a mean SL of 2.55±0.07 &mgr;m, stretches of 11.2± 1.6% of SL at a speed of 118.9± 5.9 nm s–1 were applied to the activated myofibrils (pCa2+ = 4.75). SLs along the myofibril were non–uniform before, during and after the stretch, but with few exceptions, they remained constant during the isometric period before stretch, and during the extended isometric period after stretch. Sarcomeres never lengthened to a point beyond thick and thin filament overlap. We conclude that sarcomeres are non–uniform but generally stable on the descending limb of the force–length relationship.

Force enhancement in single skeletal muscle fibres on the ascending limb of the force-length relationship.

SUMMARY It has been assumed that force enhancement in single fibres of skeletal muscles only occurs on the descending, and not the ascending or plateau region, of the force–length relationship. This assumption has been based, however, on theoretical considerations or isolated experiments, in which neither stretch conditions nor fibre lengths were optimized for force enhancement. Therefore, the purpose of this study was to investigate the residual, steady-state force enhancement following active stretch in single muscle fibres of frog on the ascending limb of the force–length relationship. Twenty-nine stretch experiments on ten single fibres from the lumbrical muscle of the frog Rana pipiens were carried out on the ascending limb of the force–length relationship. Force enhancement was observed in 28 out of the 29 tests. Moreover, the force produced for stretch experiments finishing at optimal fibre length always exceeded the force obtained for an isometric contraction at optimal length. We conclude from these results that steady-state force enhancement occurs systematically on the ascending limb of the force–length relationship, and that the steady-state force in the enhanced state can easily exceed the maximal isometric force of the fibre.

Pre-power stroke cross bridges contribute to force during stretch of skeletal muscle myofibrils

When activated skeletal muscle is stretched, force increases in two phases. This study tested the hypothesis that the increase in stretch force during the first phase is produced by pre-power stroke cross bridges. Myofibrils were activated in sarcomere lengths (SLs) between 2.2 and 2.5 μm, and stretched by approximately 5–15 per cent SL. When stretch was performed at 1 μm s−1 SL−1, the transition between the two phases occurred at a critical stretch (SLc) of 8.4±0.85 nm half-sarcomere (hs)−1 and the force (critical force; Fc) was 1.62±0.24 times the isometric force (n=23). At stretches performed at a similar velocity (1 μm s−1 SL−1), 2,3-butanedione monoxime (BDM; 1 mM) that biases cross bridges into pre-power stroke states decreased the isometric force to 21.45±9.22 per cent, but increased the relative Fc to 2.35±0.34 times the isometric force and increased the SLc to 14.6±0.6 nm hs−1 (n=23), suggesting that pre-power stroke cross bridges are largely responsible for stretch forces.

Time course of neuromuscular adaptations to knee extensor eccentric training.

This study investigated the chronology of neural and morphological adaptations to knee extensor eccentric training and their contribution to strength gains in isometric, concentric and eccentric muscle actions. 20 male healthy subjects performed a 12-week eccentric training program on an isokinetic dynamometer, and neuromuscular evaluations of knee extensors were performed every 4 weeks. After 12 training weeks, significant increases were observed for: isometric (24%), concentric (15%) and eccentric (29%) torques; isometric (29%) and eccentric (33%) electromyographic activity; muscle thickness (10%) and anatomical cross-sectional area (19%). Eccentric and isometric torques increased progressively until the end of the program. Concentric torque and muscle mass parameters increased until the eighth training week, but did not change from this point to the twelfth training week. Eccentric and isometric activation increased at 4 and 8 training weeks, respectively, while no change was found in concentric activation. These results suggest that: 1) the relative increment in concentric strength was minor and does not relate to neural effects; 2) eccentric and isometric strength gains up to 8 training weeks are explained by the increased neural activation and muscle mass, whereas the increments in the last 4 training weeks seem to be associated with other mechanisms.

The mechanisms of the residual force enhancement after stretch of skeletal muscle: non-uniformity in half-sarcomeres and stiffness of titin

When activated skeletal muscles are stretched, the force increases significantly. After the stretch, the force decreases and reaches a steady-state level that is higher than the force produced at the corresponding length during purely isometric contractions. This phenomenon, referred to as residual force enhancement, has been observed for more than 50 years, but the mechanism remains elusive, generating considerable debate in the literature. This paper reviews studies performed with single muscle fibres, myofibrils and sarcomeres to investigate the mechanisms of the stretch-induced force enhancement. First, the paper summarizes the characteristics of force enhancement and early hypotheses associated with non-uniformity of sarcomere length. Then, it reviews new evidence suggesting that force enhancement can also be associated with sarcomeric structures. Finally, this paper proposes that force enhancement is caused by: (i) half-sarcomere non-uniformities that will affect the levels of passive forces and overlap between myosin and actin filaments, and (ii) a Ca2+-induced stiffness of titin molecules. These mechanisms are compatible with most observations in the literature, and can be tested directly with emerging technologies in the near future.

Force produced by isolated sarcomeres and half-sarcomeres after an imposed stretch

When a stretch is imposed to activated muscles, there is a residual force enhancement that persists after the stretch; the force is higher than that produced during an isometric contraction in the corresponding length. The mechanisms behind the force enhancement remain elusive, and there is disagreement if it represents a sarcomeric property, or if it is associated with length nonuniformities among sarcomeres and half-sarcomeres. The purpose of this study was to investigate the effects of stretch on single sarcomeres and myofibrils with predetermined numbers of sarcomeres (n = 2, 3. . . , 8) isolated from the rabbit psoas muscle. Sarcomeres were attached between two precalibrated microneedles for force measurements, and images of the preparations were projected onto a linear photodiode array for measurements of half-sarcomere length (SL). Fully activated sarcomeres were subjected to a stretch (5-10% of initial SL, at a speed of 0.3 μm·s(-1)·SL(-1)) after which they were maintained isometric for at least 5 s before deactivation. Single sarcomeres showed two patterns: 31 sarcomeres showed a small level of force enhancement after stretch (10.46 ± 0.78%), and 28 sarcomeres did not show force enhancement (-0.54 ± 0.17%). In these preparations, there was not a strong correlation between the force enhancement and half-sarcomere length nonuniformities. When three or more sarcomeres arranged in series were stretched, force enhancement was always observed, and it increased linearly with the degree of half-sarcomere length nonuniformities. The results show that the residual force enhancement has two mechanisms: 1) stretch-induced changes in sarcomeric structure(s); we suggest that titin is responsible for this component, and 2) stretch-induced nonuniformities of half-sarcomere lengths, which significantly increases the level of force enhancement.

History-dependent properties of skeletal muscle myofibrils contracting along the ascending limb of the force-length relationship

There is a history dependence of skeletal muscle contraction: stretching activated muscles induces a long-lasting force enhancement, while shortening activated muscles induces a long-lasting force depression. These history-dependent properties cannot be explained by the current model of muscle contraction, and its mechanism is unknown. The purposes of this study were (i) to evaluate if force enhancement and force depression are present at short lengths (the ascending limb of the force–length (FL) relationship), (ii) to evaluate if the history-dependent properties are associated with sarcomere length (SL) non-uniformity and (iii) to determine the effects of cross-bridge (de)activation on force depression. Rabbit psoas myofibrils were isolated and attached between two microneedles for force measurements. Images of the myofibrils were projected onto a linear photodiode array for measurements of SL. Myofibrils were activated by either Ca2+ or MgADP; the latter induces cross-bridge attachment to actin independently of Ca2+. Activated myofibrils were subjected to three stretches or shortenings (approx. 4% SL at approx. 0.07 µm s−1 sarcomere−1) along the ascending limb of the FL relationship separated by periods (approx. 5 s) of isometric contraction. Force after stretch was higher than force after shortening at similar SLs. The differences in force could not be explained by SL non-uniformity. The FL relationship produced by Ca2+- and MgADP-activated myofibrils were similar in stretch experiments, but after shortening MgADP activation produced forces that were higher than Ca2+ activation. Since MgADP induces the formation of strongly bound cross-bridges, this result suggests that force depression following shortening is associated with cross-bridge deactivation.

ATTENUATION OF MYOSIN LIGHT CHAIN PHOSPHORYLATION AND POSTTETANIC POTENTIATION IN ATROPHIED SKELETAL MUSCLE

Abstract Previously we have demonstrated that the absence of staircase potentiation in atrophied rat gastrocnemius muscle is accompanied by a virtual absence of phosphorylation of the regulatory light chains (R-LC) of myosin. It was our purpose in the present study to determine if posttetanic potentiation and corresponding R-LC phosphorylation were also attenuated in disuse-atrophied muscles. Two weeks after a spinal hemisection (T12), twitch and tetanic contractile characteristics were measured in situ in control, sham-treated and atrophied (hemisected) muscles. Posttetanic potentiation 20 s after a 2 s tetanic contraction (200 Hz) was depressed in atrophied muscles (128.7 ± 2.6%; mean ± SEM) when compared to sham-treated (149.9 ± 2.4%) and control (142.9 ± 2.7%) muscles. Atrophied muscles demonstrated a significant increase in R-LC phosphorylation from rest (0.05 ± 0.04 moles of phosphate/mole of R-LC) to posttetanic conditions (0.21 ± 0.03 moles of phosphate/mole of R-LC), and less phosphorylation than control and sham-treated muscles (0.43 ± 0.06 and 0.49 ± 0.03 moles of phosphate/mole of R-LC, respectively) after tetanic stimulation. The preservation of the potentiation-phosphorylation relationship in atrophied muscles is consistent with the hypothesis that R-LC phosphorylation may be the principal mechanism for twitch potentiation.