Myosin cross-bridge kinetics slow at longer muscle lengths during isometric contractions in intact soleus from mice

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

Sensitivity of muscle force response of a two-state cross-bridge model to variations in model parameters

Muscle models based on the cross-bridge theory (Huxley-type models) are frequently used to calculate muscle forces for different contractile conditions. Dynamic and nonlinear characteristics of muscle forces produced during isometric, concentric, and eccentric contractions can be represented to a limited extent by using cross-bridge models. Cross-bridge models use various parameters to simulate force responses. However, there remains uncertainty as to the effect of changes in model parameters on force responses in Huxley-type models. In this study, we aimed to analyze the sensitivity of force response to changes in model parameters in Huxley-type models. A two-state Huxley model was used to determine the cross-bridge attachment distributions and forces for shortening and lengthening contractions. Sensitivity of muscle force to changes in attachment rate, detachment rate, and cross-bridge binding distance was examined within a range of ±20% of the nominal value using Monte Carlo simulations. Changes in the detachment rate influenced the predicted muscle forces the most for lengthening contractions, while changes in attachment rate and binding distance affected forces the most for shortening contractions. These results show once more the asymmetry between shortening and lengthening contractions and the difficulty in using a single cross-bridge model to predict forces during shortening and elongation accurately.

How velocity impacts eccentric force generation of fully activated skinned skeletal muscle fibers in long stretches

Eccentric muscle contractions are fundamental to everyday life. They occur markedly in jumping, running, and accidents. Following an initial force rise, stretching of a fully activated muscle can result in a phase of decreasing force ('Give') followed by force redevelopment. However, how the stretch velocity affects 'Give' and force redevelopment remains largely unknown. We investigated the force produced by fully activated single skinned fibers of rat extensor digitorum longus muscles during long stretches. Fibers were pulled from length .85 to 1.3 optimal fiber length at a rate of 1, 10 and 100% of the estimated maximum shortening velocity. 'Give' was absent in slow stretches. Medium and fast stretches yielded a clear 'Give'. After the initial force peak, forces decreased by 11.2% and 27.8% relative to the initial peak force before rising again. During the last half of the stretch (from 1.07 to 1.3 optimal fiber length, which is within the range of the expected descending limb of the force-length relationship), the linear force slope tripled from slow to medium stretch and increased further by 60% from medium to fast stretch. These results are compatible with forcible cross-bridge detachment and re-development of a cross-bridge distribution, and a viscoelastic titin contribution to fiber force. Accounting for these results can improve muscle models and predictions of multi-body simulations.

Myosin cross-bridge kinetics slow at longer muscle lengths during isometric contractions in intact soleus from mice

Muscle contraction results from force-generating cross-bridge interactions between myosin and actin. Cross-bridge cycling kinetics underlie fundamental contractile properties, such as active force production and energy utilization. Factors that influence cross-bridge kinetics at the molecular level propagate through the sarcomeres, cells and tissue to modulate whole-muscle function. Conversely, movement and changes in the muscle length can influence cross-bridge kinetics on the molecular level. Reduced, single-molecule and single-fibre experiments have shown that increasing the strain on cross-bridges may slow their cycling rate and prolong their attachment duration. However, whether these strain-dependent cycling mechanisms persist in the intact muscle tissue, which encompasses more complex organization and passive elements, remains unclear. To investigate this multi-scale relationship, we adapted traditional step-stretch protocols for use with mouse soleus muscle during isometric tetanic contractions, enabling novel estimates of length-dependent cross-bridge kinetics in the intact skeletal muscle. Compared to rates at the optimal muscle length (L_o), we found that cross-bridge detachment rates increased by approximately 20% at 90% of L_o (shorter) and decreased by approximately 20% at 110% of L_o (longer). These data indicate that cross-bridge kinetics vary with whole-muscle length during intact, isometric contraction, which could intrinsically modulate force generation and energetics, and suggests a multi-scale feedback pathway between whole-muscle function and cross-bridge activity.