Crossbridge viscosity in activated frog muscle fibres

Impact of lengthening velocity on the generation of eccentric force by slow-twitch muscle fibers in long stretches

After an initial increase, isovelocity elongation of a muscle fiber can lead to diminishing (referred to as Give in the literature) and subsequently increasing force. How the stretch velocity affects this behavior in slow-twitch fibers remains largely unexplored. Here, we stretched fully activated individual rat soleus muscle fibers from 0.85 to 1.3 optimal fiber length at stretch velocities of 0.01, 0.1, and 1 maximum shortening velocity, vmax, and compared the results with those of rat EDL fast-twitch fibers obtained in similar experimental conditions. In soleus muscle fibers, Give was 7%, 18%, and 44% of maximum isometric force for 0.01, 0.1, and 1 vmax, respectively. As in EDL fibers, the force increased nearly linearly in the second half of the stretch, although the number of crossbridges decreased, and its slope increased with stretch velocity. Our findings are consistent with the concept of a forceful detachment and subsequent crossbridge reattachment in the stretch's first phase and a strong viscoelastic titin contribution to fiber force in the second phase of the stretch. Interestingly, we found interaction effects of stretch velocity and fiber type on force parameters in both stretch phases, hinting at fiber type-specific differences in crossbridge and titin contributions to eccentric force. Whether fiber type-specific combined XB and non-XB models can explain these effects or if they hint at some not fully understood properties of muscle contraction remains to be shown. These results may stimulate new optimization perspectives in sports training and provide a better understanding of structure-function relations of muscle proteins.

Muscle as a molecular machine for protecting joints and bones by absorbing mechanical impacts

We hypothesize that dissipation of mechanical energy of external impact to absorb mechanical shock is a fundamental function of skeletal muscle in addition to its primary function to convert chemical energy into mechanical energy. In physical systems, the common mechanism for absorbing mechanical shock is achieved with the use of both elastic and viscous elements and we hypothesize that the viscosity of the skeletal muscle is a variable parameter which can be voluntarily controlled by changing the tension of the contracting muscle. We further hypothesize that an ability of muscle to absorb shock has been an important factor in biological evolution, allowing the life to move from the ocean to land, from hydrodynamic to aerodynamic environment with dramatically different loading conditions for musculoskeletal system. The ability of muscle to redistribute the energy of mechanical shock in time and space and unload skeletal joints is of key importance in physical activities. We developed a mathematical model explaining the absorption of mechanical shock energy due to the increased viscosity of contracting skeletal muscles. The developed model, based on the classical theory of sliding filaments, demonstrates that the increased muscle viscosity is a result of the time delay (or phase shift) between the mechanical impact and the attachment/detachment of myosin heads to binding sites on the actin filaments. The increase in the contracted muscle's viscosity is time dependent. Since the forward and backward rate constants for binding the myosin heads to the actin filaments are on the order of 100s(-1), the viscosity of the contracted muscle starts to significantly increase with an impact time greater than 0.01s. The impact time is one of the key parameters in generating destructive stress in the colliding objects. In order to successfully dampen a short high power impact, muscles must first slow it down to engage the molecular mechanism of muscle viscosity. Muscle carries out two functions, acting first as a nonlinear spring to slow down impact and second as a viscous damper to absorb the impact. Exploring the ability of muscle to absorb mechanical shock may shed light to many problems of medical biomechanics and sports medicine. Currently there are no clinical devices for real-time quantitative assessment of viscoelastic properties of contracting muscles in vivo. Such assessment may be important for diagnosis and monitoring of treatment of various muscle disorders such as muscle dystrophy, motor neuron diseases, inflammatory and metabolic myopathies and many more.

The Huxley-Simmons manoeuvre: is still lacking the experimental evidence that the quick release is a pure elastic phenomenon

The analysis of the quick release is usually made by fitting a straight line to the first few experimental points of the tension-length curve. The line is then extrapolated to zero force. The fact that the tension-length curve can be represented by a straight line does not grant, however, that the quick release is a pure elastic process. As a matter of fact the experimental precision is not such to exclude a small nonlinearity from the curve and thus to mistake a visco-elastic process for an elastic one. At least two are the consequences of such a mistake: (1) stiffness is overestimated; (2) energy balance is incorrect.

Preferred movement patterns during a simple bouncing task

Elastic tissues in the human body can store and return mechanical energy passively, reducing the metabolic cost of cyclical movements. However, it is not clear whether humans prefer movement patterns that optimize this storage and return. We investigated the preferred movement pattern during a bouncing task for which non-invasive techniques can identify the resonant frequency, which is the least metabolically costly. We quantified the preferred and resonant bounce frequencies for three mechanical conditions. During 'normal' trials, subjects bounced while reclined on a sled that moves along a track. During 'added mass' trials, mass was added to the sled. During 'added stiffness' trials, a spring was attached between the sled and the supporting frame, parallel to the track. Subsequently, we quantified the preferred bounce frequencies during ischemia, a technique that disrupts the available sensory feedback. Mechanical condition had a significant effect on both the preferred and resonant frequencies. Changes in preferred frequency scaled with resonant frequency, but the preferred frequency was significantly lower than the resonant frequency. These results indicate that humans adapt their preferred bouncing pattern in response to changes in mechanical condition. Humans may prefer a lower than resonant frequency because of an inability to sense metabolic cost during our relatively short trials. In contrast, during ischemia the preferred bounce frequency remained constant even when mechanical condition was varied, indicating that feedback is necessary to adapt the preferred frequency to changes in mechanics. These findings suggest that disrupted sensory feedback may prevent humans from choosing the optimal movement pattern.

Water and muscle contraction

The interaction between water and the protein of the contractile machinery as well as the tendency of these proteins to form geometrically ordered structures provide a link between water and muscle contraction. Protein osmotic pressure is strictly related to the chemical potential of the contractile proteins, to the stiffness of muscle structures and to the viscosity of the sliding of the thin over the thick filaments. Muscle power output and the steady rate of contraction are linked by modulating a single parameter, a viscosity coefficient. Muscle operation is characterized by working strokes of much shorter length and much quicker than in the classical model. As a consequence the force delivered and the stiffness attained by attached cross-bridges is much larger than usually believed.

Crossbridge and non-crossbridge contributions to tension in lengthening rat muscle: force-induced reversal of the power stroke

Lengthening of active muscle is an essential feature of animal locomotion, but the molecular processes occurring are incompletely understood. We therefore examined and modelled tension responses to ramp stretches (5% fibre length, L0) over a wide range of velocities (0.1-10 L(0) s(-1)) of tetanized intact rat muscle fibre bundles (L0 approximately 2 mm) with a resting sarcomere length of 2.5 microm at 20 degrees C. Tension rose to a peak during stretch and decayed afterwards to a level which was higher than the prestretch tetanic tension. This residual force enhancement was insensitive to velocity. The tension rise during stretch showed an early transition (often appearing as an inflection) at approximately 1 ms. Both the stretch (L1) and the tension rise at this transition increased in proportion to velocity. A second transition, marked by a reduction in slope, occurred at a stretch of approximately 18 nm per half-sarcomere; the rise in tension at this transition increased with velocity towards a plateau. Based on analyses of the velocity dependence of the tension and modelling, we propose that the initial steep increase in tension arises from increasing strain of all attached crossbridges and that the first transition reflects the tension loss due to the original post-stroke heads executing a reverse power stroke. Modelling indicates that the reduction in slope at the second transition occurs when the last of the heads that were attached at the start of the ramp become detached. Thereafter, the crossbridge cycle is largely truncated, with prepower stroke crossbridges rapidly detaching at high strain and attaching at low strain, the tension being borne mainly by the prestroke heads. Analysis of the tension decay after the ramp and the velocity dependence of the peak tension suggest that a non-crossbridge component increasingly develops tension throughout the stretch; this decays only slowly, reaching at 500 ms after the ramp approximately 20% of its peak value. This is supported by the finding that, in the presence of 10 microm N-benzyl-p-toluene sulphonamide (a myosin inhibitor), while isometric tension is reduced to approximately 15%, and the crossbridge contribution to stretch-induced tension rise is reduced to 30-40%, the peak non-crossbridge contribution and the residual force enhancement remain high. We propose that the residual force enhancement is due to changes upon activation in parallel elastic elements, specifically that titin stiffens and C-protein-actin interactions may be recruited.