Do cross-bridges contribute to the tension during stretch of passive muscle? A response

Acute Effects of Local High-Frequency Percussive Massage on Deep Fascial and Muscular Stiffness and Joint Range of Motion in Young Adult Men

BACKGROUND

Local high-frequency percussive (HFP) massage has recently found widespread application in physical therapy. Although HFP massage reportedly improves range of motion (ROM), the mechanism underlying its action has not yet been proven. This study aimed to clarify whether a 5-minute percussive massage regimen affects muscular or connective tissues, such as the deep fascia and deep intermuscular fascia and the change in joint ROM.

METHOD

The study sample was calculated using G*Power analysis program, and this study enrolled 15 healthy men who underwent 5-minute HFP massage to the medial gastrocnemius muscle. Shear-wave elastography was used to measure tissue stiffness in the deep fascia, muscle, and deep intermuscular fascia through shear-wave velocity as well as the ROM of the volunteers' ankle joint dorsiflexion before and after the HFP massage. A value of P < .05 was used to declare statistical significance, and post hoc was used to calculate the effect size using G*Power.

RESULTS

Shear-wave velocity revealed a significant change in the deep fascia (P = .003; shear-wave velocity: -0.7 m/s) and significant increase in ROM of ankle dorsiflexion (P = .002; increase in ROM: 3.0°) after 5 minutes of HFP massage. However, the muscle and deep intermuscular fascia did not exhibit any significant changes.

CONCLUSIONS

HFP massage for 5 minutes modified the stiffness of the deep fascia and concurrently improved the ankle joint-dorsiflexion ROM. This method can be used as an intervention to decrease stiffness of the deep fascia and increase the ROM efficiently.

Thixotropy and rheopexy of muscle fibers probed using sinusoidal oscillations

Length changes of muscle fibers have previously been shown to result in a temporary reduction in fiber stiffness that is referred to as thixotropy. Understanding the mechanism of this thixotropy is important to our understanding of muscle function since there are many instances in which muscle is subjected to repeated patterns of lengthening and shortening. By applying sinusoidal length changes to one end of single permeabilized muscle fibers and measuring the force response at the opposite end, we studied the history-dependent stiffness of both relaxed and activated muscle fibers. For length change oscillations greater than 1 Hz, we observed thixotropic behavior of activated fibers. Treatment of these fibers with EDTA and blebbistatin, which inhibits myosin-actin interactions, quashed this effect, suggesting that the mechanism of muscle fiber thixotropy is cross-bridge dependent. We modeled a half-sarcomere experiencing sinusoidal length changes, and our simulations suggest that thixotropy could arise from force-dependent cross-bridge kinetics. Surprisingly, we also observed that, for length change oscillations less than 1 Hz, the muscle fiber exhibited rheopexy. In other words, the stiffness of the fiber increased in response to the length changes. Blebbistatin and EDTA did not disrupt the rheopectic behavior, suggesting that a non-cross-bridge mechanism contributes to this phenomenon.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Short-range mechanical properties of skeletal and cardiac muscles

Striated muscles are disproportionately stiff for small movements. This facet of their behavior can be demonstrated by measuring the force produced when the muscle is stretched more than about 1% of its initial length. When this is done, it can be seen that force rises rapidly during the initial phases of the movement and much less rapidly during the latter stages of the stretch. Experiments performed using chemically permeabilized skeletal and cardiac muscles show that the initial stiffness of the preparations increases in proportion with isometric force as the free Ca²(+) concentration in the bathing solution is raised from a minimal to a saturating value. This is strong evidence that the short-range mechanical properties of activated muscle result from stretching myosin cross-bridges that are attached between the thick and thin filaments. Relaxed intact muscles also exhibit short-range mechanical properties but the molecular mechanisms underlying this behavior are less clear. This chapter summarizes some of the interesting features of short-range mechanical properties in different types of muscle preparation, describes some of the likely underlying mechanisms and discusses the potential physiological significance of the behavior.

Muscle performance in a soft-bodied terrestrial crawler: constitutive modelling of strain-rate dependency

Experimental data on the passive mechanical properties of the ventral interior lateral muscle of the tobacco hornworm caterpillar, Manduca sexta, are reported. The stress-deformation response of the Manduca muscle is shown to be nonlinear pseudo-elastic, capable of large deformations and subject to stress softening during initial loading cycles. The muscle passive mechanical properties also depend on multiple time-dependent processes. In particular, we show new experimental data from cyclic loading tests of an unstimulated muscle with constant maximum stretch and different, constant engineering strain rates. Then, on the basis of these data a constitutive model is derived to reproduce the main characteristics of this behaviour. In formulating the constitutive model, we consider the muscle as a complex macromolecular structure with fibrous components at numerous size scales. The model uses a phenomenological approach to account for different mechanisms by which passive force changes during applied deformation and how the muscle properties recover after unloading.

Human motor compensations for thixotropy-dependent changes in resting wrist joint position after large joint movements

AIM

Resting tension of relaxed skeletal muscle fibres held at a given length varies with the immediate previous history of length changes and contractions. The primary aim of this study was to explore the motor control consequences of this history-dependency in healthy subjects.

METHODS

Angular position and passive torque were recorded from the intact wrist joint. Integrated surface electromyography (IEMG) was recorded from wrist extensor and flexor muscles.

RESULTS

In relaxed subjects, wrist joint position was displaced towards dorsiflexion after a single high-amplitude dorsiflexion movement combined with a strong flexor/extensor co-contraction (dorsiflexion conditioning), whereas after volarflexion conditioning there was a shift towards volarflexion. These after-effects could be abruptly cancelled by short periods ( approximately 5 s) of rapid flapping hand movements or forceful isometric co-contractions, findings indicative of muscle thixotropy. The IEMG-evaluated motor after-effects were as follows. A slowly subsiding wrist flexor contraction was needed to restore and maintain the original resting wrist position after dorsiflexion conditioning whereas a slowly subsiding extensor contraction was needed for the same goal after volarflexion conditioning. Furthermore, ongoing wrist extensor IEMG activity required to actively hold the wrist in a moderate dorsiflexed position or to resist a constant volar torque at resting position was temporarily reduced after dorsiflexion conditioning and enhanced (not significantly) after volarflexion conditioning.

CONCLUSION

The results provide evidence that during voluntary maintenance of a desired wrist joint position the motor commands to the position-holding muscles are unconsciously adjusted to compensate for thixotropy-dependent variations in the resting tension of the muscles.

Sarcomeric visco-elasticity of chemically skinned skeletal muscle fibres of the rabbit at rest

The giant muscle protein titin (connectin), contained in the gap filament that connect a thick filament to the Z-line in a sarcomere, is generally considered to be responsible for the passive force (tension) and visco-elasticity in resting striated muscle. However, whether it can account for all the features of the resting tension response remains unclear. In this paper, we examine the basic features of the 'sarcomeric visco-elasticity' in a single resting mammalian muscle fibre and attempt to account for various tension components on the basis of known structural features of a sarcomere. At sarcomere length of approximately 2.6 microm, the force response to a ramp stretch of 2-5% is complex but can be resolved into four functionally different components. The behaviour displayed by the components ranges from pure viscous type (directly proportional to stretch velocity, ranging from 0.1 to 30 lengths s(-1)) to predominantly elastic type (insensitive to stretch velocity at 1-2 s time scale); simulations show two components of visco-elasticity with characteristically different relaxation times. The velocity-sensitive components (only) are enhanced by filament lattice compression (dextran - 500 kD) and by increased medium viscosity (dextran - 12 kD); also, the relaxation time of visco-elasticity is longer with increased medium viscosity. Amplitude of all the components and the relaxation time of visco-elasticity are increased at longer sarcomere length (range approximately 2.5 - 3.0 microm). The study, and quantitative analyses, extend our previous work on intact muscle fibres and suggest that the velocity-sensitive tension components in intact sarcomere arise from interactions between sarcomeric filaments, filament segments and inter-filamentary medium; the two components of visco-elasticity arise from distinct regions of titin (connectin) molecules.

History-dependent mechanical properties of permeabilized rat soleus muscle fibers

Permeabilized rat soleus muscle fibers were subjected to repeated triangular length changes (paired ramp stretches/releases, 0.03 l(0), +/- 0.1 l(0) s(-1) imposed under sarcomere length control) to investigate whether the rate of stiffness recovery after movement increased with the level of Ca(2+) activation. Actively contracting fibers exhibited a characteristic tension response to stretch: tension rose sharply during the initial phase of the movement before dropping slightly to a plateau, which was maintained during the remainder of the stretch. When the fibers were stretched twice, the initial phase of the response was reduced by an amount that depended on both the level of Ca(2+) activation and the elapsed time since the first movement. Detailed analysis revealed three new and important findings. 1) The rates of stiffness and tension recovery and 2) the relative height of the tension plateau each increased with the level of Ca(2+) activation. 3) The tension plateau developed more quickly during the second stretch at high free Ca(2+) concentrations than at low. These findings are consistent with a cross-bridge mechanism but suggest that the rate of the force-generating power-stroke increases with the intracellular Ca(2+) concentration and cross-bridge strain.

Passive mechanical properties of the medial gastrocnemius muscle of the cat

1. This is a report on the history dependence of the passive mechanical properties of the medial gastrocnemius muscle of the anaesthetised cat. 2. The muscle was conditioned with an isometric contraction at the test length, or at 3 mm longer than the test length and then returned to the test length, where the level of resting tension was measured, as well as tension changes during a slow stretch. 3. The level of resting tension depended on the form of conditioning and, at the optimum length for active tension, the history-dependent component was 9 % of the total passive tension. 4. During a slow stretch, tension initially rose steeply up to a yield point, beyond which it rose more gradually. The shape of the tension rise depended on the form of conditioning. The level of tension at the yield point consisted of a stretch-dependent component, the 'short-range tension' plus the resting tension for that length. 5. The short-range tension increased with muscle length to peak close to the optimum for active tension. The slope of the tension rise during a stretch, the short-range stiffness, peaked at 2 mm beyond the optimum. 6. The short-range tension was small immediately after a conditioning contraction but grew in size as the interval was increased up to 60 s, with a time constant of 9.9 +/- 0.6 s. After a series of conditioning movements, it recovered more rapidly, with a time constant of 6.6 +/- 0.5 s. 7. The history-dependent changes in passive tension and the response to stretch are interpreted in terms of the presence, in sarcomeres of resting muscle fibres, of crossbridges between actin and myosin which have very slow formation rates, both at rest and during movements.