011: What is Viscoelasticity?

biomechanics passive stretching Aug 19, 2021

"Viscoelasticity" is a word that refers to the fact that muscle is a viscoelastic material that exhibits both viscous and elastic characteristics when it is subjected to tensile and compressive forces (Zatsiorsky & Prilutsky, 2012). The word "viscous" describes a fluid that resists deformation (change of its shape), and the term "elastic" refers to a material that returns to its original shape after the deforming force is removed.

Viscoelasticity is the stress (force) and subsequent strain (deformation/change of shape) in the muscle dependent on the rate of loading. Therefore, the timing of the applied force will affect how the muscle responds. It is important to understand that viscoelasticity is a characteristic of passive muscle; the viscoelasticity displaying during tensile loading (i.e., the application of a stretching force) is caused by the passive properties of the protein titin (Wang et al., 1993; Toursel et al., 2002).

How fast the muscle lengthens affects the passive properties of the muscle fibres significantly (Rehorn, Schroer, & Blemker, 2014). As the speed of lengthening increases, the peak temporary stress within the muscle fibres also increases, even at very slow velocities. In other words, faster stretching results in a higher stiffness than a slower stretch in passive muscles. Hence, slow stretching exercises can minimise the increase in force in the muscle-tendon unit for a given amount of stretch.

Other important properties of viscoelasticity include creep, stress relaxation, and hysteresis. Hysteresis can be observed when a stretched muscle returns to its pre-stretched length; the length-tension characteristics will be different during loading (when the external force is applied) and unloading (when the external force is removed) (Ramos et al., 2017). The hysteresis value is the area on a stress-strain curve between the loading and unloading phases, and it represents the amount of mechanical energy lost (converted to thermal energy and dissipated as heat) in the recovery from the stretch. The more mechanical energy lost as heat, the more pronounced the hysteresis value on the stress-strain curve will be. A higher hysteresis value also represents a higher metabolic cost of returning tissues to their pre-stretched state because the tissues use less stored mechanical energy. Thus, hysteresis is also an indicator of the energy efficiency of tissues. Generally speaking, the relationship between hysteresis and the speed of muscle stretch (or how often the muscles are stretched) can be presented in the following formula:

E = kωn

E is the dissipated mechanical energy (the area between the loading and unloading curves on the stress-strain curve, also called the hysteresis area), ω is the oscillation frequency, and k and n are empirical constants. If n=1, the dissipated energy is proportional to the stretch velocity, and when n=0, the dissipated energy is independent of velocity. In the latter case, the resistance acts like friction, and it will be the same at any given velocity. In human joints, the exponent n is often close to zero. Therefore, muscle hysteresis is generally independent of oscillation frequency compared to classic viscoelastic materials like rubber. In simple terms, this means that the amount of mechanical energy "lost" as (converted to) heat is relatively independent of the speed of muscle length change.

Recent studies have examined the effects of ageing and sex on muscular viscoelastic characteristics in athletes (Westing, Seger, & Thorstensson, 1990). Ageing is commonly associated with increased muscle tone and muscle stiffness, which can potentially cause an increased risk of falling, injuries, and general decreases in muscle performance, which consequently limits the ability of a person to perform activities of daily living (ADLs).

When a muscle is constantly stretched for a sufficiently long period, it will exhibit creep, which is a passive lengthening reaction (Ryan et al., 2010). Fast stretching occurs initially and is followed by a slower increase in deformation. Stress relaxation is the decrease in resistance (the tension felt when the muscle is stretched) over time when a constant stretch is held (Knudson, 2007). For example, holding a static passive stretch at a specific joint angle results in a gradual decrease in tension in the muscle from stress relaxation - the stretch feels more comfortable as it is held for a certain length of time. Creep and stress relaxation have important implications for stretching and risk of injury in repetitive tasks.

Muscles also have thixotropic properties, which are distinct from their elastic and viscous properties. Thixotropy is a type of "sticky friction," and it is dependent upon the history of muscular contractions (Proske, Morgan, & Gregory, 1993). Thixotropy can be observed when fluid materials appear semi-solid at rest but temporarily become more fluid when shaken or stirred to decrease their viscosity. Thixotropy occurs because of long-term stable cross-bridges between the myofilaments of both extrafusal and intrafusal muscle fibres. Synovial fluid found in human joints is thixotropic, so performing movements upon waking up in the morning makes joints more flexible. Under some conditions, proprioception and muscle force are affected by thixotropy (Proske, Tsay, & Allen, 2014).

A newly developed image-based technique called elastography is capable of measuring passive mechanical properties of skeletal muscles. When researchers use MRI or ultrasound, the determination of stiffness and elastic properties of the muscles are based upon two principles: 1) for a given stress (force), stiffer tissue strains (deforms) less, and 2) mechanical waves (specifically shear waves) propagate (travel) faster through stiffer tissues. In vivo methods for measuring muscle mechanical properties are developing all the time. However, it's important to remember there are still some uncertainties and limitations to such methods (see post 004 for more information). Nevertheless, understanding the typical mechanical properties of muscle (including viscoelasticity) is crucial to our ability to differentiate abnormal patterns of function and determine the causes of problems encountered following injury to provide appropriate stimuli to return the tissues to pre-injury levels. In addition, we can also potentially optimise those properties for improved athletic performance.

References

Zatsiorsky, V. M. & Prilutsky, B. (2012) Biomechanics of Skeletal Muscles. Champaign, IL: Human Kinetics.

Wang, K. et al. (1993) Viscoelasticity of the Sarcomere Matrix of Skeletal Muscles. The Titin-Myosin Composite Filament is a Dual-Stage Molecular Spring. Biophysics Journal vol. 64, no. 4. pp. 1161-1177.

Toursel, T. et al. (2002) Passive Tension of Rat Skeletal Soleus Muscle Fibres: Effects of Unloading Conditions. Journal of Applied Physiology vol. 92, no. 4, pp. 1465-1472.

Rehorn, M. R., Shroer, A. K., & Blemker, S. S. (2014) The Passive Properties of Muscle Fibres Are Velocity Dependent. Journal of Biomechanics vol. 47, no. 3, pp. 687-693.

Ramos, J. et al. (2017) Hysteresis in Muscle. International Journal of Bifurcation and Chaos vol. 27, no. 1: 1730003.

Westing, S. H., Seger, J. Y., & Thorstensson, A. (1990) Effects of Electrical Stimulation on Eccentric and Concentric Torque-Velocity Relationships During Knee Extension in Man. Acta Physiologica Scandinavica vol. 140, no. 1, pp. 17-22.

Ryan, E. D. et al. (2010) Viscoelastic Creep in the Human Skeletal Muscle-Tendon Unit. European Journal of Applied Physiology vol. 108, no. 1, pp. 207-211.

Knudson, D. (2007) Fundamentals of Biomechanics. Journal of Sports Science & Medicine vol. 6, no. 3, p. 384.

Proske, U., Morgan, D. L. & Gregory, J. E. (1993) Thixotropy in Skeletal Muscle and in Muscle Spindles: A Review. Progress in Neurobiology vol. 41, no. 6, pp. 705-721.

Proske, U., Tsay, A., & Allen, T. (2014) Muscle Thixotropy as a Tool in in the Study of Proprioception. Experimental Brain Research vol. 232, no. 11, pp. 3397-3412.