As more and more fitness professionals incorporate dynamic movement into their training protocols, it is important that they understand how the storage of elastic energy in the body can significantly aid such movement. This article seeks to highlight and explain the mechanisms that control how the body stores and recoils elastic energy during movement. Such mechanisms can dramatically reduce metabolic cost and are fundamental to the functional efficiency and performance of all the clients and athletes we train.
In Part 1, we will explore the component parts of the body’s muscular and connective tissues that contribute to the storage of elastic energy. Next, we will consider muscular and tendon fiber types and lengths, pennation angle, orientation and collagen content.
R. McNeill Alexander, the great biomechanist and leading researcher in the role of stored elastic energy, likened the human gait to a pogo stick bouncing up and down. The spring compresses and then returns the energy harnessed. If the spring conforms to Hooke’s Law — that stress and strain are proportionate and that no energy is lost to hysteresis (dissipation of energy, generally as heat) — then, theoretically, the pogo stick could continue to bounce forever. The problem with the body is that the muscles and connective tissues have different levels of viscoelasticity. This means that their level of hysteresis and conformity to Hooke’s Law do, in fact, change. As a result, the level of returned energy is reduced and there is a metabolic energy cost to the body.
The need to conserve energy becomes even more vital when we factor in athletic needs. The body needs to keep the metabolic cost as low as possible; the principle of conservation of energy requires it, and has therefore devised muscular and connective tissue arrangements that fulfill these roles. We will explore these arrangements below.
Model of the Muscle Complex
Muscle is comprised of a contractile element and a non-contractile element. This, however, is a simplistic view. We cannot truly understand muscle mechanics without including connective tissue such as tendons.
Levin and Wyman (1927) break muscle mechanics down into series elastic components (SEC), parallel elastic components (PEC), and the contractile components (CC). Simply put, the contractile components are the muscles themselves, and the SEC work in a line with the muscles. Imagine two vertical straight lines one on top of the other; together they help provide force in the same direction. They include tendons (SEC), cross bridges (CC), myofilaments (CC), titin (CC)and Z-discs (CC). The PEC is comprised of sheaths around muscle and resting cross bridges. Cavagna (1977) believes that the PEC contains little stored elastic energy and contributes little to the energy balance of exercise. Of all the SEC elements, Suzuki and Sugi (1983) feel the myofilaments provide the greatest contribution.
The traditional view of the tendon as simply connecting muscle to bone may be understating its contribution to the SEC. The tendon may have a much larger role to play in elastic energy storage and subsequent metabolic energy conservation in the body.
Below we will explore the muscle complex more deeply by looking at titin and tendons. We will also explore collagen and elastin, the elastic building blocks of tendons and muscles.
Model of muscular contractile components
Titin (see diagram below) is a giant protein molecule that spans half the sarcomere. It forms an integral part of the thick myosin filament and creates a flexible connection between the thick filament and the Z-disks. It is now generally accepted that titin has a large part to play in the elasticity of striated muscle, since elasticity is one of striated muscle's most important properties (Gajdosik, 2001). While the myofilaments (actin and myosin) are already well-known for their lengthening ability, the titin section, located near the I-band, is the most functionally extensible.
Diagram of titin's location in the muscle complex (Image Source)
Different muscles exhibit variations in elasticity. This could be in part because of differences in the extensible properties of endosarcomeric proteins (Granzier and Irving, 1995). The compliance or stiffness of the individual titin isoforms impact protein extensibility.
Titin's role in muscle passive tension (stretch) is now well-established. It is at the heart of the molecular mechanisms that control long-range elasticity in skeletal muscle. Tskhovrebova and Trinick feel, “The rich variety of isoforms also provide a simple basis for the variation of sarcomere length working range observed in different types of muscle ﬁbres” (2002).
Differing titin elastic properties would impact on the performance of stiffer and also more compliant muscle types in elastic energy storage and recoil. This author would like to see more research into the individual titin properties of specific muscles to better understand its impact on the viscoelastic properties of the muscle as a whole and the resulting contribution to elastic energy storage.
Tendons store and return energy. As R. McNeill Alexander states, “Metabolic energy can be saved in locomotion if tendons stretch and then recoil, storing and returning elastic strain energy” (2002).
The body always wants to conserve its energy stores; it has found a way of doing this by using the elastic ability of the tendons. Tendons require less stored energy than if the work was done by our muscles. Tendons can return up to 93% of the elastic energy applied to them in their recoil (Benett et al., 1986). Cavagna (1977) finds that much of the muscle activity in running is associated with tendons.
The less the muscle attached to the tendon changes length (dynamically) the less thermodynamically (energy transferred in heat) expensive it is. Alexander states, “The metabolic rate is higher when the muscle is shortening, doing work” (2000). This is because cross bridge detachment and re-attachment splits ATP and therefore dissipates energy as heat in each cycle. Alexander feels that the metabolic cost is “is lower when the muscle is being forcibly stretched” (2000).
Some feel that the muscle remains relatively isometric while the tendon does the work. Later in the article, when we factor muscle stiffness into this model, this may not be precisely what is happening. This is because of the implications of long-term isometric contraction.
Tendons display non-linear elastic properties. They are collagenous, meaning they are viscoelastic, and although they do not simply follow Hooke’s Law, this may have a greater correlation with tendon performance than other collagenous structures that are less compliant or, in other words, less willing to lengthen. This is highlighted by the fact that the greater the velocity and magnitude of stretch to the tendon the more the elastic storage (Rack and Westbury, 1974). Compliance is vital to the storage and payback of elastic energy.
The type 1 collagen fibers in tendons have an absence of hydroxyproline and proline residues at specific locations in the amino acid sequence. This allows the formation of bends and loops in the triple helix, which gives the tendon more flexibility. The multi-stranded nature of the fibrils and fascicles also contribute to tendon flexibility. The linear and parallel nature of tendon fibers also means they are perfect for simple lengthening and shortening and. therefore, linear force transmission.
Tendon fibers contain small concentrations of elastin that will contribute to their compliance and also restore collagen fibers after stretching (Minns et al., 1973). This elastic recoil will help return the energy applied to the tendon.
Ker et al. (1987) find that the foot in gait behaves like a spring. It flattens and then gives 78% of energy return in elastic recoil. This is mainly comprised of the Achilles tendon and spring-like action of the arch of the foot.
How do the muscles figure into the elastic energy model along with the tendon? It may be that there is a different orientation of tendon length, muscle fiber type, length and pennation angle according to the need for conservation of metabolic energy through the tendon or more dynamic but energy consumptive storage and explosive recoil of elastic energy from the contractile components of the muscle.
Let’s first look at the thermodynamically efficient model. If it were more efficient to allow the tendon to deform rather than the muscle, how would the muscle structure and arrangement go about helping allow this? Imagine pulling on two structures joined together, one cotton and one elastic, the elastic one will stretch but the cotton one won't. The cotton is the muscle, the elastic the tendon, they are joined but have different elastic properties due to structural makeup.
Collagen again plays a key role in allowing tendon elasticity, this time in a muscular context. Slow twitch (ST) muscle fibers have twice the concentration of collagen (Kovanen, 1984). Collagen's viscoelastic property of stiffness means that ST fibers will resist deformation especially under higher loads. This is in line with the model of thermodynamic efficiency that requires a mainly isometric contraction of the muscle. Instead of remaining isometric, however, the muscles are able to eccentrically lengthen and concentrically shorten, but with much reduced ranges. This is due to the stiffness of the collagen content, thus allowing the tendon to compliantly deform, store and recoil elastic energy.
This may seem like a minor detail until you factor in blood supply to the tissue and proprioceptive information flow from the muscular contractile components and the tendon. With sustained isometric contraction, the build-up of intramuscular tissue pressure will lead to ischemia (restriction in blood supply). The same restricted blood supply will also affect the ability to remove the metabolic build up of lactate and provide other energy sources to where they are needed.
ST muscle fibers also have a longer cross bridge attachment time (Bosco, 1982). This strongly cross-linked collagen would lead to less muscle lengthening, allowing for the deformation and elastic energy storage to come from the more compliant tendon.
Some element of muscle lengthening may also allow for more directional control. The linear nature of tendons is far more suited to force production between two points rather than the multidimensional control that muscle with multidirectional fibers are capable of.
Fiber length will also play a part in reducing muscular deformation. The shorter the fibers, the less distance the contractile protein elements can travel. Muscle velocity is related to fiber length for the same reason. The less far it can travel, the less velocity it can generate. This all equals less deformation and elastic energy storage from the muscle fibers and more contribution from the tendon to elastic energy over the same joint excursion.
Pennate fibers are arranged obliquely to a tendon that runs along the longitudinal axis of the muscle. The pennation angle means that less force can be applied along the longitudinal axis of the muscle. This is not an ideal mechanism compared to the longitudinal or linear muscle fiber arrangement; so why, then, has the body created this arrangement?
Simply put, if less movement comes from the stiffer muscle fibers in series with and in the same direction as the tendon, then the more compliant tendon will have more force applied to it and go through greater deformation storing and recoiling elastic energy.
We could describe this as a counterforce. Imagine two people are holding an elastic band. If they moved together in the same direction, the band would not lengthen. If one person stayed still while the other moved away, then the band would lengthen. In the body, the muscle is playing the role of the person staying still, while the tendon lengthens just like the elastic band.
The classic pennation angle concept is that a larger number of muscle fibers can be packed into the same cross sectional area than with a longitudinal fiber arrangement. This provides more force in general to overcome the disadvantage of less force being generated along the longitudinal axis. This is a valid mathematical concept but one that does not take into account the tendinous model of elastic energy storage and recoil.
The tendon in a bipennate muscle arrangement penetrates deep into the muscle with two sets of fibers arranged obliquely to the central tendon. This enables the tendon to be longer and as we have seen with muscle fibers the longer the tendon the more it can lengthen and shorten and store and recoil elastic energy. Now couple this with less muscle deformation along the longitudinal axis because the fibers are now running in two distinct directions, neither of which is along the longitudinal axis in series with the tendon.
This is truly a powerful arrangement to favor the storage of elastic energy from the tendon. Different levels of pennation angle coupled with being either pennate, bipennate or multipennate may give an indication of how much elastic energy is designed to come from the tendon.
Although the predisposition of the arrangement seems to be to resist muscular lengthening, there will also be a neurological control of muscular stiffness through the muscle spindles. They will receive information from the joint proprioceptors and be able to vary the stiffness accordingly for the demand of the task. The Golgi tendon organs that control tendon length will interact with the muscle spindles to allow the right proportion of lengthening from both structures.
If we return to the example of the two people holding an elastic band example again, this would mean that the person staying still could vary the amount they moved (controlled by the central nervous system) and thereby vary the lengthening of the tendon and muscle.
In Part 2, we will put these theories into context. By looking at two different muscles we can understand their individual properties that contribute to differing elastic energy storage and recoil. Part 2 will also provide practical application for personal training programming.
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