In Part 1 we looked at the properties of muscles and tendons in regards to elastic energy storage and recoil. In Part 2 we will look at elastic energy storage and recoil from a more muscular context and apply this to two muscles. Practical application points for program design are also explored to help you integrate this important physical process into your personal training sessions.
The first part of this article described how the tendon is the primary storage area for elastic energy through its simple collagen structure, which is ideal for the task. The muscle complex helps this through its ST muscle fiber type and short length, high pennation angle and increased tendon length. Let’s now put this into context and see how this theory holds water when applied to a muscle/tendon arrangement.
The soleus is connected to the Achilles tendon, which is the thickest and strongest tendon in the body. It is ideal to store and recoil energy, especially in an area that has a huge work rate when we factor in the universal human function of gait. Much of the research into elastic energy has focused on this area.
The soleus should then, in theory, display some or all of the characteristics highlighted above.
Let’s first look at the fiber type of the soleus. Philip et al. (1974) found that the mean of the soleus fiber type for all subjects in their study was 80% ST fibers. This collagenous tissue will display stiffness to resist deformation through stronger cross bridge attachment. ST fibers also have a shorter length that also allows less deformation. The average fiber length of the soleus being 3cm versus the 16cm of the semitendinosus (Winter, 1990).
The soleus is a bipennate muscle with a large pennation angle of 30 degrees (Winter, 1990). This appears to be the largest pennation angle in the lower limbs; the gastronemeus displays a pennation angle of 15 degrees and also attaches to the Achilles tendon. The gastrocnemius also has a more even split of slow twitch to fast twitch (FT) fibers and a longer fiber length. This may mean that it has a different role to play in terms of elastic energy storage.
The soleus fits neatly into the model outlined previously, in which elastic energy storage and recoil come primarily from the tendon. Now let’s turn our attention to a muscular model of elastic energy storage and recoil.
Muscular Elastic Energy Storage and Recoil
Not all muscles have a pennate nature, an ST fiber type, and a tendon arrangement that favors elastic energy storage from the tendon. ST fibers and tendon arrangements that store elastic energy may be more adapted to storing and recoiling their energy in slow contractions, since the stiff nature of collagen would resist fast, compliant contractions (Bosco, 1982). This fits with the efficiency of prolonged work levels undertaken by the areas with such arrangements.
As a result, fast, compliant and powerful elastic energy must be stored in other ways and may come from within the muscle itself. How would the muscle go about doing this? A longitudinal arrangement that displays the fibers in series will allow a greater degree and velocity of movement. When this is combined with a longer fiber length (to generate more lengthening and shortening force) and a predominance of less-collagenous type ll fibers, the muscle fibers themselves are more compliant. The actin and myosin in the myofibril are known to be compliant structures that can stretch and store elastic energy. This may come from the rotation of the meromysin heads of the cross bridges and elongation of the tail; a helical structure that would promote extensibility (Huxley, 1974).
This may also be related to variations in length of titin isoforms that are expressed in different muscle types (Labbeit and Kolmerer, 1995). It would make sense that longer titin isoforms were present in FT muscles to aid extensibility, but more research is required to contribute to this hypothesis.
This also brings into play the amortization phase between eccentric and concentric loading.
Amortization is the period in between the lengthening and shortening of the muscle and could also be described as depreciation. This is because the energy available reduces as the amortization period becomes longer.
Stored elastic energy will be dissipated through reduced cross bridge attachment after a long delay (Edman et al., 1978). A fast-loading motion must be quickly followed by an unload to fully utilize elastic energy stored in a more muscular context.
The semitendinosus of the hamstring group is a good example of a muscle that stores and recoils elastic energy. The hamstrings contain a relatively higher proportion of more compliant and faster-contracting type II fibers (Garrett et al., 1984). The fiber length on average is a healthy 16cm in the semitendinosus (Winter, 1990) and displays a longitudinal fiber arrangement without a long central tendon.
Here we have discussed muscles at two distinct ends of the elastic energy spectrum. The caveat is that there are many shades of gray in the human body and it does not easily fit into overly simplistic models. As always, the principle of individuality must be applied. The Biceps Femoris (BF) may be a good example of this.
The BF short head displays a fiber length of 7cm, while the long head has a fiber length of between 9-14cm. Fiber type of the BF have been reported as both mainly ST (66.1%) and FT (50.5-60.4%). Winter (1990) reported no pennation angle in the long head of the BF while others feel it is a pennate muscle.
Yamaguchi et al. (1990) measured a pennation angle of between 7-17 degrees in the BF although did not differentiate between the long and short heads.
We may have in the BF a case for the two heads having differing roles when it comes to both elastic energy and direction control. It may also highlight that some muscles may exhibit a fiber type and arrangement that could fulfill to some degree roles in between the two ends of the elastic energy spectrum and do not fit easily into a restricted model. Add to this a number of varying muscular arrangements acting around a joint and we have a complex interplay of muscular roles and forces occurring with regards to elastic energy.
We must also take into account the individuality of elastic energy storage between people in regards to the same muscle. Nature and function will both have a role to play in creating the individual characteristics of muscles.
A question you may ask at this point is, “How do I apply this to my training programs?” A good place to start would be to first understand the predominant movements of the sport or desired function.
In this section we will look at two examples of sports classification — one leg-driven example and one arm-driven example — that will encompass the vast majority of sports.
First we will look at a superb leg-driven example: a hop. Nearly all leg-driven and most arm-driven sports involve running or jumping in some format. This usually involves motion on a single leg at a time, as in the hop. By adjusting the training variables we can better suit the specifics of our function.
Long distance runners may require a more linear directionality to their movement. Their ground contact time may be longer and, therefore, amortization time will also be longer. Their range of movement may also be larger per stride, with a slower cadence and more vertical displacement of center of mass. This allows the long distance runner to take advantage of tendinous elastic energy storage and, therefore, metabolic energy conservation. This would mean we would have mainly sagittal plane hops that have larger, slower movements to train the predominant muscle/tendon arrangements.
A sprinter would have sharper, stiffer hops at a much higher cadence. The ground contact time would be very short, as would amortization time. This means they emphasize more explosive power from different muscular arrangements and areas.
For multidimensional sports, a multidimensional hop matrix maybe applicable. Arm drivers can also be added for upper body authenticity to functional activity.
Below is a framework for training examples of two ends of the running spectrum. This can be adapted for the many running distances in between.
|Hops for distance runners
||Hops for sprinters
|Larger range (1 meter)
||Shorter range (0.5 meters)
|Longer ground contact time (1 sec)
||Shorter ground contact time (0.5 sec)
Throwing is a good example of arm-driven function. A similar movement pattern should be employed to the functional activity.
In throwing, the retraction of the arm generally creates elastic energy in the muscles of the anterior portion of the body, and also causes rotation of the trunk, taking stress away from the shoulder complex and generating more energy in the trunk rotators. Less muscular/tendon arrangements exist in the upper body for metabolic energy conservation as exist in the lower body. The deltoids do have a multipennate nature, but with a much smaller tendon length. This may have to do with the universal function of gait and elastic energy stored from repetitive ground reaction.
Throwing usually involves high-force movements. This means that speed of movement should be fast and amortization time low. Range of motion (ROM) for safety may want to be graduated from smaller to larger to take into account individual flexibility levels. Adding external load would create more momentum and therefore additional force to be paid back, although too heavy and it may compromise amortization time and movement pattern authenticity.
The ability of the human body to adjust its stiffness according to different surfaces is a truly astounding feat. The variability of the contractile components with regards to stiffness means that we can alter the bias between the contractile components and tendons in relation to surface stiffness and elastic energy storage and recoil. The millisecond adjustment that allows proprioceptive information from the joints to the muscle spindles and Golgi tendon organs to achieve this is quite amazing! This means that we can also maintain consistent ground reaction forces; ground contact time and vertical displacement of center of mass. This leads to consistency in the dynamics of running.
What implications does this have then for the stiffness of shoes selected for running? This means the wrong shoe selection could impact on the metabolic energy expenditure and therefore affect the overall function of the body.
Individual joint excursions and muscle flexibility levels will also affect the ability of a person to store and recoil elastic energy. A good assessment protocol will enable the personal trainer to maximize the client’s flexibility and elastic energy storage and recoil. Over long distances the endurance ability of the individual will also affect elastic energy storage. Flexibility along with endurance ability may be a limiting factor in prolonged homeostasis in elastic energy storage and recoil.
The storage and recoil of elastic energy is involved in all the body’s functional movements and is especially important in the sporting arena. Enhancing our body’s ability to do so will decrease the need for metabolic energy thus improving our performance and endurance. Popular exercise such as running relies heavily on the processes outlined above.
This article has sought to highlight the mechanisms behind this process and the structures involved. By increasing our understanding of elastic energy and integrating it into our training programs, we can help our clients improve their performance.
We see that the body has at least two distinct ways of storing and paying back elastic energy, although with all the different muscular and tendinous arrangements, there could be many more. The body may do this in different ways according the power or efficiency demands of the task undertaken. The soleus is an excellent example of a muscular/tendinous arrangement that favors efficiency. The hamstrings could store energy more within the muscle complex itself, suited to fast and compliant energy storage and recoil that is powerful but dissipates quickly and is more metabolically consumptive. Larger, slower motions with more transient periods between stretching and shortening have been shown to favor ST fiber loading while rapid, shorter range loading and less amortization time has been shown to favor FT fiber loading (Bosco, 1982).
So what implication does this have for personal trainers? It means that function-specific exercise variables will impact on the types of muscular elastic energy arrangements utilized. If we do not consider the elastic needs of the specific client/athlete functions when programming training variables, then essentially the performance of elastic energy production in the body will go untrained. Elastic energy production to some extent will be influenced by genetics, but like any system in the body can be improved by training.
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