This article is going to seem a little “out there” for most of my readers, but try your best to understand the information as it’s very important for exercise science and I will be referring to this article as time goes on. For the past six months I’ve really delved into the literature involving structural proteins. This has been a very intriguing process. First I will discuss some important concepts in musculotendinous modeling, then I’ll delve into some exciting new research involving titin and myotilin – two exciting proteins that play a huge role in athletic performance.

Contributors to Force – The CC, PEC, and SEC 

Below is a figure that will help you understand the primary components that contribute to the development of force. You will notice that this model contains three components.

The first part of the model is the contractile component which is symbolised by the square box in models A and B. The contractile component represents a conversion of electrical stimulation into force production via the actin and myosin filaments.

The second component of the model is the parallel elastic component which is symbolised by a spring- denoting its elastic like qualities. When muscle is stretched beyond resting length, the parallel elastic component contributes to tension. The structures associated with the parallel elastic component are the fascia surrounding the muscle such as the perimysium, endomysium, and epimysium, as well as the sarcolemma.

The third part of the model is the series elastic component (SEC), which once again is symbolised by a spring and occurs in series with a contractile component. When muscles contract, the series elastic component lengthens and stores energy, and therefore can contribute to total tension. The SEC is primarily the tendon which is responsible for transferring the forces developed by the contractile component or the cross-bridges to the bone and thereafter subsequent movement. These three components contribute to active and passive tension within the muscle. The active tension, via the contractile component, is mediated by the nervous system; whereas the passive tension, which comprises of the parallel elastic component and the series elastic component, are not.

What I love about this model is that it allows you to understand what’s going on at a smaller and larger scale when it comes to force production. The top picture could represent force contribution at a microscopic level, where the PEC could represent deep fascia cells, the CC could represent a single sarcomere, and the SEC could represent the titin strands. The bottom picture could represent force contribution at a macroscopic level, where the PEC could represent elastic tissues such as the deep fascia and sarcolemma, the CC could represent an entire muscle belly, and the SEC could represent the tendons. This model does a good job of illustrating the fact that active tension coincides with passive tension both parallel and in series on a smaller scale as well as a larger scale.

We’ve been taught that muscles produce force through cross-bridge formation (the sliding filament theory) and that muscle force gets weaker as the muscle is stretched. Many have heard of the length-tension relationship, which dictates that force will decrease as a muscle is stretched to long lengths due to the fact that less cross bridges will form. While this is true and active force does in fact diminish, passive force rises to “pick up the slack.” Below is a model of a rabbit psoas muscle.

As you can see, the contractile force does indeed diminish, but elastic force rises as the sarcomere is stretched. Researchers have always wondered where this extra passive force is coming from, and have long suspected that it comes from a molecular protein called titin.

King Titin

Titin is actually the largest protein in existence and consists of 34,350 amino acids! Here’s the way we used to think of titin during active stretching (think eccentric contraction):

Just recently Kiisa Nishikawa published a fantastic article in The Proceedings of Royal Society B. What she proposes is that titin first binds to actin like this:

Then, titin winds up during the active contraction like this:

This model effectively explains why concentric contractions require so much more energy than eccentric contractions and why we’re much stronger eccentrically. The contractile components do all the work winding up and stretching titin when the sarcomere shortens, and then the incredibly strong and stiff titin recoils and unleashes elastic energy upon eccentric lengthening. Titin therefore provides much stability at long muscle lengths and helps protect the sarcomeres from damage. Furthermore, titin acts as a spring, so it’s not just the tendons that provide “springiness” or “elasticity”; it’s also the muscle fascia and especially the titin within the sarcomeres.

What’s very interesting is that our titin isoforms grow shorter and stiffer as we age from babies to adults (to allow us for more stability and greater force production), and powerlifters, Olympic weightlifters, and sprinters have all been shown to possess titin alterations that lead to enhanced force production. The bottom line is that training at long muscle lengths will lead to favorable adaptations which will dramatically enhance force production through stronger titin and greater elastic recoil.

Added Muscle Length

Two different phenomena have been shown to add length to muscles; prolonged lengthening and eccentric contractions. Individuals who receive casts with their muscles in long positions will see an increase in sarcomeres in series which quickly returns to normal after the cast is removed. This illlustrates how long duration stretching can add length to the muscles, but it’s a slow process. Walking downhill (pure eccentric contractions) versus walking uphill (pure concentric contractions) has been shown to result in increased sarcomeres in series in the downhill walkers compared to uphill walkers. Researchers have known this for quite some time – that eccentric contractions produced an increase in sarcomeres, but they weren’t quite sure as to why this occurred. 

Furthermore, it’s been shown that training at long lengths and performing eccentrics shifts the region where a muscle develops optimal torque to longer lengths. For example, training the hamstring muscles at long lengths and with eccentrics will cause the muscles to be stronger at long lengths and weaker at shorter lengths.  What explains these phenomena?

Myotilin

Lena Carlsson in Neuromuscular Disorders showed that during eccentric contractions, a protein called myotilin plays a key role in myofibrillar remodeling and the creation of new sarcomeres in series. Sarcomerogenesis can occur in two ways; through a split of a single Z-disc, or by alteration of a single sarcomere, which is shown below.

The take home point is that this new length is only added through eccentrics and this helps prevent muscles from injury. This has implications ranging from postural to athletic enhancement.

Conclusion

I realize that this material is quite complicated, but just know that full range movements that include the eccentric component are critical in the enhancement of mobility and stability. Many practitioners are aware of the motor control and neural aspects of mobility and stability, but are unaware of the structural aspects involved. I believe that this research is of enormous importance to the physical therapy and sport science fields.