Most of our previous knowledge of muscle energetics theory came from the work of AV Hill and Meyerhof in the first half-mid 20th Century. Their work led to the introduction of the sliding filament theory of muscle contraction.
The Sliding Filament Theory
When you analyse skeletal muscle under a microscope you can see that is striated in appearance. This striped pattern is formed by series’ of sarcomeres that are stereotyped in both in-parallel and in-series domains, and repeated throughout the muscle cell. Within each sarcomere are two protein myofilaments called actin (thin) and myosin (thick).
When a stimulus is presented to the muscle, these two filaments interact and a muscle contraction occurs. Essentially, the actin (which is located at the lateral aspect of the Z line) is pulled inward towards the relatively constant lying myosin (located within the A band) which acts as a ratchet for actin to attach to, and muscle tension increases (shortening the length of the I band)- subsequently shortening the sarcomere and the muscle as a whole.
The bonding between the globular head of a myosin molecule and the actin filament, repeatedly formed during muscle contraction to draw it into the A band of a sarcomere is often referred to as the ‘cross-bridging’ effect. See diagram 1.
Whilst the sliding filament model answered many questions relating to muscle contraction (prior to that there were some strange theories relating to balloons and animal spirits!), it also left some unresolved- for example the reasons why force increases as the muscle stretches/decreases in force as the muscle shortens, and the low cost of force production during active stretch. All of these questions suggest that our knowledge of the process of muscle contraction remained incomplete.
The Role of Titin
When the sliding filament theory is revisited, it seems evident that active muscles behave in a manner that suggests the existence of an internal ‘spring’. Furthermore this spring is seemingly capable of storing and returning elastic potential energy allowing transfer from stored to kinetic energy (ESE>EKE). This may in part go some way to explaining some of the questions that remained from the sliding filament theory model.
In striated muscle the internal spring mechanism appears to be due to a protein called ‘titin’ which surprisingly was discovered back in 1976. However as it was discovered after the introduction of the sliding filament theory it was not factored into the model. Titin, also referred to as connectin, is a giant protein that functions as a molecular spring responsible for the passive elasticity of muscle. It is composed of a large number of individually folded proteins that unfold when the protein is stretched and refold when the tension is removed.
Diagram 2 illustrates titin (as seen in yellow) and its relationship to the other myofilaments. It spans approximately half the length of the sarcomere and intercepts from Z disc to M line, allowing it to maintain sarcomere integrity and therefore contribute to passive tension (additionally force transmission also limits the range of motion in the sarcomere, and consequently contributes to passive stiffness of the muscle). Titin is composed of two spring-like elements- the PEVK element (proline, glutamic acid, valine, and lysine) and a tandem immunoglobin (Ig) element.
The interplay of PEVK and tandem Ig allows the folded Ig domain to increase passive tension as it elongates at relatively short sarcomere length, and at longer lengths the PEVK segment allows passive tension to increase substantially. Elongation of the PEVK segment largely determines the passive elasticity of skeletal muscle fibres. In contrast to the sliding filament theory, the so-called ‘winding filament’ theory not only explains force regulation in isometric and concentric contractions, but also explains force regulation and stiffness energetics for eccentric muscle contractions as well.
The Winding Filament Theory
Due to the emerging information regarding the isolation and process behind titin, the winding filament theory proposes a new addition- the ‘swinging’ cross-bridge theory as opposed to the satndard cross-bridge one: a two-step ‘winding filament’ model of muscle sarcomeres, in which titin is ‘activated’ mechanically by the influx of calcium, and then is wound upon the thin filaments by the cross-bridges, which not only translate but also rotate the thin filaments.
The Winding Filament Theory and Athletic Performance
Evidence suggests a role for titin in skeletal muscle mechanics in that human locomotion involves the use of stored elastic energy. There is currently much work being done in-vivo to ascertain this proposed model and also its role in athletic training. Presently however there is limited data analyzing the functional role of titin and the winding filament theory in athletic performance. There is however evidence of it relating to particular characteristics in sports involving power and strength activities (as tested utilizing 1RM and CMJ by McBride et al, 2003).
An interesting future direction will be to collect data relating such theories of muscle mechanics to not only power based sports but also to skeletal muscle hypertrophy models as well. Additionally, future research may also benefit from analyzing the expression of various titin isoforms as the result of resistance exercise or power training, and the subsequent influence on muscle elasticity.
Boundless. Sliding Filament Model of Contraction. Boundless Anatomy and Physiology. Boundless, 08 Jan. 2016. Retrieved 29 Mar.
McBride et al. Characteristics and titin in strength and power athletes. Eur J App Physiol. 2003. 88: 553-557
Monroy et al. What is the role of titin in active muscle. Ex Sport Sci Reviews. 2012
Nishikawa et al. Is titin a ‘winding filament’? a new twist on muscle contraction. Proceedings of the royal society. March 2012; Vol 279. Issue 1730