I may have misunderstood this picture of new research related to the stretch-shortening cycle on the microscopic scale.
The Hill Muscle Model shows the functional components of a muscle on the smallest scale. It provides a way to visualize what is going on and to think about how active muscle components (actin & myosin) and passive muscle stretch component (recently also the Titin protein molecule) might function.
Recently, new research has emphasized the role played by passive Titin, the largest protein molecule, also located within the sacomere in parallel with the active Actin and Myosin structures. Recently it has been considered that Titin in each muscle cell
provides the main
stretch capability of the muscle. An older theory views stretch as an overall muscle stretch to include tendons.
Actin, Myosin Animation (no Titin) - Active Muscle Shortening
Sarcomere with Titin Illustrations
Actin, Myosin & Titin Illustrations
Report on Stretch Shortening Training with Biopsied Human Muscle Measurements. Added 2/5/2013
Powerpoint presentation Stretch Shortening Cycle including Titin. Added 2/5/2013
New research on Titin
This report in Figure 7 proposes a new way that Titin might be interacting with Actin to provide stretch functions.
I don't understand this last research but maybe it implies that a stretch can be deliberately activated at various lengths of the muscle.
This new research might be especially important along with other research that indicates muscle shortening might be faster
if 'passive' stretch is employed instead of active muscle shortening. (the Actin -Myosin animation above even looks slow).
In Biomechanics of Advanced Tennis
(2003). Elliott said
"10-20% of additional racket head speed is achieved following a stretch shortening cycle."
(This publication is now 10 years old so there may be different views in 2013.)
Is that a simple addition to racket head speed or is passive stretch derived muscle shortening the only
mode that can shorten that fast with control and reproducibility? Main principle of athletic movement?
Titin-based contribution to shortening velocity of rabbit skeletal myofibrils
University of Manchester (2010) Proc Physiol Soc 19, SA12
History dependence of muscle force production: structural non-uniformities, cross-bridge action or something altogether different?
1. Kinesiology, University of Calgary, Calgary, Alberta, Canada.
Medline articles by:
Figure 1: Mean (
When a muscle is actively stretched (shortened) its steady-state force following the stretch (shortening) is increased (decreased) relative to the purely isometric force at the same length. This history dependence of muscle force production was first described systematically more than half a century ago (1), but cannot be explained by the reigning paradigm of muscle contraction: the cross-bridge theory (2, 3). For the past three decades, history dependence had been explained with structural non-uniformities; specifically the development of sarcomere length non-uniformities when muscles were stretched (shortened) actively on the “unstable” descending limb of the force-length relationship (4, 5). The sarcomere length non-uniformity theory allowed for precise predictions, including that force enhancement following an active stretch cannot occur on the ascending part of the force-length relationship and that the enhanced forces can never exceed the maximal isometric forces at obtained on the plateau of the force-length relationship. However, these two basic predictions were shown to be not satisfied in a series of experiments from different laboratories (e.g. 1, 6). Recently, we discovered that passive forces following an active stretch of muscles, fibres and myofibrils were increased (7). When eliminating titin from isolated myofibril preparations, this passive force enhancement was abolished indicating that titin might play a force regulatory role. Stretching troponin C depleted myofibrils (to inhibit cross-bridge connections between the contractile proteins actin and myosin) in solutions of increasing calcium concentration resulted in an increase in passive forces, suggesting that titin is a molecular spring whose stiffness can be modulated by calcium (e.g., 7). Unfortunately, the increase in force associated with titin’s calcium sensitivity only accounted for a few percent of the observed increases in passive force with active muscle stretching. When stretching single myofibrils passively (low calcium concentration) and actively (high calcium concentration) beyond actin-myosin filament overlap, forces in the actively stretched condition were 3-4 times greater at lengths where cross-bridge forces were absent, and these differences reached values approximately 2-3 times the maximum active isometric force at the plateau of the force-length relationship. How can such high forces be explained in the absence of actin-myosin based cross-bridges forces? When eliminating titin, these force difference are abolished. Calcium activation alone (when cross-bridge attachments are inhibited) merely accounts for a tiny amount of the observed force increases. However, when myofibrils are actively stretched from different parts of the descending limb of the force-length relationship, and thus from different force levels, the increase in force beyond actin-myosin filament overlap is proportional to that force (Figure 1). From these results we conclude that titin is a strong regulator of force in skeletal muscle and becomes particularly important at long sarcomere lengths. Titin’s force regulation depends on the amount of active force, but is essentially independent of calcium concentration. We tentatively suggest that titin’s force regulation is caused by a force-dependent interaction of titin with actin which causes the free spring length of titin to become smaller thereby increasing its stiffness, and thus force upon stretching.
Titin force is enhanced in actively stretched skeletal muscle
Krysta Powers, Gudrun Schappacher-Tilp, Azim Jinha, Tim Leonard, Kiisa Nishikawa, Walter Herzog
Journal of Experimental Biology (Impact Factor: 3). 08/2014; 217(20). DOI: 10.1242/jeb.105361
ABSTRACT The sliding filament theory of muscle contraction is widely accepted as the means by which muscles generate force during activation. Within the constraints of this theory, isometric, steady-state force produced during muscle activation is proportional to the amount of filament overlap. Previous studies from our laboratory demonstrated enhanced titin-based force in myofibrils that were actively stretched to lengths which exceeded filament overlap. This observation cannot be explained by the sliding filament theory. The aim of the present study was to further investigate the enhanced state of titin during active stretch. Specifically, we confirm that this enhanced state of force is observed in a mouse model and quantify the contribution of calcium to this force. Titin-based force was increased by up to four times that of passive force during active stretch of isolated myofibrils. Enhanced titin-based force has now been demonstrated in two distinct animal models, suggesting that modulation of titin-based force during active stretch is an inherent property of skeletal muscle. Our results also demonstrated that 15% of titin's enhanced state can be attributed to direct calcium effects on the protein, presumably a stiffening of the protein upon calcium binding to the E-rich region of the PEVK segment and selected Ig domain segments. We suggest that the remaining unexplained 85% of this extra force results from titin binding to the thin filament. With this enhanced force confirmed in the mouse model, future studies will aim to elucidate the proposed titin-thin filament interaction in actively stretched sarcomeres.