 How do muscles really work? In this video I'll explain the sliding filament theory of muscle contraction as well as the effects of muscle length and rate of shortening on muscle force capabilities. Muscles are made up of small bundles of muscle fibres called fascicles and muscle fibres contain contractile organelles called myofibrils with all sarcomeres arranged in series. We often see a sarcomere represented like this in 2D. The myofibril contains various myofilaments including myosin in red here and actin in blue. According to the sliding filament theory the different myofilaments slide past each other during a cross bridge cycle when a muscle is stimulated calcium ions bind to troponin. This moves tropomyosin out of the way and uncovers binding sites for myosin on the actin myofilaments. ADP and phosphate are attached to the myosin head from the previous cycle of movement but the phosphate is released as the myosin head attaches to the exposed binding site on actin forming a cross bridge. Energy stored in the myosin head is used to move the head causing actin to slide past. ADP is released as myosin moves. The actin myosin bond is broken when an ATP molecule binds to the myosin head and this ATP is broken down into ADP and phosphate releasing energy which is stored in the myosin head and will be used later for movement. The head returns to its upright position and is ready to bind to actin again. If calcium ions are still present the entire sequence is repeated. In reality muscles are not two-dimensional so each myosin filament supplies six actins and each actin is supplied by cross bridges from three myosin filaments forming a hexagonal array. A cross section would therefore look something like this and the full 3D cross bridge cycle would look something like this. Thank you to Drew Barry for permission to use his video which I'll link to in the description below. The force-length relationship describes how much force a muscle can exert as a function of the muscle length when maximally active and at isometric steady states. This relationship is at least partly dictated by the overlap between myosin and actin filaments and hence by the number of possible cross bridges that can be formed. At the optimal muscle length an optimal overlap between actin and myosin enables the maximum number of cross bridges to be formed and so the maximum force. At slightly longer muscle lengths less force can be produced as actin and myosin overlap less and so less of them are able to bind. When there is no actin and myosin overlap there is no active force as the myosin heads cannot interact with actin anymore. On the opposite end of the relationship as the muscle decreases in length the filaments can become more closely packed and are able to form fewer cross bridges. The terms ascending limb plateau region and descending limb are often used to describe the various regions of this relationship. In addition to the active force length relationship it should be noted that the passive tension in muscle shown here in blue increases when stretched beyond resting lengths. The force velocity relationship describes the maximum steady state force and muscle can exert at optimal fascicle length as a function of its rate of shortening. Concentrically maximal force decreases at increasing rates of shortening. This is believed to be caused by both the increasing likeliness of actin and myosin pairs passing each other without forming cross bridges and the increasing proportion of cross bridges that will not be disassociated in time generating a force in the opposite sense of muscle shortening. The maximal velocity of unloaded shortening is found at the point where negative forces equal positive forces and so net force is zero. Enhanced force production during eccentric contractions may be due to an increased number of cross bridges attached to actin, increased force per cross bridge or a combination of these. We've seen this as concentric velocity increases force decreases. This means that power which is the product of these two variables is greatest somewhere in the middle where there is a moderate force multiplied by a moderate velocity rather than one of the two values being low. For much more information including practical applications and theoretical underpinnings as well as the effect of a prior muscle stretch or shortening on muscle force, check out this full lecture by Professor Walter Herzog.