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Sarcomere

Muscles are responsible for making movements through muscle contractions. Muscles consist of many different fibers, which consist of myofibrills. Myofibrills in turn consist of many sarcomeres in series. A sarcomere is the smallest functional unit in a muscle and is the structure that makes the muscle contraction possible.

A sarcomere consists of thin actin filaments and thick myosine filaments, which can slide past each other when the sarcomere contracts. The actin filaments are connected to the z-lines which form the connections between the sarcomeres in series. The myosin filaments are in the middle of the sarcomere in the center of several actin filaments. Each myosin filament has myosinheads portruding from the filament, which can bind to special binding places on the actin filaments. Through this interaction between actin and myosin filaments, sarcomeres are able to exert force.

When a contraction is started, the myosin heads are activated and form so called cross-bridges by binding at the binding places at the actin filaments. After forming the cross-bridge, the myosinhead pulls at the actin filament, and the filaments slide past each other, shortening the sarcomere. After this, the myosinhead unbinds and returns to its original position, ready to form another crossbridge with the next binding place.Muscle structure

References:

-Wilmore, J.H., Costill, D.L., Kenney, W.L. (2008). Structure and function of exercising muscle. Physiology of Sport and Exercise Fourth Edition. USA Human Kinetics.

Muscles and Muscle Fiber Types

The limbs of the body are able to move because muscles contract and excert force on the skeleton. Muscles consist of muscle fibers and a single muscle fiber in turn is made up of smaller units called sarcomeres Muscle and muscle fiber structure

Muscle Contraction

A muscle consist of sarcomeres which can contract in three different ways, namely concentric, isometric and eccentric. But what is the difference in these types of muscle contraction?

Force Velocity Relationship

The amount of force a sarcomere can exert is, outside of the relative length at which it currently functions, dependant at the speed the sarcomere is contracting. Making the cross bridges necessary for contraction, evidently takes time. Logically, when the filaments are being moved at a higher velocity, less myosinheads can bind to the actin filaments at a given time and as a result total force is lower. The Force Velocity relationship can be seen in the figure below. A higher speed during a concentric contraction, results in a lower force. At speed 0, or an isometric contraction, the force is greater. When the contraction velocity turns negative and the sarcomere is stretched, also known as a eccentric contraction, the force a sarcomere increases even further. This can be explained by the force required to stretch passive structures and lengthen the muscle.

Force velocity

References:

-Wilmore, J.H., Costill, D.L., Kenney, W.L. (2008). Structure and function of exercising muscle. Physiology of Sport and Exercise Fourth Edition. USA Human Kinetics