Tag Archives: force-length relationship
Improving Running Economy
Running 
is a very popular and effective method to increase endurance and improve health. An increasing number of people participate in large running events such as a Marathon, Half-Marathon or other distances. Aside from oxygen uptake (VO2max) running economy is one of the most important variables that influences how well you perform at running events. This article will tell you why and how you should be improving your running economy.
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.
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.
Range of Motion
The range of motion is a term which is used to describe the range through which a joint or muscle can move. The range of motion of a joint is dependent on several factors such as the stiffness of tendons or ligaments, activity level, age, gender and structure of the particular joint.
During training it is important to move through an as large as possible part of the available and safe range of motion. This results in a more effective training for the active muscles and flexibility is maintained or even increased. A limited range of motion can be detrimental to performance in sports and is related to developing physical problems such as low back pain. In addition a limited range of motion also increases risk of injury.
See also:
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 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 Length Relationship
The amount of force that a sarcomere and consequently a muscle can deliver is strongly dependant on the length of the sarcomere at which it is delivering force. This is caused by the amount of overlap between the myosine and actin filaments at a given length. The greater the number of myosin head that can bind to the actin filaments, the higher the force that can be produced. However, when the sarcomere is maximally shortened, it can occur that the oposite actin filaments colide with one another and slide past each other. This results in less cross bridges being formed between actin and myosin filaments and extra resistance during contraction and therefore a reduction in produced force. In the figure below can be seen that a sarcomere builds up strength during shortening until it peaks and decreases due to a decrease in crossbridge forming surface on the filaments. 
References:
-Gordon, A.M., Huxley, A.F., and Julian, F.J. (1966). The variation in isometric tension with sarcomere length in vertebrate muscle fibers. J. Physiol., 184, 170-192.
-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