Category Archives: Encyclopedia

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


-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

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. Force Length relationship

-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


EPOC or Excess Post-Exercise Oxygen Consumption is the amount of oxygen a body takes in above the normal oxygen needs following exercise. It is often seen as an oxygen debt, built up during intense exercise which could not be sustained by the aerobic system alone. However there are other mechanisms that increase EPOC as well.


Cortisol, also known as a stress-hormone, is a hormone produced by the adrenal cortex and plays an important role in glucose regulation. Since nerve cells can only produce their energy from carbohydrates and glycogen, when blood sugar levels and glycogen stores are low, cortisol stimulates conversion of protein to carbohydrates. This results in increased protein breakdown and it inhibits protein synthesis, effectively breaking down muscle tissue to produce carbohydrates. Cortisol is secreted during times of stress but high intensity resistance exercise stimulates cortisol production as well. Since resistance exercise also stimulates testosterone and growth hormone production, this is not a negative effect, since these more than counter the influence on hypertrophy of cortisol. In fact, it is believed that cortisol plays an essential role in remodeling muscle tissue after training.


The Body Mass Index (BMI) is a measure which gives an indication whether someone has a healthy bodyweight, underweight, overweight or obese. The BMI can be calculated with the following formulae:BMI_formula_EnglishBMI formula metric The number that is calculated from one of these formulae can be looked up in the table below and determines if someone has a healthy weight or is nderweight, overweight or even obese.

Body Composition

The body composition or  fat percentage is the relative amount of bodyfat. Although the fat percentage is more difficult to assess than for exmaple the BMI, it does give a more information about someone’s bodycomposition and wheter this person has a healthy weight or is over- or underweight.

Anaerobic System

The anaerobic system consists of several energy systems that generate ATP without the aid of oxygen. The anaerobic system is responsible for generating ATP when the intracellular supply of ATP has been depleted. It will generate ATP until the aerobic system is activated and able to meet energy demands. The anaerobic systems can generate ATP at a higher rate than the aerobic system and start up more quickly. The greatest disadvantage of the anaerobic systems is that they are quickly exhausted. In addition, when exercise intensity is too high for the aerobic system to generate ATP quickly enough, the muscles are inhibited by acidosis because H+ ions are released faster than the aerobic system can use them in the oxidative fosforylation.

Aerobic System

The aerobic system, also called the oxidative system (aerobic means a proces which functions with the aid of oxygen) is the most important source of ATP in rest and during lower intensity exercise. This system generates ATP from both carbohydrates and fats. When the carbohydrate stores are depleted protein can also be used. Depending on the intensity of exercise, a certain ratio of carbohydrates and fats is used. At rest this ratio is around 70% fat and 30% carbohydrates. When exercise starts, the use of fat as an energy substrate decreases and carbohydrate use increases with the increase in intensity. When intensity rises to 100% VO2max, almost all ATP will be generated from carbohydrates.


Adenosine triphosphate (ATP) is the fuel that provides energy for all energy demanding cellular processes. ATP is the fuel that allows contraction of the muscles. Below is the equation which depicts the reaction occuring within the muscles and makes contraction possible. This reaction is reversible so that ATP supply can be resynthesized.

1 Repetition Maximum

Training intensity guidelines for strength training are mostly expressed in a percentage of the 1 Repetition Maximum. Often abbreviated to 1RM, the 1 repetition maximum is the highest amount of weight or resistance with which someone can make one repetition with proper form in a particular exercise. When you know 1RM for a certain exercise, it is possible to estimate the necessary amount of weight to reach a prescribed number of repetitions or intensity.