Concentric Muscle Contractions – Active Shortening of the Muscle
A concentric contraction is that type of contraction that determines the muscle to become shorter while contracting, during physical exercise, for instance. Whenever a muscle must lift a weight that is lighter than the tetanic tension which can be triggered by it, that particular muscle is actively shortening. Such contractions which allow the muscle to reduce its length are known as concentric contractions.
When it comes to concentric contractions, the muscle produces a force that is necessarily inferior to the maximum (Po) of that muscle. If the weight lifted by the muscle decreases, the speed of the contraction intensifies. This phenomenon is available until the maximum contraction speed of the muscle (Vmax) is obtained. If you exercise a constant series of speed shortening contractions, you could be able to establish a relationship between force and speed.
More clearly, this type of concentric contraction takes place during physical exercise when you lift a weight in your hand, causing the biceps to curl, while the elbow gets to be fully flexed instead of staying straight. This is actually the standard type of contraction which is present in every kind of physical exercise’
Eccentric Muscle Contractions – Active Lengthening of the Muscle
These types of contractions are exactly the opposite of the concentric ones, causing the muscle to lengthen while it gets contracted. Eccentric contractions are not so common because they imply the deceleration or control of the action started by that eccentric muscles agonist. Normally, muscles stay active almost all the time during their lengthening. This happens mostly during walking, when the knee extensors or quadriceps must be active right after heel strike, with the knee flexing at the same time. Such a contraction may also take place during exercise, when you have to put an object down slowly. The arm muscles called flexors need to stay active in order to prevent the object from falling onto the ground.
Another good example would be that when you have to kick a football. By doing so, the muscle called quadriceps suffers a concentric type of contraction in order to set the knee straight, whereas the hamstrings perform an eccentric contraction to slow down the movement of the lower part of the limb. This contraction is the one that causes most muscle injuries, especially during physical exercise, because it presupposes too much strain going through the muscle.
When the load on a certain muscle gets heavier and heavier, it ends up reaching a stage where the power induced by that muscle is inferior to that coming from the exterior. In this case, the muscle is compelled to lengthen because of that external force, even if it is already activated. This is what an eccentric contraction is. It is also important to realize that, in this context, contraction does not have to mean shortening.
Isometric Muscle Contractions – Holding the Muscle Active at a Steady Length
Whenever the contracting muscle does not suffer any change in length it means that muscle undergoes an isometric contraction. Such a contraction takes place during an exercise which presupposes holding an item in front of the body, with its weight pulling down the arms. In this case, the muscles get contracted in order to keep the object steady, at the same height.
This is exactly what happens when you carry a load in front of your body. Gravity will cause the object to pull down, whereas the muscles in your arms will use an equal force to stop it from going down. During this type of exercise, your biceps will perform an isometric contraction because your arms maintain the same position. The same thing happens when you want to lift a tennis racket. Your hand joints do not move, however the muscles perform isometric contraction to generate enough force for you to be able to hold on to the object.
The isometric contraction generates a force that is totally depending on the muscle length during that contraction. The maximum of isometric tension is generated at the optimum length of that muscle, meaning onto the plateau of that curve formed by length and tension.
The force produced by a muscle while isometrically contracting is given by the muscle length at the contraction point. Each different muscle has its own optimum length where a maximum isometric force could be generated.
Passive Muscle Stretch – Passive Lengthening of the Muscle
Apart from the three types of contractions mentioned above, the muscle can also suffer a passive stretch. When this is the case, the muscle remains passive, but it lengthens at the same time, without any stimulation to get contracted. This contraction could be exemplified by the kind of physical exercise that requires touching your toes with the tips of your fingers. In this case, the hamstrings undergo such contraction.
