Background

For an alternative description of the contraction types from Biodex.

Concentric Contraction:
Concentric contraction occurs when a load is lifted and the muscles involved shorten. Positive work also occurs at this time, when the product of the muscle moment arm and the angular velocity are positive.

When the arm is lifting a load, the flexor muscles of the arm shorten, resulting in concentric contraction. The product of the moment and velocity of the arm are positive, resulting in positive work	When the arm is lowering or pulling against a spring, the extensor muscles of the arm shorten, resulting in concentric contraction. The product of the moment and velocity of the arm are positive, resulting in positive work

Eccentric Contraction:
When a weight is being lowered in a controlled manner, the muscles involved are lengthening in a controlled manner (Fleck et al.). Negative work also occurs at this time, when the product of the muscle moment arm and the angular velocity are negative.

When an external force is applied, or the weight is being lowered, the flexor muscle is being lengthened, resulting in an eccentric contraction. The product of the moment and velocity of the arm are negative, resulting in negative work	When an upward external force is applied, the extensor muscle is being lengthened, resulting in an eccentric contraction. The product of the moment and velocity of the arm are negative, resulting in negative work

Eccentric and Concentric contractions are demonstrated below, using the triceps and biceps

The eccentric contraction of the bicep occurs when trying to lower a heavy object in a controlled manner. The force exerted by the ball (Fb) is greater then the muscular force (Fm), resulting in the arm lowering and the biceps lengthening. When the weight becomes less or the biceps exert a stronger force, the ball is lifted and concentric contraction occurs.

The concentric contraction of the tricep occurs when pulling against a spring. If the force exerted by the tricep (Ft) is greater then that of the spring (Fs), the arm moves down and the triceps experience concentric contraction. If the spring constant is increased or if the muscle is fatigued, the limb moves upward while still contracting, resulting in eccentric contraction.

Isometric Contraction:
Isometrics, or static resistance training, refers to a muscular action during which no change in the length of the muscle takes place. This type of resistance training is normally performed against an immovable object such as a wall, a barbell, or a weight machine loaded beyond an individual's maximal concentric strength.(Fleck et al.)

Isotonic Contraction:
Isotonics is traditionally defined as a muscular contraction in which the muscle exerts a constant tension (Fleck et al.). The velocity is variable. As a subject's muscle contracts, the force acting on the muscle will remain constant despite the angular velocity. This type of contraction is most often seen in free weight lifting.

Isokinetic Contraction :
Isokinetics refers to a muscular action performed at constant angular limb velocity (Fleck et al.). The resistance is variable. As the subject encounters a painful section of ROM with a given resistance, the subject will slow down in order to accommodate for the pain. The isokinetic machine will sense the change in velocity and lower the applied resistance. It will thereby allow the subject to move through the painful section of their ROM and to maintain the same angular velocity throughout the exercise.

Analysis & Questions
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