Isometric contractions produce force without changing muscle length, whereas isotonic contractions involve muscle shortening or lengthening. Isokinetic contractions involve movement at a constant angular velocity because resistance adapts throughout the range of motion.
How the contractions differ
Muscles produce tension when activated. The contraction type is classified by what happens to muscle length, joint movement, and movement velocity.
| Contraction type | Mechanism | Example |
|---|---|---|
| Isometric | Muscle tension increases, but muscle length and joint angle remain effectively constant. No visible movement occurs. | Holding a plank or pausing while holding a weight with the elbow fixed. |
| Isotonic concentric | The active muscle shortens while producing tension, creating movement against resistance. | The biceps brachii shortens during the upward phase of a biceps curl. |
| Isotonic eccentric | The active muscle lengthens while producing tension, usually to control or decelerate movement. | The biceps brachii lengthens while controlling the downward phase of a biceps curl. |
| Isokinetic | The muscle changes length while joint movement occurs at a constant angular velocity. Resistance accommodates changes in force throughout the movement. | Knee extension performed using a specialized isokinetic dynamometer. |
During isotonic movement, the concentric and eccentric phases normally occur in sequence. In a squat, the quadriceps contract eccentrically during the controlled downward phase and concentrically during the upward phase.
A common misconception is that isokinetic means the same as isotonic. It does not: isotonic describes a shortening or lengthening contraction traditionally associated with constant external load or tension, while isokinetic specifically requires constant movement velocity. True isokinetic exercise generally needs specialized equipment that adjusts resistance as the athlete's force changes.
