Skeletal muscles are the ones that really concern us and are the muscles that cover the entire skeleton. Without them we could not move and they make up around 50% of our body weight.
Each muscle can only be contracted, and so can only move in one direction. Every muscle has an opposing muscle which runs either side of the bone, and works in unison. When one muscle contracts, the other relaxes.
- In concentric contraction, the force generated is sufficient to overcome the resistance, and the muscle shortens as it contracts. This is what most people think of as a muscle contraction.
- In eccentric contraction, the force generated is insufficient to overcome the external load on the muscle and the muscle fibers lengthen as they contract. An eccentric contraction is used as a means of decelerating a body part or object, or lowering a load gently rather than letting it drop.
- In isometric contraction, the muscle remains the same length. An example would be holding an object up without moving it; the muscular force precisely matches the load, and no movement results.
- In isotonic contraction, the tension in the muscle remains constant despite a change in muscle length. This can occur only when a muscle's maximal force of contraction exceeds the total load on the muscle