>2. Muscle: Types of Contractions

In physiology, muscle contraction is not synonymous with muscle shortening. Muscles can be contracting without a change in muscle length, such as when holding a heavy object. The variables that describe muscle contractions are length, and tension. Additionally all contractions produce some damage to muscle cells due to the chemical and mechanical stress they endure (which is part of what you adapt to).

Muscle contractions can be broadly classified into two types:

<Isometric Contractions
In isometric contractions, muscle length stays constant while muscle tension is increased. Isometric muscle contraction is present in static exercises, such as planks.

<Isotonic Contractions
Isotonic contractions can be further categorized into two groups:

Concentric Contractions
Occur when tension remains constant as muscle length shortens. For example pushing a barbell up during a bench press, or pulling yourself up on a pullup bar.

Eccentric Contractions
Occur as muscle length increases while muscle tension remains constant. This can be thought of as decelerating action, such as lowering yourself from a pull up bar. The mechanism through which muscle cells produce force in eccentric contractions is different than isometric and concentric contractions, and not currently understood. This different mechanism allows eccentric contractions to be 40% stronger than concentric, however with significantly more cellular damage. This difference is thought to be why delayed onset muscle soreness (DOMS) is so prevalent with eccentric exercises. The body can adapt to eccentric contractions with proper training, resulting in significantly less damage cellular damage and DOMS.