> 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.

< Auxotonic Contractions (non-isometric, non-isotonic)

Most exercises have fluctuation in muscle tension, and the contractions involved could technically be classified as auxotonic. Auxotonic contractions occur when both muscle tension and length changes. 

For fitness purposes, there's generally no need to differentiate between isotonic and auxotonic contractions. However it may be useful to think about auxotonic contractions when it comes to exercise selection or carryover and the change in muscle tension throughout the movement.