Classification of Exercise and Muscle Action Types (Dynamic vs. Static)

The source classifies exercise and its muscle action types by whether the exercise is dynamic or static, and by what happens to muscle length:

Dynamic exercise involves movement and comprises concentric (muscle length decreases) and eccentric (muscle length increases) actions. Static exercise involves no movement and uses isometric action (no change in muscle length).

• Dynamic + Concentric → muscle length decreases.
• Dynamic + Eccentric → muscle length increases.
• Static + Isometric → no change in muscle length.

• concentric-action — dynamic, decreasing length.
• eccentric-action — dynamic, increasing length.
• isometric-action — static, no length change.
• muscle-action-terminology — why these are "actions," not "contractions."