Final answer:
In case the load is equal to or exceeds the muscle's force, an isometric muscle contraction occurs, characterized by muscle tension without change in length.
Step-by-step explanation:
When the load attempted by a muscle is equal to or greater than the force it can generate, an isometric muscle contraction occurs. An isometric contraction is defined by the muscle producing tension without a change in length, meaning the muscle does not shorten or lengthen because it is unable to move the load. In contrast, during an isotonic contraction, the muscle changes length, which can be seen during actions that involve moving a load with enough muscle tension, such as lifting a weight. Isometric contractions are often involved in maintaining posture and the stability of bones and joints.
There are two types of isotonic contractions: concentric and eccentric. A concentric contraction involves the muscle shortening to move a load, while an eccentric contraction occurs as the muscle tension diminishes and the muscle lengthens.