Which contraction results from a force exceeding the muscle's momentary force?

When a muscle experiences an eccentric contraction, it is effectively lengthening under tension while opposing a force that exceeds its current momentary force production capacity. This type of contraction occurs when a muscle is forced to stretch while it is actively contracting, typically in response to an external load that is greater than the force the muscle can generate at that moment.

In practical terms, an example of an eccentric contraction is when a weight is slowly lowered, such as during the descent of a bicep curl. The biceps muscle lengthens while it is still engaged, as it attempts to control the weight being lowered. This is crucial for activities that require deceleration and control, such as running downhill or lowering objects safely.

Concentric contractions, on the other hand, involve shortening of the muscle while generating force, as seen when lifting a weight. Isometric contractions occur when the muscle generates force without changing length, like holding a static position. Isotonic contractions are a broader category that includes both concentric and eccentric contractions, referring to muscle contractions that produce movement with a constant load. Therefore, the eccentric contraction is the precise response to the situation described in the question.