If a muscle is developing tension as it lengthens, this is referred to as a(n) muscle action.

eccentric contraction

Explanation:

When a muscle contracts under any load or tension then such muscle action is referred to as eccentric contraction. One of the common most example is squat exercise in which muscles lengthen under the effect of eccentric contraction in the downward position

This usually happens when the total length of the muscle increases with the increase in tension produced in the muscles.