Questions: Which statement describes the muscle response at a stimulation interval of 120 ms (about 8 stimuli per second)? The muscle was only partially relaxed before each stimulus, and the contractions generated more force than individual twitches. The muscle was only partially relaxed before each stimulus, and each contraction generated the same force as an individual twitch. The muscle had time to completely relax before each stimulus, and each twitch generated the same force. The muscle was in the process of contracting when each stimulus arrived, and the contractions generated more force than individual twitches.

The answer is the first one: The muscle was only partially relaxed before each stimulus, and the contractions generated more force than individual twitches.

Explanation for each option:

1. The muscle was only partially relaxed before each stimulus, and the contractions generated more force than individual twitches.

   • This statement is correct. At a stimulation interval of 120 ms (about 8 stimuli per second), the muscle does not have enough time to completely relax between stimuli. This leads to a phenomenon known as temporal summation, where the force of each subsequent contraction is greater than that of an individual twitch because the muscle is still partially contracted from the previous stimulus.

2. The muscle was only partially relaxed before each stimulus, and each contraction generated the same force as an individual twitch.

   • This statement is incorrect. If the muscle is only partially relaxed before each stimulus, the force generated by each contraction would typically be greater than that of an individual twitch due to the additive effect of the stimuli.

3. The muscle had time to completely relax before each stimulus, and each twitch generated the same force.

   • This statement is incorrect. At a stimulation interval of 120 ms, the muscle does not have sufficient time to completely relax before the next stimulus arrives. Complete relaxation would occur at longer intervals between stimuli.

4. The muscle was in the process of contracting when each stimulus arrived, and the contractions generated more force than individual twitches.

   • This statement is partially correct but less precise than the first option. While it is true that the muscle is in the process of contracting when each stimulus arrives and the contractions generate more force than individual twitches, the key detail is that the muscle is only partially relaxed before each stimulus, which is better captured in the first option.

Summary: The correct statement is that the muscle was only partially relaxed before each stimulus, and the contractions generated more force than individual twitches. This is due to the phenomenon of temporal summation, where the force of each contraction builds upon the previous one when the muscle does not fully relax between stimuli.