Final answer:
When summation occurs, a second stimulus increases the force generated due to the additional release of calcium ions, which activate more sarcomeres for a stronger muscle contraction.
Step-by-step explanation:
The force generated by a second stimulus in muscle contraction is larger during summation because the extra stimulation causes more calcium ions (Ca++) to be released. These additional calcium ions are available to activate extra sarcomeres, which are the functional units within a muscle fiber responsible for contraction. Since the muscle hasn't fully relaxed from the initial stimulus, the second contraction adds to the residual tension, resulting in a greater overall contraction.
Incomplete tetanus is relevant here as well; if the stimulation frequency is sufficiently high, the muscle contractions can start to fuse together, leading to a continuous contraction. However, the key factor in the context of summation and increased force generation by the second stimulus is the increased calcium concentration.