Final answer:
Muscular endurance is achieved when muscles can sustain effort over time without fatigue, and the correct answer is option 4: muscles are able to engage in sustained effort while resisting fatigue.
Step-by-step explanation:
Muscular endurance is defined as the ability of a muscle to continue contracting over a long period without tiring. This is distinct from muscular strength, which is the capacity of a muscle to exert force during a contraction. When we consider the four options provided, muscular endurance has occurred when muscles are able to engage in sustained effort while resisting fatigue. This means that the correct answer is option 4. Understanding muscle fatigue helps us appreciate endurance better. Muscle fatigue limits the muscle's ability to contract and is influenced by ATP depletion, lactic acid buildup, ion imbalances, and damage to the sarcoplasmic reticulum (SR) and sarcolemma. Sustained exercises, such as aerobic exercises, increase cellular mitochondria and myoglobin, enhancing endurance by allowing muscles to utilize oxygen more efficiently and sustain effort for longer durations.