There are fast twitch muscles and slow twitch muscles.
Slow twitch muscles are for endurance and fast twitch are for short bursts of strength. The heart is a slow twitch muscle and will continue to beat for years on end, needing only to slow down once in a while to rest.
Runners develope a lot of slow twitch muscle in their legs and can do marathon runs, but if you ask them to do squats with a squat bar, they will not be able to do very many--unless of course, they have been doing a lot of squats in addition to running.
Look at a chicken. The white meat in the breast is fast twitch and the dark meat in the legs is slow twitch. The chiken can walk around all day but can only fly short distances.