When people begin new exercise, their body must have time to adjust. There is a learning curve for people to start new types of exercise. Muscles that have not been previously recruited may not be strong enough yet to perform. It takes times for muscles, such as the quadriceps to adjust to running. With any new exercise, the addition should be gradual. For example, if you just started running, you should not expect to run 5 miles on the first day. A great way to build up strength and endurance is to interval train. For example, run for 5 minutes and then walk for 10, then run for 5 minutes. Every new activity should start with just a few minutes and with repetition, the duration of the exercise can be increased. Also the surface of the activity must be considered. For example, running on a track or flat pavement is going to be less challenging that learning how to run on beach sand or up hills.