Duration

Increasing the duration of an activity is the second way to overload the systems. The effect of increasing duration is easiest to demonstrate in cardiovascular activities. Most people can walk the length of a house without noticing fatigue or breathlessness. As one increases the length of the walk, however, the muscles need more oxygen and the heart must work harder, even when one walks at an even pace.

Strength training uses duration in two different ways—increasing the amount or time one holds a muscle contraction or, more commonly, increasing the number of contractions. Isometric contractions (ones in which you hold a position) may be timed. An example is the use of wall sits to strengthen your legs. You may start by holding a wall sit position for 30 seconds, gradually increasing the duration to 60 seconds. I do not usually recommend strength training by using a lot of long isometric contractions, because this method can have a negative effect on blood pressure. Though not normally thought of as a means of increasing duration, increasing the number of repetitions for a given movement also boosts the overload to the muscular system. Usually as the number of repetitions goes up, the amount of resistance (intensity) goes down, and the result is more improvement in muscular endurance. In general, 8 to 10 repetitions of a strength movement improve both muscular strength and endurance.