Lets start with the relationship between duration and intensity: duration is the amount of time in which you participate in a single bout of exercise, and intensity is a measure of how hard you work during a single workout. These two components are inversely related: the higher the intensity of the activity you are doing, the shorter amount of time you are able to maintain that intensity. Why? The higher the intensity, the closer you are to your maximal performance, meaning that you are working as close to 100% effort as possible. Your body simply doesn’t have enough energy to fuel this intensity for an extended period of time, as the fuel sources stored in muscle is expended within 10-30 seconds of intense activity.  

Next comes frequency: how often in a given time frame an exercise is performed.  Frequency affects the recovery period following each workout. The more often you workout, the shorter available time there is for the body to recover from the stimulus provided during exercise. The higher the intensity of your workout, the more time your body needs to recover. 

A good exercise regimen requires a balance between the frequency of your workouts, the duration of each session, and how intensely you perform each exercise. Too often people adopt the mindset that working harder and longer will provide you with the most benefits, when actually having too large a workload can be detrimental to your exercise program, and prevent your body from optimally adapting to the stimulus provided by exercise. Always take a careful look at your exercise routine to make sure there is a balance between frequency, duration, and intensity.