The Overload Principle

 One of my favorite professors from college taught by emphasizing definitions.  This is one of his favorite phrases to define. 

Overload Principle:  the requirements necessary to bring about improvement in the various systems of the body.  As the body is subject to loads greater than they are accustomed to, the systems adjust and increase their capacity to perform physical work.
Definition from Strength Training 4th ed. by Philip Allsen 

This principle applies to cardiovascular and muscular training.  Basically, if you do the same exact workout all the time, your body will not adjust and increase its capacity.  Here are some ways you can mix up a workout so that you can continue to challenge your body and improve.

Muscular Training

• increase the resistance
• increase the number of repetitions
• increase the number of sets
• decrease the time of rest between sets for a given exercise (rest time should still be at least a minute, however)
• increase workout frequency (go from 2 to 3 days per week)
• switch up your routine (don't do the same exercise for each body part all the time)
• try circuit training (alternating resistance exercise with short bouts of high intensity aerobic exercise like rope jumping, jumping jacks, running, etc.)

Cardiovascular Training

• increase your mileage/time
• increase your intensity by upping your pace
• increase workout frequency (start at 3 days and gradually increase to 5 or 6 days per week)
• incorporate hills into your workout (or increase the incline or resistance on machines)
• interval training (where you alternate between high and low intensity)
• try circuit training (alternating resistance exercise with short bouts of high intensity aerobic exercise like rope jumping, jumping jacks, running, etc.)
• try a new mode of exercise (if you're a runner, mix in some biking, swimming, aerobic dance, etc.)

Hopefully these tips will help you incorporate the overload principle and get you out of your exercise routine rut!