Progressive overload is the gradual increase of stress placed upon the body during exercise training. It was developed by Thomas Delorme, M.D. while he rehabilitated soldiers after World War II. This technique is recognized as a fundamental principle for success in various forms of strength training programs including fitness training, weight lifting, high intensity training and physical therapy programs.
 Scientific Principles
A common goal for any strength training program is to increase, or at least maintain the user’s physical strength or muscle mass.
In order to achieve new results, as opposed to maintaining the current level of achievement, the muscles (see skeletal muscles) need to be overloaded, which stimulates the natural adaptive processes of the human body, which develops to cope with the new demands placed on it. Conversely, decreased use of the muscle results in the loss of mass and strength, known as muscular atrophy. (see atrophy, and muscle atrophy.)
The fact is that the adaptive processes of the human body will only respond if continually called upon to exert a greater magnitude of force to meet higher physiological demands. .
In order to minimize injury and maximize results, the novice begins at a comfortable level of muscular intensity and advances towards overload of the muscles over the course of the exercise program. 
Progressive Overload requires a gradual increase in volume and intensity in order to achieve the targeted goal of the user. In this context, volume and intensity are defined as follows:
- Volume is the total number of repetitions multiplied by the resistance used as performed in specific periods of time.
- Intensity is the percent value of maximal functional capacity, or expressed as percent repetition maximum.
- Overtraining is the decline in training performance over the course of a training program, often resulting increases the risk of illness or injury or decreased desire to exercise. In order to help avoid this problem, the technique of periodization is applied.
- Periodization in the context of fitness or strength training programs is the scheduling of provisions for adequate recovery time between training sessions, variety over the course of a long-term program, and motivation – avoiding monotony when repeating identical exercise routines.
- ^  Optimizing Strength Training
- ^ a b American College of Sports Medicine position stand. Progression models in resistance training for healthy adults. Med Sci Sports Exerc. 34(2):364-80, 2002 Feb, PMID:11828249.
- ^ a b The Team Physician and Conditioning of Athletes for Sports: A Consensus Statement- 02/01/2006, the American College of Sports Medicine
- ^ Overtraining With Resistance Exercise, ASMC Jan 2001, Andrew C. Fry, Ph.D., the American College of Sports Medicine
Which leads us to the next thing.
The aim of a fitness programme can be an improvement in one or more of the components of fitness, i.e. stamina, strength, suppleness, speed or skill. The result will be seen in physiological changes to the body achieved by progressive increases in the intensity of the overload, applied in the following ways:
DEGREE OF RESISTANCE – The amount of resistance is increased by lifting heavier weights or leads by applying greater farce. An example would be to add a few more weights on the bar when doing bench presses or, when doing step-ups, carry a dumb-bell in each hand and then gradually increase the weight.
WORK RATE – The work rate is increased by performing the exercise or task in a shorter period of time. Instead of walking a mile in 20 minutes, cover the distance in 15 minutes.
WORK DURATION – The work period is increased but the same work Intensity is retained. An example would be to increase your 4 mph walk to cover five miles at the same speed In one hour and 15 minutes.
REST INTERVALS – The rest periods taken between training repetitions or set exercise periods is reduced, for instance when carrying out training perhaps comprising walking for 800 yards, then jogging for 200 yards, gradually reducing the walking phase to 400 yards and increasing the jogging phase to 300 yards. When doing step-ups in three sets perhaps of 20 reps, reduce the rest period between each set.