The intensity of exercise in a resistance training program is the amount of resistance (weight) imposed on the contracting muscle during each repetition of an exercise. The amount of resistance is also referred to as the exercise load (training load), that is, the extent to which the muscle is loaded or how much weight is lifted, lowered, or held.

Remember, consistent with the overload principle, to improve muscle performance the muscle must be loaded to an extent greater than loads usually incurred. One way to overload a muscle progressively is to gradually increase the amount of resistance used in the exercise program. The intensity of exercise and the degree to which the muscle is overloaded is also dependent on the volume, frequency, and order of exercise or the length of rest intervals.

Submaximal Versus Maximal Exercise Loads

Many factors, including the goals and expected functional outcomes of the exercise program, the cause of deficits in muscle performance, the extent of impairment, the stage of healing of injured tissues, the patient’s age, general health, and fitness level, and other factors determine whether the exercise is carried out against submaximal or maximal muscle loading. In general, the level of resistance is often lower in rehabilitation programs for persons with impairments than in conditioning programs for healthy individuals.

Submaximal loading. Exercise at moderate to low intensities is indicated:
• At the beginning of an exercise program to evaluate the patient’s response to resistance exercise, especially after extended periods of inactivity
• In the early stages of soft tissue healing when injured tissues must be protected
• After periods of immobilization when the articular cartilage is not able to withstand large compressive forces or when bone demineralization may have occurred, increasing the risk of pathological fracture
• For most children or older adults
• When the goal of exercise is to improve muscle endurance
• To warm up and cool down prior to and after a session of exercise
• During slow-velocity isokinetic training to minimize compressive forces on joints

Near maximal or maximal loading. High-intensity exercise is indicated:
• When the goal of exercise is to increase muscle strength and power and possibly increase muscle size
• For otherwise healthy adults in the advanced phase of a rehabilitation program after a musculoskeletal injury in preparation for returning to high-demand occupational or recreational activities
• In a conditioning program for individuals with no known pathology
• For individuals training for competitive weight lifting or body building

PRECAUTION: The intensity of exercise should never be so great as to cause pain. As the intensity of exercise increases and a patient exerts a maximum or near-maximum effort, cardiovascular risks substantially increase. A patient needs to be continually reminded to incorporate rhythmic breathing into each repetition of an exercise to minimize these risks.

Initial Level of Resistance (Load) and Documentation of Training Effects

It is always challenging to estimate how much resistance to apply manually or how much weight a patient should use during resistance exercises to improve muscle strength particularly at the beginning of a strengthening program. With manual resistance exercise the decision is entirely subjective, based on the therapist’s judgment during exercise. In an exercise program using mechanical resistance the determination can be made quantitatively.

Repetition Maximum

One method of measuring the effectiveness of a resistance exercise program and calculating an appropriate exercise load for training is to determine a repetition maximum. This term was first reported decades ago by DeLorme in his investigations of an approach to resistance training called progressive resistive exercise (PRE). A repetition maximum (RM) is defined as the greatest amount of weight (load) a muscle can move through the available range of motion (ROM) a specific number of times.

Use of a repetition maximum. There are two main reasons for determining a repetition maximum: (1) to document a baseline measurement of the dynamic strength of a muscle or muscle group against which exercise-induced improvements in strength can be compared and (2) to identify an exercise load (amount of weight) to be used during exercise for a specified number of repetitions. DeLorme reported use of a 1 RM (the greatest amount of weight a subject can lift through the available ROM just one time) as the baseline measurement of a subject’s maximum effort but used a 10 RM (the amount of weight that could be lifted and lowered exactly 10 times) during training.

In the clinical setting, a practical, time-saving way to establish a baseline RM for a particular muscle group is for a therapist to select a specific amount of resistance (weight) and document how many repetitions can be completed through the full range before the muscle begins to fatigue. Remember, a sign of fatigue is the inability to complete the available ROM against the applied resistance.

Despite criticism that establishing a 1 RM involves some trial and error, it is a frequently used method for measuring muscle strength in research studies and has been shown to be a safe and reliable measurement tool with healthy young adults and athletes as well as active older adults prior to beginning conditioning programs.

Percentage of Body Weight as an Initial Exercise Load

• Universal bench press: 30% body weight
• Universal leg extension: 20% body weight
• Universal leg curl: 10% to 15% body weight
• Universal leg press: 50% body weight

PRECAUTION: Use of a 1 RM as a baseline measurement of dynamic strength is inappropriate for some patient populations because it requires one maximum effort. It is not safe for patients, for example, with joint impairments, patients who are recovering from or who are at risk for soft tissue injury, or patients with known or at risk for osteoporosis or cardiovascular pathology.

Alternative Methods of Determining Baseline Strength and a Beginning Exercise Load

Cable tensiometry and isokinetic or handheld dynamometry are alternatives to a repetition maximum for establishing a baseline measurement of strength. A percentage of body weight also has been proposed to estimate how much resistance (load) should be used in a strengthening program. The percentages indicated are meant as guidelines for the advanced stage of rehabilitation and are based on 10 repetitions of each exercise at the beginning of an exercise program. Percentages vary for different muscle groups.

Training Zone

After establishing the baseline RM, the amount of resistance (exercise load) to be used at the initiation of resistance training is often calculated as a percentage of a 1 RM for a particular muscle group. At the beginning of an exercise program the percentage necessary to achieve training-induced adaptations in strength is low (30% to 40%) for sedentary, untrained individuals or very high (80% to 95%) for those already highly trained. For healthy but untrained adults, a typical training zone usually falls between 60% and 70% of an RM. The lower percentage of this range is safer at the beginning of a program to enable an individual to focus on learning exercise form and technique.

Exercising at a low to moderate percentage of the established RM is also recommended for children and the elderly. For patients with significant deficits in muscle strength or to train for muscular endurance, using a low load, possibly at the 30% to 50% level, is safe yet challenging.

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