The three elements of muscle performance strength, power, and endurance can be enhanced by some form of resistance exercise. To what extent each of these elements is altered by exercise depends on how the principles of resistance training are applied and how factors such as the intensity, frequency, and duration of exercise are manipulated. Because the physical demands of work, recreation and everyday living usually involve all three aspects of muscle performance, most resistance training programs seek to achieve a balance of strength, power, and muscular endurance to suit an individual’s needs and goals. In addition to having a positive impact on muscle performance, resistance training can produce many other benefits.

Potential Benefits of Resistance Exercise
• Enhanced muscle performance: restoration, improvement or maintenance of muscle strength, power, and endurance
• Increased strength of connective tissues: tendons, ligaments, intramuscular connective tissue
• Greater bone mineral density or less bone demineralization
• Decreased stress on joints during physical activity
• Reduced risk of soft tissue injury during physical activity
• Possible improvement in capacity to repair and heal damaged soft tissues due to positive impact on tissue remodeling
• Possible improvement in balance
• Enhanced physical performance during daily living, occupational, and recreational activities
• Positive changes in body composition: lean muscle mass or body fat
• Enhanced feeling of physical well-being
• Possible improvement in perception of disability and quality of life

Muscle strength is a broad term that refers to the ability of contractile tissue to produce tension and a resultant force based on the demands placed on the muscle. More specifically, muscle strength is the greatest measurable force that can be exerted by a muscle or muscle group to overcome resistance during a single maximum effort.Functional strength relates to the ability of the neuromuscular system to produce, reduce, or control forces, contemplated or imposed, during functional activities, in a smooth, coordinated manner. Insufficient muscular strength can contribute to major functional losses of even the most basic activities of daily living.

Strength training. The development of muscle strength is an integral component of most rehabilitation or conditioning programs for individuals of all ages and all ability levels. Strength training (strengthening exercise) is defined as a systematic procedure of a muscle or muscle group lifting, lowering, or controlling heavy loads (resistance) for a relatively low number of repetitions or over a short period of time. The most common adaptation to heavy resistance exercise is an increase in the maximum force-producing capacity of muscle, that is, an increase in muscle strength, primarily as the result of neural adaptations and an increase in muscle fiber size.

Muscle power, another aspect of muscle performance, is related to the strength and speed of movement and is defined as the work (force × distance) produced by a muscle per unit of time (force × distance/time). In other words, it is the rate of performing work. The rate at which a muscle contracts and produces a resultant force and the relationship of force and velocity are factors that affect muscle power.Because work can be produced over a very brief or an extended period of time, power can be expressed by either a single burst of high-intensity activity (such as lifting a heavy piece of luggage onto an overhead rack or performing a high jump) or by repeated bursts of less intense muscle activity (such as climbing a flight of stairs). The terms anaerobic power and aerobic power, respectively, are sometimes used to differentiate these two aspects of power.

Power training. Many motor skills in our lives are composed of movements that are explosive and involve both strength and speed. Therefore, re-establishing muscle power may be an important priority in a rehabilitation program. Muscle strength is a necessary foundation for developing muscle power. Power can be enhanced by either increasing the work a muscle must perform during a specified period of time or reducing the amount of time required to produce a given force. The greater the intensity of the exercise and the shorter the time period taken to generate force, the greater is the muscle power. For power training regimens, such as plyometric training or stretch-shortening drills the speed of movement is the variable that is most often manipulated.

Endurance is a broad term that refers to the ability to perform low-intensity, repetitive, or sustained activities over a prolonged period of time. Cardiopulmonary endurance (total body endurance) is associated with repetitive, dynamic motor activities such as walking, cycling, swimming, or upper extremity ergometry, which involve use of the large muscles of the body.

Muscle endurance (sometimes referred to as local endurance) is the ability of a muscle to contract repeatedly against a load (resistance), generate and sustain tension, and resist fatigue over an extended period of time. The term aerobic power is sometimes used interchangeably with muscle endurance.237 Maintenance of balance and proper alignment of the body segments requires sustained control (endurance) by the postural muscles. In fact, almost all daily living tasks require some degree of muscle and cardiopulmonary endurance.

Although strength and muscle endurance, as elements of muscle performance, are associated, they do not always correlate well with each other. Just because a muscle group is strong, it does not preclude the possibility that muscular endurance is impaired. For example, an individual in the workplace who is strong has no difficulty lifting a 10-pound object several times—but does the worker have sufficient muscle endurance in the upper extremities and the stabilizing muscles of the trunk and lower extremities to lift 10-pound objects several hundred times during the course of a day’s work without excessive fatigue or potential injury?

Endurance training. Endurance training (endurance exercise) is characterized by having a muscle contract and lift or lower a light load for many repetitions or sustain a muscle contraction for an extended period of time. The key elements of endurance training are low-intensity muscle contractions, a large number of repetitions, and a prolonged time period. Unlike strength training, muscles adapt to endurance training by increases in their oxidative and metabolic capacities, which allows better delivery and use of oxygen. For many patients with impaired muscle performance, endurance training has a more positive impact on improving function than strength training. In addition, using low levels of resistance in an exercise program minimizes adverse forces on joints, produces less irritation to soft tissues, and is more comfortable than heavy resistance training.

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