The ability of the muscles to apply maximum force in the least amount of time.

Endurance

Endurance is related to the ability to perform work over an extended period of time. Children, for example, can play actively for hours. We need endurance to perform repetitive activities of daily living, such as stirring food while cooking, using a blow dryer to dry our hair, or walking up steps. Recreational and job-related tasks also often require a high level of endurance.

Endurance can be affected by an individual muscle, a muscle group, or the total body. Total body endurance usually refers to cardiopulmonary endurance, reflecting the ability of the heart to deliver a steady supply of oxygen to working muscle. Muscle endurance reflects the ability to sustain repeated muscle contraction and is related to muscle strength. Developmentally, muscle endurance has been shown to increase linearly in boys between 5 and 13 years of age, after which a spurt is observed. A steady linear increase in muscle endurance is seen in girls.24 Endurance decreases in older adults, with elite senior marathon runners demonstrating diminished endurance after the age of 50 years.43,44

Endurance

Endurance is the ability to sustain an activity for extended periods of time and usually refers to aerobic ability. Local muscle endurance is best described as the ability to resist muscular fatigue and describes how a given type of contraction can be sustained, typically measured in terms of the number of repetitions. Like aerobic endurance, muscular endurance relies on aerobic metabolism. Muscle endurance and muscle strength, both important for everyday life, together constitute muscular fitness. Improving muscular fitness makes everyday activities easier and decreases the risk of injury with activity. Activities that improve cardiovascular endurance also improve muscular endurance. Muscular endurance training programs can produce small, but measurable, gains in muscular strength. As with any fitness program to initiate specific gains, improving muscular endurance requires the application of the overload principle to muscular endurance activities.

Examples of muscular endurance include how many times a full squat, sit-up, or bicep curl with light to moderate weight before breaking form can be performed. Activities that require muscular endurance include sustained walking or running, cycling, resistance training, calisthenics, swimming, circuit training, aerobics, dance, and rope jumping. There are various protocols for muscular endurance training. But, in general, the load applied is relatively low and the number of repetitions is high, as in climbing the stairs.2 When training for muscular endurance, the number of repetitions and the length of time the muscle or group of muscles contract are more important than the resistance/load or intensity/speed at which the physical activity is performed. However, a minimum training threshold (intensity, frequency, and duration) is required for improvement.

Aerobic endurance

Endurance is another important factor that should be carefully determined. Assessing general endurance provides an idea about an older adults’ ability to generate adequate force during tasks that require continued effort, such as walking for a long distance. The 6-minute walk test (described in Chapter 17) is a commonly used quantitative test to assess endurance. The 6-minute walk test can assess endurance in frail older adults.86 In this test, the patient walks up and down a premeasured walkway, for example, a hospital corridor, at his or her normal pace for 6 minutes, resting as needed. The distance covered after 6 minutes is recorded as well as perceived exertion. Fatigue in older adults has been related to increased mortality rates.87

5 Multiple Types of Exercise Endurance Tests

Cardiorespiratory endurance, or aerobic endurance is: “the ability of the whole body to sustain prolonged, rhythmic exercise” (quotations are taken from Wilmore and Costill82 in this paragraph). Two tests of cardiorespiratory endurance—submaximal and maximal—appear in the literature. The test of “Submaximal (cardiorespiratory) endurance capacity is more closely related to actual competitive endurance performance and likely is determined by both the individual's VO2max and his or her lactate threshold.” Maximal cardiorespiratory endurance capacity (aerobic power) is defined as “the highest rate of oxygen consumption attainable during maximal or exhaustive exercise.” For animal studies to mimic human standards for maximal cardiovascular endurance tests, a determination of VO2max or VO2peak can be obtained, as described by Britton and Wisloff.83–85 Maximal endurance capacity tests are ramped by nature, progressively increasing workload every 3–5 min past the lactate threshold until approaching or exceeding maximal workload. When exercise capacity tests are performed without VO2 determinations as the “gold” standard for proof of maximal cardiorespiratory endurance capacity, the test endpoint relies upon an animal's motivation to avoid electric shock, which is not a surrogate for VO2max or VO2peak. Muscular endurance is “skeletal muscle's ability to repeatedly develop and sustain near-maximal or maximal forces.”

Cardiorespiratory endurance capacity declines with aging. For example, times for top marathoners increased exponentially with age. For ages 30 → 50 → 70 → 90 years old, pace (min/mile) for men increased from 4:46 → 4:55 → 6:02 → 8:22, respectively, and for women from 5:10 → 5:36 → 7:26 → 11:04, respectively.86

Endurance

Endurance refers to the ability to sustain a force and to repeat the application of a force over time. Aspects of the AT system design can minimize the effect of fatigue by requiring low-energy expenditure for activation. An example is a joystick for a wheelchair that requires very little travel and small force. In some neuromuscular disabilities, such as myasthenia gravis, initial strength may be within a normal range. However, as the individual repeats a movement, there is a continual decrease in performance until total fatigue occurs. Other disorders that can involve muscle weakness (e.g., ALS, MS,) can also limit endurance. Pain (chronic or associated with movement) can also affect endurance. Sometimes pain is consistent; in other cases, it changes throughout the day in response to activity or medication.

Protein Needs of Endurance Athletes

Endurance athletes often have different muscle mass goals than strength athletes. They mainly desire to maintain an optimal lean muscle mass that will complement but not hinder performance and therefore do not often train to promote large increases in muscle mass.5 Because protein contributes only 1% to 6% to total energy costs during endurance exercise,49 it is not the main dietary focus for many endurance athletes. Whether endurance exercise will increase daily protein needs depends on the intensity of training, duration of training, nutritional intake of the athlete (i.e., lower total energy or carbohydrate intake, or both, put a greater reliance on protein for energy), and state of training (i.e., with training, adaptations occur that appear to lower the body's use of protein as an energy source during endurance exercise).49 Additionally, research has shown that when endurance athletes consumed carbohydrates during exercise, this attenuated any increase in amino acid oxidation that would naturally occur with exercise as carbohydrate stores were used up.50 Total energy intake is also an important factor determining how much protein an endurance athlete will oxidize during exercise. With adequate energy intake, the body's reliance on protein as a fuel source during exercise decreases.51 These two latter points highlight the importance of an endurance athlete meeting both his or her daily total energy and carbohydrate needs for optimal performance and sparing protein for muscle mass repair and maintenance.

Overall, recreational endurance athletes—those participating in low to moderate intensity and duration exercise—likely do not have higher protein needs than the general population. Those athletes participating in moderate inten-sity endurance exercise (4 to 5 days weekly for longer than 60 minutes) likely have approximately 25% higher protein needs than those of the general population, while elite endurance athletes have needs approximately double those of the general population (see Table 17-4). Women's protein needs have been suggested to be close to 15% to 20% lower than those of males, although more research is necessary to confirm or refute this.49

5.8 Summary

Endurance improves well in response to training; however, genetic factors also determine success. Endurance can be divided into categories such as aerobic, anaerobic lactic, and anaerobic alactic due to differences in metabolic processes. The main energy sources for ATP synthesis are fat and carbohydrates in aerobic, carbohydrates in anaerobic lactic, and CP in anaerobic alactic processes. Aerobic metabolism provides more energy, thus the longer that aerobic metabolism can provide energy and burn fat, the more successful an endurance athlete can be. Therefore, VO2max mainly determines endurance. At a young age males have an average VO2max 50 mL/kg/min, professional athletes have 70–85 mL/kg/min, and females have values 10% less. VO2max is determined by cardiac output, RBC count, A/VO2 difference, vascularization of skeletal muscle, muscle fibers type, and the amount of mitochondria and the activity of oxidative enzymes.

Intensity associated with anaerobic threshold is another important factor that determines endurance. It marks an intensity level where aerobic and anaerobic processes are in equilibrium; over this point, anaerobic processes dominate. This point is at an intensity of 20%–30% higher (VO2max) in professional athletes compared to less trained individuals. The main goal of endurance training is to increase the anaerobic threshold, because this improves training efficiency. Determining factors of VO2max react differently to different training intensity and duration, and genetic factors also determine the VO2max of an individual.

▪ HIP AND KNEE ARTHRITIS

Endurance and strengthening exercises have been prescribed for hip and knee osteoarthritis and have been studied in the literature. Quadriceps weakness is known to correlate with the presence of arthritis, and with greater pain and disability, but not with progression of the disease.13 Both endurance and strengthening programs improve pain and function when prescribed for persons with hip or knee arthritis. Additionally, participation in endurance activities has been shown to decrease depressive symptomatology in this patient group.13 As discussed, transient increases in pain due to exercise occur and the likelihood of injury as a result of exercise participation is very small.13 One of the most fundamental exercises in a knee arthritis physical therapy program is the quad set, in which the patient lies or sits with the affected knee extended and the shank supported, and isometrically contracts the quadriceps muscle. This exercise has been shown to reduce pain and improve function (Fig. 127-2).28 Straight leg raises, quadriceps and gluteal strengthening, hamstring stretches, joint conservation instruction and cycling or water aerobics can be added to the script, as well as gait training with a cane (Fig. 127-3).

Endurance

Endurance refers to the ability to sustain a force and to repeat the application of a force over time. In some neuromuscular disabilities, such as myasthenia gravis, initial strength may be within a normal range. However, as the individual repeats a movement, there is a continual decrease in performance until total fatigue occurs. Aspects of the AT system design can minimize the effect of fatigue in several ways by requiring low-energy expenditure for activation. An example is a joystick for a wheelchair that requires very little travel and small force. Careful consideration of the strength and endurance available to move an effector is crucial to the successful application of AT systems. The presence of pain also affects controller use. Sometimes pain is consistent; in other cases, it changes throughout the day in response to activity or medication.

Cardiorespiratory

Endurance performance is dependent on the ability of the cardiorespiratory system to deliver and use oxygen at a rate that meets the energetic demands of muscle activity. In community-dwelling adults, the age-associated decreases in maximal aerobic capacity (VO2max) increase progressively from 3% to 6% in the third and fourth decades of life to greater than 20% per decade after age 70 years.37 In master athletes, reductions in VO2max range from −1% to −4.6% per year for men and −0.5% to 2.4% per year for women beginning in the fourth decade.38 The relative rate of decline in VO2max is similar between endurance athletes and sedentary adults (Fig. 28.6). However, in absolute terms, athletes experience greater decrements in VO2max compared with sedentary counterparts.3 The decline in VO2max in athletes is highly dependent on the continued magnitude of the training stimulus (i.e., volume and intensity).2,7,8,39 The majority of the athletes reduced their training levels over time, resulting in longitudinal reductions in VO2max two to three times as large as those predicted by cross-sectional analyses or longitudinal responses seen in their sedentary peers.3,39 Despite similar relative and greater absolute reductions in VO2max, masters athletes still possess superior cardiorespiratory fitness levels compared with age-matched nonathletes.

The capacity of the cardiorespiratory system to transport and use oxygen depends on both central and peripheral factors (Fig. 28.7).2,7 Among the central factors contributing to endurance performance, the largest declines with age are seen in cardiac output (stroke volume [SV] × heart rate [HR]) (Table 28.3).2 SV and HR experience similar responses with about a 10% decline with age when young (28 years) and master (60 years) endurance-trained athletes were compared.2 Declines in maximal HR (HRmax) occur at a rate of an estimated 0.7 beats/min per year beginning during early adulthood.2 Although HRmax is a major contributor to cardiac output, decrements in HRmax in master male and female endurance runners was not correlated with change in VO2max.38 In addition to cardiac output, ventilation also seems to play a key role in the decline in VO2max in aging elite runners.40 For example, Everman and coworkers40 demonstrated that decreases in maximal minute ventilation (VEmax) was a significant predictor of the decrease in VO2max over a 45-year span.40

The reported peripheral factors contributing to reduced VO2max in senior athletes consist of lower arteriovenous oxygen difference (a-vO2 diff) and loss of muscle mass.7 The age-related changes associated with peripheral factors are slightly less than those observed for central factors.2 The peripheral factors determine the muscles’ capacity to extract oxygen from the blood to be used for energy synthesis by the mitochondria. The mechanisms of age-related muscle loss have been discussed previously in this section (see the Neuromuscular section). The changes in a-vO2 diff with age is influenced by oxygen availability (i.e., oxygen delivery), oxidative enzyme concentrations, and mitochondrial and capillary density. For example, in highly active male endurance cyclists (aged 55–79 years), capillarity density and capillary-to-fiber ratio of the vastus lateralis were significantly correlated with VO2max.41 Thus, interventions that promote peripheral adaptations to maintain or improve muscle oxygen delivery and utilization may be advantageous for delaying declines in aerobic capacity or endurance performance.