Funded By:
Exercise testing in this review is defined as the measurement of cardiopulmonary responses produced by brief and limited treadmill or bicycle exercise. Aerobic exercise training is defined as prolonged high repetitive, low resistive exercise such as walking, running and swimming. This is in contrast to high resistive, low repetitive strengthening activities such as weight lifting. Results covered in this review were based, in part, on data obtained during a ten-year Rehabilitation Research & Training Center study (University of California, Davis.)
Responses and Adaptations in Slowly Progressive Neuromuscular Diseases.
From the standpoint of tolerance to high repetitive aerobic exercise, there are two types of slowly progressive NMDs. The rare metabolic myopathies, such as carnitine palmityl-transferase and carnitine deficiency, are characterized by fatigue with muscle bulk and strength preserved. In the other slowly and rapidly progressive NMDs, such as muscular dystrophy, spinal muscular atrophy (SMA), hereditary motor sensory neuropathy (HMSN), limb-girdle syndrome (LGS) and amyotrophic lateral sclerosis (ALS), there is a marked and progressive loss of strength and muscle mass. In a University of California, Davis, Research & Training Center study of 20 individuals with HMSN (7), LGS (3), and myotonic dystrophy (10), subjects underwent graded exercise testing (GXT) using a semi-recumbent cycle ergometer. A continuous progressively incremental protocol was used to achieve a volitional maximal aerobic effect. This technique, based on the criteria of age related to 70% of maximum heart rate, normalized the exercise testing effort for each individual. Results, measured at 100% peak VO2, and compared to predicted reference values, when available, were as follows:
Work capacity (peak power output): 103 +/- 37 Watts. 60% of predicted.
Cardiovascular function:
|
- Peak heart rate:
|
151 ± 15 beats/min. 80% predicted and proportional to work rate. |
|
- Peak blood pressures
(systolic/diastolic): |
167 / 93 ± 32 / 19 mm Hg. Proportional to work rate. |
Respiratory/metabolic function:
| - Peak O2 uptake (VO2): | 18 ± 5 ml/kg/min. 50% of predicted. |
| - Peak VCO2: | 23 ± 1.4 ml/kg/min. |
| - Respiratory exchange ratio: | 1.3 ± 0.04 |
Pulmonary function:
| -VC/FEV1 (Vital capacity/ forced expiratory volume): |
3.8 / 3.1 ± 1.1 / 0.8 liters 90% predicted. |
| -Ventilatory rate: | 43 ± 4 liters/min. 50% predicted. |
| -Breathing frequency: | 40 ± 2.6 breaths/min. |
| -Tidal volume: | 1.15 ± 0.18 liters/breath. |
Disease duration was 20 ± 12 years, age 37 ± 13 years, and peak concentric isokinetic leg strength 102 ± 59 N-m; about 50% predicted compared to non-disabled controls. There was a high correlation between peak power output (PPO) and leg strength. Pulmonary function measurements, heart rate and blood pressure were within predicted normal limits. These results of significantly reduced work capacity (PPO), VO2, and ventilatory rate, as well as decreased cardiac output, are similar to those reported by other investigators (Carroll: Archives Neurology 36:457, 1979; Florence: Medical Science in Sports and Exercise 16:460, 1984).
While some of these changes with exercise testing may be found in the metabolic myopathies, there are, in addition, disproportionate increases in serum creatine kinase, lactate and pyruvate, a higher anaerobic threshold, and failure to achieve a steady state during exercise testing (Brooke: Neurology 29:636, 1979; Carroll: Muscle & Nerve 1:103, 1978; Haller: Neurology 33:1283, 1983).
In the University of California RRTC study, 8 of the 20 individuals underwent a submaximal 12-week aerobic walking program to determine if a training program would result in increased functional capacity, would be safely tolerated, and whether subjects could adhere to a self-monitored home-based program (Wright: Aerobic walking in slowly progressive neuromuscular diseases: Effect of a 12-week walking program. Archives Physical Medicine & Rehabilitation, in press). Subjects walked 15-30 minutes 3-4 days per week at 50-60% of their heart rate reserve, and underwent graded exercise testing before and after the training program. In addition to the laboratory measurements, subjective responses of the degree of exertion were recorded as a rating of perceived exertion (RPE). Participants reported completing 83 ± 21% of their prescribed exercise sessions, and the average RPE was between fairly light and somewhat hard on the RPE scale. None reported excessive soreness or fatigue. There were significant decreases in submaximal heart rate and systolic blood pressure, and nonsignificant increases in peak power output and VO2. The results from this study were consistent with those from previous investigations (Florence: Endurance exercise in neuromuscular disease. In Serratrice: Neuromuscular Diseases, 1984; Jones: Journal of American Medical Association 261:2255, 1989.)
Responses and Adaptations in Rapidly Progressive Neuromuscular Diseases.
There have been several studies regarding responses to exercise testing in the rapidly progressive NMDs. In Duchenne dystrophy (DMD), exercise testing on a treadmill was limited by reduced cardiopulmonary capacity and peripheral oxygen utilization, with marked differences in response to submaximal and maximal exercise testing. During both, work capacity, maximum oxygen uptake, cardiac output, and pulmonary ventilation were lower than in non-disabled controls. During maximal exercise testing, work rate, endurance, heart rate, and respiratory exchange ratio were also reduced (Sockolov: Archives Physical Medicine & Rehabilitation 58:195, 1977). Even a few minutes of passive exercise testing resulted in an increase in cardiac cost (Eickelberg: Journal of Human Ergology 3:157, 1975).
Similar findings to the University of California DMD study have been reported in amyotrophic lateral sclerosis. Maximum oxygen uptake and work capacity decreased in proportion to the loss of muscle strength and functional score, and the oxygen cost of even submaximal exercise increased (Sanjak: Neurology 37: 1217, 1987).
There have not been any reported studies on adaptations to aerobic high-repetitive exercise training in individuals with rapidly progressive NMDs.
Summary.
Diseases affecting skeletal muscle appear to produce two patterns of exercise testing intolerance (Haller: Medical Science in Sports & Exercise 16:456, 1984; Lewis: Med Sci Sports Exerc 16:466, 1984). In disorders of muscle energy metabolism, muscle bulk and strength are relatively well preserved, but an imbalance in muscle energy production and utilization during exercise testing and training results in exertional muscle pain, cramping, weakness, or fatigue. Dynamic exercise is limited in disorders of oxidative metabolism and static exercise is impaired by disorders of anaerobic glycolosis. In the other neuromuscular disorders, there is a progressive loss of skeletal muscle fibers that results in reduced VO2 and power output (physical work capacity) due to loss of functional muscle mass.
Training effects, while lower, are similar to those found in non-disabled individuals indicating similar improvement in peripheral and central adaptations to exercise programs. Regardless, studies show that moderate intensity aerobic exercise programs can safely produce improvements in physical work capacity, aerobic capacity, and in several cardiovascular variables in individuals with NMD. Any improvement in cardiopulmonary fitness has the potential to reduce the strains of daily living by increasing reserve capacities, allowing the physiological demands required for activities of daily living to be met more effectively.
