Physical activity level has been associated inversely with the prevalence of low back pain. Reported values of maximal oxygen uptake (VO2max) in patients with chronic low back pain are contradictory. Some authors consider that this is due to the interference of limiting factors of maximum effort during ergometric tests.
Low back pain has been, and remains, a serious health problem in today's society. The highest prevalence has been reported in industrialized countries with high levels of sedentarism, while it is almost nil in rural communities of developing countries where physical activity levels are significantly higher. These findings have suggested an inverse relationship between fitness level and the risk of back pain.
Although only a small percentage of patients with low back pain evolves to chronicity, this group accounts for 75-90% of societal costs of back pain (Nachemson 1992). Some authors have blamed physical inactivity and its consequent physical deconditioning as a contributing factor for the evolution of back pain towards chronicity (Hazard 1989, Mayer 1988). Treatment of low back pain is a challenge in medical practice and although it has been proven not be useful as a part of treatment, prolonged rest is still frequently prescribed. At present, rehabilitation of patients with chronic low back pain (CLBP) is based on functional restoration programs that prescribe physical training, based on an assumption that patients are deconditioned.
Maximal aerobic capacity
Maximal oxygen uptake (VO2max) is defined as the maximal capacity to carry oxygen from ambient air to the mitochondria for energy production in the body, and it is considered to be the physiological parameter that best represents the maximal physical capacity of an individual. A high VO2max is associated with better quality of life and decreased risk of disease. The VO2max value depends on how active the subject is: the greater the physical activity, the higher the VO2max, and, conversely, the greater the physical inactivity, the lower the VO2max.
Data reported in the literature concerning the VO2max in patients with CLBP are contradictory (Smeets 2006). Some authors have found the VO2max in CLBP patients to be equivalent to that of physically active healthy subjects. Others have reported the VO2max value in CLBP patients to correspond with that of poorly conditioned healthy subjects. However, performing a maximum effort during an incremental test does not guarantee that the subject has achieved the maximum value of VO2.
The criteria for having achieved VO2max (Howley 1995) are fairly simple: VO2max is considered valid when the patient achieves the primary criterion or two of the three secondary criteria:
- Variation < 2 ml/kg/min with respect to the VO2 from the last test charge increment
- Lactatemy >= 6.7 mmol/L
- Respiratory exchange ratio >= 1.13
- Maximal heart rate >= 90% of the theoretical maximum
If the individual does not meet the primary criterion or at least two of the secondary criteria, the obtained value is considered VO2peak.
Limiting factors of maximal effort
Previous studies have reported the inability of some patients with CLBP to perform a maximal effort during the ergometric test and suggested the interference of limiting factors of maximal effort in the measurement of VO2max. More recent studies have attempted to determine the level of physical fitness and the interference of limiting factors of maximal effort in the measurement of VO2max in patients with CLBP.
Patients in one study (Duque 2009) underwent an incremental test until exhaustion, in which the above-mentioned criteria were taken into account. In addition, exhaustion and reaching at least 90% of the maximum theoretical heart rate were established as the “maximal effort” clinical endpoints. Back pain, quadriceps pain/fatigue, and pedaling difficulty were considered “limiting factor” endpoints of maximal effort. The authors demonstrated the existence of an association between obtaining a VO2max value and the presence of “maximal effort” endpoints and an association between obtaining a peak value of VO2 and the presence of “limiting factors” endpoints.
The most prevalent limiting factor of ergometric tests in this population was quadriceps pain/fatigue; back pain was less frequently reported as a limiting factor. Keeping in mind the achievement criteria of VO2max and the limiting factors of maximal effort, this study also showed that the VO2max achieved by these patients was similar to that reported by several authors and this value, compared with the average healthy population, was equivalent to that of healthy but poorly conditioned subjects. The most likely cause of deconditioning appears to be the progressive decrease in physical activity due to pain. In the study, a significantly lower VO2max in women than in men was attributed to lower levels ??of physical activity and lower values of cardiac output and hemoglobin content.
During the transit of O2 from lungs to mitochondria, multiple organs and physiological systems, involved either independently or coupled, may be affected by physical inactivity.
The reported harmful effects of physical inactivity are mainly muscle atrophy and reduced functional capacity resulting from peripheral muscle fatigue (Paddon-Jonnes 2006). High levels of lactatemia, phosphocreatine depletion, impairment in energy substrate supply, and metabolic and structural disorganization of contractile proteins are the most frequently involved mechanisms. The decreasing use of the muscles in their full length capacity also seems to be related to an increased stiffness and contractures secondary to the damage of collagen tissue, which in turn forces the use of anaerobic metabolism derived from a decreased muscle blood perfusion. Low levels of strength and power affect the working capacity of antigravity muscles responsible for maintaining a well-aligned posture. As a result, the subject experiences a decline in participation in physical activities, both at work and at leisure, consequently leading to overload and muscle spasm during the performance of low-intensity exercise. Coordination disorders make the subject more vulnerable to muscular, ligament, and even intervertebral disk injuries.
The negative effects of bed rest on the cardiovascular system primarily involve the distribution of blood pressures and volumes. Bed rest causes a redistribution of body fluids, leading to a decrease in blood volume and plasma-mediated neurohumoral mechanisms; this explains the orthostatic intolerance. Among the cardiovascular implications, some authors propose a decreased stroke volume coupled with a compensatory increase in heart rate mediated by high levels of norepinefrine, which leads, in turn, to increased sensitivity of ?-adrenergic receptors and a decrease in vagal tone. The excessive reduction in stroke volume when sitting or standing in the upright position has been attributed to a decrease in size and compliance of the left ventricle due to physiological atrophy mediated by the myocardial plasticity mechanism.
At present, functional restoration programs for the treatment of chronic low back pain are based on intensive physical exercise programs. These programs are designed on the assumption that patients with CLBP are physically deconditioned. Therefore, it is very important to ensure an accurate exercise program prescription, taking into account the subject’s maximum aerobic capacity, as well as the limiting factors that may affect the accurate measurement of this physiological variable.
Measurement of VO2max as a part of the diagnostic workup for a patient with chronic low back pain is clinically important because it allows the establishment of the patient´s fitness level. Two main criteria should be taken into account during the performance of maximum ergometric test in patients with CLBP: the concept of VO2max in opposition to VO2peak, and the establishment of the clinical endpoint of the test. The use of these two criteria will allow a more accurate analysis of obtained data to contribute to the development of a suitable functional restoration program and to facilitate the analysis of improvements obtained with a physical exercise-based treatment. This concept could be extended in general to programs designed to combat sedentary lifestyles and their associated diseases.
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Paddon-Jones D, Sheffield-Moore M, Cree MG, Hewlings SJ et al. Atrophy and impaired muscle protein synthesis during prolonged inactivity and stress. J Clin Endocrinol Metab. 91 (2006) 4836-41.
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