Comparison between elderly and young males’ lumbopelvic extensor muscle endurance assessed during a clinical isometric back extension test.
From: J Manipulative Physiol Ther. 2009 Sep;32(7):521-6
Low back pain is one of the leading causes of disability, contributing to 40% of all workdays lost in the United States of America. Because of the cost associated with work-related low back pain disabilities, most published studies have focused on the working-aged population. However, back pain and neck pain are also common musculoskeletal disorders affecting, each month, approximately one third of adults older than 70 years. The annual prevalence of chronic low back pain ranges from 44% to 84% in adults older than 65 years. In Canada, chronic low back pain is the third and fourth most important chronic health problem in women and men, respectively, older than 65 years. Women with severe chronic low back pain are 3 to 4 times more likely than other women to have difficulty with light housework tasks and 2 times more likely to encounter problems with mobility tasks, such as climbing stairs, walking, or lifting. In Canada, it is estimated that approximately 25% of the population will be older than 65 years by 2031. Anticipating the growing impact of the ageing population, a better understanding of chronic low back pain’s impact on physical capabilities in elderly people is important.
In working aged adults, chronic low back pain has been associated with increased fatigability of the lumbopelvic extensor muscles, as demonstrated by shorter back endurance test duration. In all back endurance protocols reported in the literature, isometric testing procedures with the trunk positioned in a weight-dependent position, such as the Sorensen test, may be most suitable in clinical settings. Weight dependent position tests of muscle endurance are cost-effective, easy to perform in a clinical context, and require no special equipment. The Sorensen test is conducted with subjects lying prone, the upper body unsupported in a horizontal position relative to the ground, and the lower limbs fixed by straps. This procedure has been found to be a reliable measure of position-holding time and can discriminate between subjects with and without low back pain.
Ageing has been related to changes in the neuromuscular system. Among these changes, loss of muscle force generation capacity, a slower firing rate of motor units, and a reduction in motor unit and muscle fiber number have been observed. Together with the loss of muscle fibers, a selective decrease in fast twitch fibers has been demonstrated with advancing age, leading to alteration of muscle fiber type proportion. This shift in fiber-type proportion appears to contribute to changes in muscle fatigability in healthy elderly individuals.
Very few studies have assessed back muscle endurance in elderly subjects. The current investigation aims to evaluate back extensor muscle fatigability in healthy elderly adults by quantifying endurance time during a clinical isometric back endurance test and the posttest decrease of lumbopelvic extensor maximal force. We hypothesized that elderly subjects will experience greater fatigability of lumbopelvic extensor muscles than young subjects.
The endurance time of back extensor muscles has become an important outcome in clinical decision making to guide exercise therapy interventions, particularly in chronic low back pain patients. Indeed, reduced back muscle endurance has been identified as a potential personal risk factor for developing low back pain. However, identifying endurance deficits requires normative values and an understanding of related psychophysical mechanisms. Very few databases on back muscle endurance time in elderly subjects are available. Furthermore, there is very limited information regarding the psychophysical aspects of performance during isometric back extensor testing in young adults and the elderly.
The experimental data provided in this study support the Sorensen test to evaluate back and hip extensor muscle endurance in elderly subjects. All the elderly participants in our study were able to perform the Sorensen test until exhaustion. One could consider that they had not achieved their endurance time limit to exhaustion because they may have underestimated their physical capabilities or have altered perceptive physical exertion. We tried to account for this with other measures, such as force reduction occurring with fatigue and effort perception on the Borg scale. Force was decreased after prolonged exertion and was an indicator of muscle fatigue. Both the elderly and young adult groups presented a significant decline of maximal isometric lifting force values after the Sorensen protocol. In addition, our results demonstrated that the amount of postfatigue reduction of isometric lifting force was similar between young adults and elderly subjects. The perceived exertion during the Sorensen test increased similarly in the elderly subjects and young adults, and we did not find any difference in Borg scores during the Sorenson test. All these results support the hypothesis that the elderly subjects achieved their endurance time limit during the Sorensen test.
The authors initially hypothesized that lumbopelvic extensor muscle endurance in the elderly subjects would be decreased during the weight-dependent isometric test due to age-associated muscle strength reduction. Although strength diminution was observed with age, we did not see a significant age effect on performance during the Sorensen test. McGill et al argued that age could impact the endurance time of torso muscles. Indeed, this study sample was composed of community-dwelling, nonsymptomatic, physically active elderly subjects that could have contributed to decrease the between-group difference. Maximal isometric lifting force was associated with back extensor endurance time in the young subjects, but no such relationship was clearly established for the elderly study participants. Thus, factors other than back muscle strength may have modulated endurance time for the elderly subjects during the fatiguing protocol in our study. Yassierli et al also found lower correlations between torso extension torque and back endurance time in elderly subjects compared to young adults, using a dynamometer in an upright position. Age-related adaptations, such as decreased muscle mass and increased type I muscle fiber proportion in elderly subjects, may limit maximal force-generation capacity in elderly subjects. These neurophysiologic factors may cause a reduction of the force-endurance relationship. In the study by Yassierli et al, back extension torque explained 74% to 78% and 46% to 63% of the endurance time variance in young and elderly subjects, respectively. In the present investigation, much lower correlation values between maximal isometric lift force and endurance time were obtained during the Sorenson test. The isometric lift task involves synergistic muscle activation of knee extensor muscles and lumbopelvic extensor muscles. Motor control strategies of multiple joints and muscles during isometric lifting may also explain the low correlation observed in elderly subjects.
This study is the first to assess lumbopelvic extensor muscle endurance in a clinical isometric back extension test in elderly subjects. Lumbopelvic extensor muscle endurance seems to be modulated by different neurophysiologic or mechanical factors with age, and in light of the present findings, normative data gathered from young adults should be interpreted with caution in elderly subjects. Future studies will be needed to assess lumbopelvic extensor muscle endurance in elderly male and female subjects with or without low back pain.
