Differentiating temporal electromyographic waveforms between those with chronic low back pain and healthy controls
Introduction
Decreased lumbar spine stability has been associated with mechanical low back pain (LBP) [1], [2] and both biomechanical models and empirical testing show that a combination of trunk muscle forces, through appropriately timed muscle activity is required to maintain joint stability [1], [2], [3], [4]. Coactivating agonist and antagonist muscles in a synergistic manner is considered a key neuromuscular control strategy contributing to improved spinal stability [3], [4]. Muscle synergy in this sense includes agonists and antagonists since both contribute to the net moment about a joint to produce movement [5], [6] and to the muscle forces that provide stiffness for joint stability [1].
Onset delays in the abdominal and trunk extensor activity during arm raising tasks [7] and trunk extensor muscles during trunk flexion and extension tasks [8] provide evidence of an impaired feed forward control strategy for the LBP subjects. Amplitude differences among the abdominal EMG recordings [9] has been attributed to muscle substitution [10] associated with decreased muscle strength or endurance in selected muscles [11]. Onset times and amplitudes are discrete measures from the EMG waveforms that do not necessarily characterize the response of the muscle to changing perturbations. Different muscles could have the same onset times, or could be activated to the same relative amplitude, but have very different temporal patterns. The focus of this paper was to evaluate the temporal EMG profiles from different muscles in response to a sequence of perturbations. The problem with quantitatively comparing temporal patterns has been the huge volume of data generated. Statistical pattern recognition techniques based on orthogonal expansion theory [12] have been applied to successfully reduce EMG profiles of lower limb muscles during walking [13], [14] and the trunk muscles during flexion and extension tasks [15]. The present study used pattern recognition methodology to examine a task that was designed to challenge the neuromuscular system to respond to a sequence of perturbations. The exercise sequence referred to as the trunk stability test (TST) [16] includes an abdominal hollowing maneuver and a series of leg-lifting and lowering perturbations while maintaining a neutral lumbar–pelvic spine [17], [18]. The purpose of this study was to determine whether differences existed in the temporal activity patterns among trunk and abdominal muscle sites and between a group with LBP and a healthy control group while performing the TST.
The two main hypotheses examined whether there were significant differences in the temporal EMG patterns: (1) among the seven muscle sites illustrating a lack of synergistic coactivation; and (2) between the groups illustrating different neuromuscular control strategies between those with and those without LBP to perform a task aimed to challenge lumbar spinal stability.
Section snippets
Subjects
Subjects were recruited through advertisements posted at a military base throughout various departments, at the hospital, the gymnasium and on the internet and were the same as in two previous studies [16], [19]. Both groups included males, between the age of 20 and 55 years old. The healthy control (CON) group had no known neuromuscular, orthopaedic or cardiovascular conditions, or a history of LBP. Chronic LBP was defined as “pain between the lower ribs and gluteal folds, with minimal
Results
Twenty-four men were included in the CON group and 14 men in the LBP group. Descriptive data are in Table 1. Age was significantly different (P<0.05) between the two groups, however, none of the other demographic variables were statistically significant (P>0.05) as indicated in Table 1. Seventy-five percent of the CON group and 50% of the LBP group could perform the trunk raising forward exercise. The mean number of years reported for having LBP was 7.7 (SD 4.8), and the range was from one to
Discussion
The data reduction technique allowed us to test differences in the EMG temporal profiles between groups and to quantify the degree of synergistic activation among muscles to perform the leg-lifting and leg-lowering task without pelvic movement or increased lordosis. These differences in the temporal EMG patterns provide evidence that the LBP group did not coactivate the seven muscles examined in a synchronous temporal manner, whereas control subjects coactivated all seven sites with the same
Conclusion
Three principal patterns captured 96% of the variance in temporal EMG profiles recorded from seven muscle sites during the performance of the TST level 1 exercise. Quantitative differences were detected in the temporal patterns both between LBP and CON groups and among muscle sites for the LBP group only. These data support that the CON subjects used a very simple activation pattern, activating both agonist and antagonist muscles with a similar temporal pattern to perform this task while
Acknowledgements
The authors would like to acknowledge the financial assistance provided by the Canadian Department of National Defense. We would like to thank J. Crouse for his technical assistance, D. Braha, J. Grainger and R. Aucoin for their assistance with data collection and analysis. Finally thanks to all of the participants for their time and effort.
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