Association between spectral characteristics of paraspinal muscles and functional disability in patients with low back pain: a cohort study

Abstract

Objectives: Characteristics of muscle activity, represented by surface electromyography (EMG), have revealed differences between patients with low back pain (LBP) and healthy adults; how they relate to functional and clinical parameters remains unclear. The purpose of the current study was to examine the correlation between frequency characteristics of EMG (analysed using continuous wavelet transform (CWT) analysis) and patients' self-rated score of disability.

Design and setting: This is a case-control study with 15 patients with mechanical LBP without radicular symptoms. Patients were recruited from the orthopaedic clinic at Charing Cross Hospital. Ten healthy adults were recruited from the staff working in the hospital and associated university. Patients completed the Roland-Morris Disability Questionnaire (RMDQ) and bilateral EMG activity was obtained from erector spinae at vertebral levels L4 and T12. Subjects performed three brief maximal voluntary isometric contractions (MVICs) of the back extensors and the torque was measured using a dynamometer. CWT was applied to the EMG signals of each muscle in a 200 ms window centred around the peak torque obtained during the MVICs. The ratio (low/high frequencies) of the energy, the peak power and the frequency of the peak power were calculated for each recording site, averaged and correlated with the individual's RMDQ score.

Results: Patients had lower peak power (T12 and L4) and lower frequency of the peak power (at T12) than the healthy adults. Additionally, RMDQ positively correlated to the average ratio of energy at T12 (r=0.63; p=0.012), that is, greater self-rated disability corresponded to a dominant distribution of energy in the lower frequencies.

Conclusion: The current findings reveal alterations in EMG profile and its association with self-related back pain disability, suggesting that spectral characteristics of EMG reflect muscle function.

Keywords: back pain; electromyography; isometric; trunk muscles; wavelet transform.

Figure 1

Experimental set-up (top left) and peak torque and electromyography (EMG) activity recorded from left and right erector spinae (ES) at T12 (L ES and R ES) from a representative subject during one maximal voluntary isometric contraction (top right). A representative three-dimensional matrix of power, scale (frequency) and time calculated from a continuous wavelet transform analysis of the EMG data centred around the peak torque (bottom right). The bottom left figure represents the sum of the power collapsed across the time window. The area under the curve has been split according to the ratio 1.8 (see Data analysis) and the corresponding scale was used to split the curve derived from each patient dataset.

Figure 2

Group mean (±SEM) data showing (A) peak power and (B) its corresponding frequency from erector spinae (ES) muscles at vertebral levels T12 (ES T12) and L4 (ES L4) in the controls and in patients with low back pain. *p<0.05, between-group comparison. CLP, chronic lower back pain.

Figure 3

Correlation between the ratio of energy at low to high (low:high) frequencies and Roland-Morris Disability Questionnaire (RMDQ) in subjects with low back pain.