Published online Nov 18, 2021. doi: 10.5312/wjo.v12.i11.816
Peer-review started: May 9, 2021
First decision: June 16, 2021
Revised: June 27, 2021
Accepted: September 30, 2021
Article in press: September 30, 2021
Published online: November 18, 2021
A significant predictor of low back pain (LBP) recurrence is ‘previous LBP’. Partly, this may be due to persisting neuromuscular system activation deficits linked to strength deficits, as well as endurance deficits in patients with recurrent LBP (RLBP) even during periods of symptoms remission. LBP management clinical guidelines propose muscle re-conditioning as a prerequisite for successful management of recurrences.
Paraspinal muscle strength and endurance deficits require reliable monitoring. To overcome patient motivation or cognitive-related concerns affecting maximal strength testing, as well as endurance testing with prolonged contractions to complete exhaustion, alternative methods have been proposed in patients with RLBP, in order to establish the contribution of those parameters in neuromuscular deconditioning, to limit further recurrences.
As electromyographic (EMG)-based frequency and amplitude domain time dependent alterations, linked to the endurance characteristics of the muscles monitored have not been universally obtained for the paraspinals, a primary objective of this study was to determine the reliability of those measures. The reliability level of maximal paraspinal muscle strength performance was also examined. Furthermore, the discriminative validity of paraspinals muscle strength and time-dependent EMG frequency and amplitude domain alterations was tested.
A custom-made isomyometer was utilised to initially assess the maximum voluntary isometric contraction (MVIC) of the paraspinals in the upright trunk position. Subsequently, short duration (60-s) isometric contractions at a submaximal level of contraction (60% of MVIC) were employed, to determine the EMG-time dependent frequency [initial median frequency (IMF) and median frequency (MF) slopes] and amplitude changes [root mean square (RMS) slopes] of the paraspinal muscles with recording electrodes placed at 4 muscle sites (L2/3 and L4/5, bilaterally). The most reliable parameters were used further to test the between populations discriminative ability of the method.
For both groups, MVIC presented excellent intraclass correlation coefficient (ICC) reliability values, although statistically significant between-day increases (P < 0.01) were recorded, within a margin of 10%; test-retest error was increased for patients compared to healthy participants. The EMG reliability of the frequency parameters was good (MF slopes) to excellent (IMF), however for the amplitude parameter (RMS slope) it was poor, for both groups. Statistically significant less MVIC and less steep MF slopes were registered for the patient group. These findings confirm previous research in the field, however in a larger population of participants with a history of RLBP and a sufficiently large comparison group of healthy participants.
Although EMG time-dependent frequency parameters presented highly significant differences between the two groups, these were in the opposite than the expected direction. The validity of this finding is enhanced for two reasons; the between-group differences in MF slopes remained after statistically controlling for possible confounders and these differences were confirmed at all muscle sites monitored. Apparently, alterations in the organization of the motor commands in patients with RLBP can additionally influence the manifestations of EMG-related time-dependent indices. Therefore, the alterations in the EMG-frequency spectrum under sustained contractions cannot only be considered as indicators of peripheral fatigue or peripheral muscle atrophy.
This methodology of EMG-related alterations followed in the current experiment is reliable. The validity of the between-group differences obtained between patients with RLBP and healthy participants requires further study. In order to explain the significance of the current findings, the history of LBP has to be taken into consideration. Therefore, results from patients with varying amounts of LBP-related disability and disease duration are required, in conjunction with detailed imaging methods of peripheral muscle state and recording of the different patterns of activation utilised under controlled experimental conditions or less controlled functional tasks. Furthermore, the effect of exercise on EMG-related frequency parameters and whether the alterations registered post-exercise in the frequency domain correspond to less LBP recurrences requires examination from a clinical viewpoint.