The aim of the present study was to determine the effect of SM on the EMG activity of the paraspinal muscles and the duration of such effect over a 30 min period of time.
Interestingly, results showed that EMG activity at L2 increased only in the control group after 30 minutes. A gradual increase in the VAS scores was also observed in the same group over the 30 minute period.
The present results partly differ from previous studies. Using the same experimental paradigm, Lalanne et al.
 observed a decrease in EMG activity at the L2 level immediately following a lumbar SM at L3 level
.Similar results were also reported by Bicalho at al.
 who showed decreases in EMG activity at the L5-S1 level following a SM at L4-L5 segment
. Such results have not been reproduced in the present study. The changes observed in the above mentioned studies, however, indicated that EMG responses were mostly segmental (changes observed only at the contacted or adjacent spinal segment)
[15, 16]. Despite the fact that decreases in EMG responses immediately following SMT were not observed in the present study, significant group differences observed during at the 30 min assessment were present for the L2 segment (SMT was performed at L3) whereas changes were not observed at L5. Interestingly, these changes were not associated with changes in lumbo-pelvic kinematics. In a recent review, Millan et al.
 reported that none of the selected studies of the lumbar spine showed an immediate effect of SMT on lumbar range of motions. Future studies should include assessment for an extended period of time (hours and days) in order to better document the association between neuromuscular response to SMT and changes in lumbo-pelvic kinematics.
The combination of increased paraspinal EMG activity and increased pain observed in the control group during the last block of trials, although unexpected, raises important questions regarding the possible effects of SMT. These results suggest that a trial-to-trial “sensitization effect”, observed in the control and leading to increased paraspinal muscle activity, did not occur in the SMT group. In a recent review of literature, Millan et al.
 explored the short term effect of SM following experimentally induced pain. The review suggested both a local and regional effect of SM on pain reduction. The outcome of SM was also affected by the method of pain induction as pain induced by pressure, electricity, stretching of painful tissue, dermal irritation and spontaneous pain all responded to SMT. Such results were not observed, however, for temperature-induced pain. The specific effect of SMT on sensitization phenomenon should be further investigated in future studies.
Alternatively, changes in trunk muscle activity may also been explained by changes in paraspinal tissue properties. Olson et al.
 showed increased paraspinal muscle EMG during the flexion following cyclic flexion extension exercise over 9 minutes
. These changes were accompanied by random EMG activity (described by the authors as spasms). Changes observed in the present study may therefore result from both modifications in spinal tissues properties and muscle fatigue. Moreover, sustained flexed or semi-flexed spinal sitting postures may result in increased paraspinal muscle activity
 and provocation or aggravation of existing pain
[26, 27]. Therefore, increases in VAS scores and EMG activity following the 25 minutes of “sitting posture” may have been triggered by changes in paraspinal tissues caused by static lumbar flexion loading. Specific mechanisms underlying between group differences during flexion and full flexion and the potential role of SMT remain to be investigated.
As for all manual therapies, true blinding of participants was impossible during the experimentation. Participant’s expectations towards receiving (or not receiving) spinal manipulation may have affected the VAS scores, but one could argue that it is less likely to affect EMG activity. Besides, because spinal manipulations were delivered by a clinician, no standardization of the force and speed parameters was possible, potentially inducing a bias in the physiological response to SMT. According to Kawchuk et al.
, a typical clinician’s trial-to-trial variability can reach 37 N when a peak force of 253 Newtons is used
. Finally, it was decided, mainly for technical reasons, that all SMTs would be delivered to the same spinal segment (L3), regardless of pain localization. Because pain and EMG responses overtime seem to follow similar patterns, delivering spinal manipulation according to pain patterns may have yielded different results. SMT procedures and delivering forces at the same segment for all subjects may not reflect the usual clinical practice where a specific joint will be targeted according to manual palpation and other clinical findings. It is therefore possible that the changes observed in the present study may not reflect exactly those encountered in a clinical setting.
Assessing the clinical relevance of EMG changes following SMT remains challenging. However, the changes reported in this study (as high as 10-15% in normalized RMS values) may be viewed as significant changes in erector spinae recruitment during a typical activity of daily living (flexing the trunk). Such changes, repeated over time may lead to muscle fatigue and changes in spinal stability.
A recent review by Millan and al.
 suggested that SMT has a hypoalgesic effect both locally (segmental level only) and regionally (related to the segmental innervation). The present results, although preliminary, suggest a possible modulation of sensitization phenomenon observed in chronic low-back pain populations
. A recent study suggested that descending pain modulation may shift from descending inhibition towards descending facilitation following repetitive muscle contractions in chronic pain populations
. SMT may have, for a brief period of time (30 minutes), limited the effect of muscle fatigue on pain processing mechanisms. The exact nature and extend (magnitude) of these effects are unclear and future study regarding the SMT in presence of muscle fatigue and changes in a tissue properties should be considered.