The present study shows that people with CLBP used similar muscle pattern compared to the healthy group when reacting to an unexpected external load. On the other hand, fatigue caused muscle pattern changes regarding temporal and amount of muscle activity characteristics that were different when compared to those of the non-fatigued group. Although not statistically significant, the CLBP showed a pattern of response for some temporal variables closer to fatigued than non-fatigues healthy subjects (Fig. 4). This similarity may indicate that some behavioural characteristics could be shared between CLBP and healthy fatigued groups. Other studies have also shown similar characteristics when comparing these two groups, e.g. proprioception alteration [18, 19]. The fact that the common temporal changes are less obvious in subjects with CLBP could be caused by the greater heterogeneity in characteristics presented by this group. However, it should be taken into account that the muscular alteration in individuals with CLBP is maintained over time, unlike the With-F group who only has this condition on a temporary basis.
Greater latency in the CLBP group in the activation of the first burst for most muscles has been determined by most authors [2, 3, 20]. However, in this study, no significant differences between CLBP and healthy groups were found in the first muscular activation delay after an external perturbation. Although not statistically significant, the CLBP showed a pattern of response of the first burst latency closer to fatigued than non-fatigued healthy subjects, with the fatigued group showing a clear delay in the first burst activation (Fig. 4). This tendency could support current theories [16, 21, 22] which describe delays in muscle activation as a phenomenon that decreases the control of the spine possibly leading to chronic pain. The lack of findings in this study and other authors concerning this issue  could be explained by different factors. The small sample size and large variability among CLBP subjects  could contribute to diminished power. Future studies with a larger sample size and/or sub-classifications of CLBP would be required to clarify this issue. In addition, the specific characteristics of the test used in the different studies could also contribute to limited detection of greater latencies in CLBP as described by others [2, 3, 20]. Controlling the pre-activation trunk muscles and using a more fixed position  could imply an experimental condition where CLBP muscle deficits were not observed. In healthy people, abdominal and trunk muscle pre-activation had showed increases in spinal stiffness and stability . Stable contexts could not be those situations where CLBP subjects present deficits in spine control. Moreover, one may interpret that the more real the position of the test (this study vs. others ), the more it relates to CLBP subjects’ daily lives. The semi-squat position, used in the current study, is a recommended posture to handle physical efforts made at the spine level, and it is frequently used without external stabilization. Also, unexpected perturbations could be experienced in daily activities. Less ability to protect the spine by CLBP in this frequent situation seems more relevant than results from an unusual context.
Subsequent muscular reactions (EMG bursts) also seem to be important to guarantee spinal protection. In the current study, the CLBP group showed no differences when compared to the healthy group without fatigue. The current results can only be compared with other studies investigating the first reaction response since, to the authors’ knowledge, other research up to now, does not consider subsequent reactions. Only one study evaluated the completion time of the first burst without observing differences between the CLBP group and the healthy group in that parameter . The variability among subjects in the motor recruitment pattern  could be greater in subsequent muscular reactions rather than in the first reaction. This could be the reason for the lack of significance among the subsequent muscle reactions found in this study or other ones.
CLBP showed similar amount of activity than healthy group after the impact as well as similar co-contraction based on the calculation of the burst synchronisation after impact in the three muscle groups. However, in different tasks, other studies found in CLBP people more muscle activation in certain muscles  as well as increased agonist and antagonist activity attributing to increased muscle co-contraction . Both variables of the current study that describe the amount of muscle activity post impact (burst number and amplitude increase) were relative of pre-impact activity without considering the possible absolute group differences on the EMG normalized amplitude. That could be a reason explaining why other studies show discrepancies regarding muscular activity in CLBP subjects [4, 7]. Interpreting together those results of the amount of muscle activity, one may conclude that CLBP subjects may be using more activity than healthy ones but not a large increase of muscular activity after the external perturbation. Regarding the co-contraction parameter, this study cannot be compared directly with those studies where co-contraction was calculated as an amplitude increment of agonist and antagonist muscles . The co-contraction parameter of this study is based on muscle onset and offset times so similar co-contraction between groups is because no more burst synchronisation is found in CLBP group compared with the healthy one. The co-contraction parameter of this study may be contrasted with studies like Mehta et al. , Radebold  and Cholewicki . Mehta et al  analysed the coincidence in time of the first burst on a sudden perturbation showing a lower synchronisation in CLBP subjects. Conversely, Radebold  and Cholewicki  determined that greater co-contraction occurred in the CLBP group, as they observed less muscleagonist deactivation once the load was withdrawn when compared to the healthy group. Considering all CLBP evidence together, one might conclude that the increase of muscle activity and the presence of co-contraction are strategies used by people with CLBP to reduce pain , but are not present in all types of tasks. It appears that in those tasks that require a sudden increase in muscle activity to control the spine (this study among others ), the delay in muscle activation could make synchronisation impossible which undermines the possibility of co-contraction. Conversely, in slower tasks  or in tasks with an initial considerable co-contraction [3, 23], increased slowness in muscular reaction would not prevent the co-contraction strategy. Similarly, even though absolute EMG normalized amplitude differences between groups are not assessed in this study, muscle activity increase as a strategy by CLBP subjects to reduce pain  could be more difficult presented after sudden perturbation than in a static position  or when undertaking a slower task . A recent new theory regarding adaptation to pain supports this task dependency interpretation . In sudden perturbation, delays in muscle activation and the difficulty of using previous described strategies to control the spine [3, 6] could imply a vulnerability for CLBP subjects and may play a role in the chronification process. Muscle training to improve muscle coordination and the quickness of muscle responses could be a strategy to improve CLBP dysfunctions. Moreover, one might consider the need for functional exercises as a treatment of CLBP (semi-squat among others).
Regarding the fatigue condition, greater latencies in the activation of the first burst, and alteration in subsequent reaction times existed (earlier times in the deactivation of the first burst of the BB muscle and less co-contraction of SE, EO and IO). Healthy subjects seem to show in the fatigue condition a similar phenomenon that decreases the control of the spine as that described for temporal alteration in CLBP [16, 21, 22]. In this transitory situation of fatigue, the lack of control seems to be much more pronounced than in CLBP subjects and could lead to a tissue injury. Some authors found similar results to the current study regarding the first onset , while others have not observed latency differences in the first burst [8, 25]. Again, literature discrepancies could be explained by the specific characteristics of the perturbation, different fatigue levels achieved prior to the reaction test and the task used to induce fatigue . Future studies analysing the same subjects with different tasks and methodologies could help to resolve this issue. Contrary to the CLBP group, the fatigue condition showed signs of reduced activity after the impact compared with non-fatigue condition (smaller amount of bursts and smaller increase in muscle activation after impact of some muscles). This reduction may indicate that fatigue leads to a lesser control of the spine overloading on different structures and is more likely to result in back injury. Some considerations must be taken against fatigue to prevent spine overloading.