The aim of this systematic review was to study the relationship between changes in clinical outcome (pain, disability) and changes in physical function (range of motion, strength, muscular endurance) as a result of physical therapy and exercise interventions in cLBP. The majority of the 16 studies reviewed indicated that no such relationship exists. Changes in pain showed predominantly no significant correlation with changes in mobility (9 studies reported no significant correlation and just 1 reported a correlation), or trunk extensor strength (7 and 2 studies, respectively) or trunk flexor strength (4 studies and 1 study, respectively), and no correlation with changes in back muscle endurance (7 and 0 studies, respectively). The meta-analysis for the associations between changes in pain and mobility also supported this conclusion, although the I-squared coefficient of greater than 60% reduced the explanatory power of the pooled data. Overall, we conclude that there is not convincing evidence that changes in pain are strongly associated with changes in physical function/performance.
Similarly, for disability, a predominance of studies showed no significant correlation with changes in mobility (3 reported no significant correlation and 2 a significant correlation) and changes in trunk extensor strength (4 and 2 studies, respectively), although these findings were less consistent than for pain.
In general, these findings concur with those of other systematic reviews and individual studies [43
]. As highlighted before [16
], if specific types of exercise therapy are to be advocated—especially those that aim to target specific functional deficits—it is important to be able to establish that improvements in the clinical complaint after therapy are in some way associated with the specific changes in function elicited. It is often not clear whether changes in performance are responsible for improvements in pain/disability or whether these two simply occur coincidentally and are actually mediated by a common third factor. If a correlation between the changes in two variables (e.g., muscle strength and disability) is established, this does not necessarily prove the existence of a causal relationship; the converse, however, i.e. a reduction in disability/pain in the absence of any significant change in the performance dimension under investigation or vice versa (i.e., no correlation), would certainly imply that the two were unrelated. The latter appears to be emerging as the overarching conclusion of the studies conducted on this theme to date, and might also explain why no particular type of exercise therapy is presently considered to be superior to any other [1
], i.e., because the exercise therapy is not actually eliciting its effects by improving specific aspects of (dys)function. The assumption that the reversal of deficits in physical function—believed to either predispose to LBP or to arise due to physical deconditioning subsequent to cLBP—results in a decrease in pain/disability was hence not substantiated by this review. Instead, our findings appear more congruent with reports showing that patients with cLBP do not necessarily show marked deficits in function [71
]. Recently, the popular intervention of core-strengthening exercises (focusing on strengthening the rectus abdominus, internal and external obliques, and erector spinae muscles) was questioned in a study that sought to compare this type of exercise with a general non-specific strengthening programme [73
]. The outcomes were similar in the two treatment groups, and the authors concluded that focusing specifically on core exercises might be a potential mistake in the rehabilitation of cLBP [73
]. Furthermore, it was shown in other studies that even stretching exercises appeared to improve strength [55
], which is difficult to explain on any physiological basis. A noteworthy feature of the trials included in this review was the large variability in exercise interventions. The diversity in the activities prescribed (e.g. strength and endurance training, interventions, with or without counselling) reflects the absence of consensus on the optimal activity programme for cLBP. Guidelines report that exercises may include aerobic activity, movement instruction, muscle strengthening, posture control and stretching, but at the same time provide no information about the required intensity, frequency, loading, progression, etc. for the chosen training programme. However, it is conceivable that these same factors—that undoubtedly influence the prescription of exercise in relation to medical conditions such as hypertension or obesity—are of less relevance when prescribing exercise for cLBP. Indeed, if the main aim of exercise therapy in cLBP is to get patients moving again and be able to confront their fears and anxieties about physical activity and movement, then the method used to do this may be immaterial. And if this were indeed the case, it may have the fortuitous side-effect that it would open up the array of potential options for the type of exercise to be carried out, allowing consideration of the all-important issues of cost, access to facilities and patient-preferences.
The biological mechanisms explaining the effects of exercise therapy are not yet clear [74
], but the findings of the present review suggest that the improvements in clinical outcome do not result from local (muscle, joint, etc.) changes. Other possible explanations are that they derive from more central effects [14
], perhaps as a correction of a distorted “body schema” [4
] or altered cortical representation of the back [22
], from modification of motor control patterns as a consequence of a reweighting of sensory input [21
], or simply from a positive therapist–patient interaction/relationship [26
]. Several studies have reported a correlation between psychological status and low back pain or pain tolerance [15
]. The efficacy of treatments that solely focus on psychological targets has, however, been shown to be small [4
]. These psychological phenomena, similar to the peripheral physical deficits, may also be responses to an altered body schema in the sense of sensory–motor incongruence that causes fear [4
]. Exercise therapy seems to positively influence psychological variables such as fear-avoidance beliefs, catastrophising and self-efficacy regarding pain-control [79
], in addition to providing physical benefits. Possibly by experiencing no harm in completing exercises, patients gain trust in the function of their back and thereby adjust their irrational cognitions and appraisals [79
], whilst simultaneously improving their physical function.
Based on the findings of our review and on similar information from other systematic reviews and studies [43
], we suggest that changes in physical function are largely unable to explain changes in the clinical condition in cLBP patients, and that the important “side effects” of exercise therapy (including, amongst other things, changes in psychological variables such as fear-avoidance beliefs, catastrophising and self-efficacy regarding pain-control) should be more specifically emphasised and investigated in future rehabilitation programs.
We used a structured study protocol to guide our search strategy, study selection, extraction of data and statistical analysis. However, a number of possible limitations of this review should be noted. First, the search strategy was limited to published studies identified through the selected search engines. Second, as noted, a publication bias may have been present, as well as a language bias, given that we restricted our search to English and German language publications. Third, as there were only 12 randomised trials, we also included several observational studies, the results of which may be affected by confounding bias due to the absence of random assignment. However, as the focus of our analysis was not the relative efficacy of different treatments, this was expected to be of little consequence. The literature search for this review revealed 58 studies that potentially could have been included, but more than half of them had not conducted any correlation analyses. We tried to obtain the original data by contacting the authors of the studies that had failed to report actual correlation data, either by email, telephone or both. Unfortunately, the few who responded either no longer had access to the data or were not interested in providing their data. This undoubtedly resulted in a loss of potential information. A further problem was that most studies that did conduct correlation analyses, did not report any corresponding data (correlation coefficients) substantiating their reported non-significant correlations that would otherwise have allowed for quantitative data analysis with meta-analyses. Finally, the interventions were heterogeneous in their design and of variable quality.
Intervention strategies that focus solely on the symptom area in the lower back should be extended to apply a more global treatment approach. Both psychological and psychosocial interventions in addition to conventional exercise therapy may have a more positive effect on treatment outcome [80
]. The targeted effect of such an approach would be the development of a sense of control over pain and the elimination of pain-avoidance mechanisms, whilst simultaneously improving overall physical fitness/function. Emphasis would shift from the “reversal of specific performance deficits” to the “adoption of enjoyable health-promoting physical activity” and this would potentially be associated with a wider choice and reduced cost. The availability of and access to such treatments might also be broadened by offering, e.g. group treatment sessions in community-based (rather than medical) settings. The exercise programs might include the training of proprioception, sensorimotor control and postural balance [91
], in addition to the more conventional aspects of performance (strength, mobility, etc.). Lastly, the beneficial psychological effects of exercise should be investigated in greater detail. A better knowledge of the psychological changes induced by physical activity and training, and any accompanying “placebo” effects or educational effects due to the therapist–patient interaction, has the potential for enhancing the efficacy of exercise as a treatment for cLBP.