## 4 Discussion

Table 2: Between-groups comparison of the percentage of change from baseline to 15 days after the third session in muscle properties. brachii, showing some effect over muscles metameric and also non-metameric, related to the spinal level that suffers the intervention. However, as there was no difference in pressure pain thresholds between the manual manipulation and the placebo groups, we cannot assume that the effect occurred due to the manual manipulation. The results obtained in the placebo group could probably be related to touch-induced analgesia combined with periarticular tension [40]. As our placebo technique included human touch and pre-manipulative position, which include periarticular tension, it is possible that those components explain, at least partially, the results found in this group and not in the instrument-assisted manipulation group, in which there was no human touch and the intervention was applied with the cervical in a neutral position. Moreover, outcomes may not depend wholly on the type of treatment provided but may also be influenced by individual attitudes or beliefs regarding the treatment [41]. Since both the manual manipulation and the placebo technique induced cervical motion, the participants of these groups might have been influenced by their beliefs that cervical manipulation induces analgesia. These findings contradict the assumption of therapeutic equivalence between manual and instrumented-assisted manipulation. The results obtained regarding muscle parameters indicate that cervical manipulation did not induce any changes at the short or mid-term in tone, elasticity, or stiffness. Our results are consistent with previous studies [42], meaning that changes observed in pain and disability after the three sessions were not related to changes in muscle mechanic parameters. Our study had some limitations that should be acknowledged. The small sample size and the lack of a previous sample-size calculation limit the generalizability of our results. Nonetheless, this study generated outcomes that can be used to determine the sample size in subsequent larger studies aiming to clearly ascertain the findings of this study. The results of this study are restricted to the immediate and mid-term, as we only had 15 days of follow-up. Future studies are needed to determine whether the positive effects on pain and disability are maintained in the long term. Additionally, a source of variability is the different postures and cervical motions used in the different study groups (e.g., while manual manipulation was applied in supine with a rotatory force, the instrument-assisted manipulation was applied with the participants positioned in prone and the cervical in a neutral position). Overall, the results of this study reaffirm the positive effects of manual manipulation recorded in previous trials and further extend understanding of the efficacy of spinal manipulation applied at a non-specific spinal level, without a previous clinical examination of motion restriction. These preliminary findings both encourage further study with these two methods of spinal manipulation, and inform the design and conduct of subsequent studies.