Eligibility criteria

Eligibility criteria were developed using the Sample, Phenomenon of Interest, Design, Evaluation, Research Type (SPIDER) search concept tool [38].

Inclusion criteria

S–the sample population was humans (of any age) and inanimate objects (e.g. instrumented tool, manikin);

PI–the phenomenon of interest was manually delivered SMob, delivered by any regulated health professional (e.g. physiotherapist or chiropractor) or student enrolled at an accredited institution;

D–observational study designs (e.g. case series studies, cohort and case-control studies);

E–kinetic variables of the intervention (e.g. force-time profile); and

R–original quantitative research data from studies utilizing SMob as either the sole intervention or as a comparator.

Exclusion criteria

Exclusion criteria were: i) studies that reported on SMob delivered by a mechanical instrument or device (e.g. Activator device or mechanical robot); ii) studies that reported on all other therapeutic modalities (e.g. soft tissue treatment, massage therapy); iii) manuscript not published in English, French or German; and iv) studies that had been retracted, were secondary analyses, trial registrations, protocols, clinical practice guidelines, commentaries, editorials, conference proceedings or single case studies.

Search strategy

The search strategy was created by subject specific and methodological experts, with the assistance of an experienced medical and health sciences librarian. MEDLINE(Ovid), Embase, CINAHL, ICL, PEDro and Cochrane Library databases were searched from inception to 4 October 2022. The following search terms and derivatives were adapted for each search engine: (spine, spinal, manipulation, mobilization or mobilisation, musculoskeletal, chiropractic, osteopathy, physiotherapy, naprapathy, force, motor skill, biomechanics, dosage, dose-response, education, performance, psychomotor, back, neck, spine, thoracic, lumbar, pelvic, cervical, sacral). Search strategies for all databases are provided in S1 File.

Study selection process

Records retrieved from the electronic searches were exported to the Rayyan© online platform (2022) [39] and duplicates were removed. Beginning with title and abstract review, groups of two authors (LG and LN; LG and IP; LG and MP) independently screened studies. Full-texts of the remaining studies were then retrieved and screened independently by groups of two authors (LG and LN; LG and IP). The first author (LG) screened the reference lists of included studies to examine whether all relevant literature was captured. No further studies were revealed by this process. Disagreements regarding study inclusion that could not be resolved by consensus were resolved by a third author (MD).

Data extraction

Data were extracted from eligible studies by groups of two independent authors (LG and LN; LG and MP). These data included: i) general study characteristics (e.g. title, author, year and country of publication and type of study); ii) general study information (e.g. individual who delivered the intervention [e.g. clinician, student], professional qualification of individual delivering the intervention [e.g. physiotherapist, chiropractor], years of clinician experience/number of student hours, number of clinicians/students who delivered SMob, recipient [e.g. human, manikin], number of recipients, whether the intervention was SMob [and grade of mobilization], the region treated [e.g. cervical, thoracic] and the measurement equipment used to record force-time characteristics of the intervention [e.g. type and metrological details]); and iii) force-time characteristics of SMob (e.g. peak force, SMob duration and frequency and force amplitude).

Definitions

In this study, the following definitions were used:

• Peak force: the maximum applied force during a single SMob, reported as the mean of the force peaks that occurred during a specified period of the intervention.
• Duration: the time period of SMob delivery.
• Frequency: the rate of force oscillation during repeated applications.
• Force amplitude: the difference between the minimum and maximum forces applied during the intervention (i.e. the difference between a peak force and trough), reported as the mean of the force amplitudes that occurred during a specified period of SMob.
• Metrological details: descriptions of the suitability (e.g. accuracy, precision, sensitivity) of the measurement equipment to quantify the force-time characteristics of SMob [40].

Data synthesis

Descriptive statistics (mean, standard deviation and range) were used to report data. Any deviations from this (such as the use of 95% confidence intervals or the reporting of median and interquartile range) are explicitly indicated and reflect how the data were reported in the original studies. Microsoft Excel (Office 365, Microsoft Corporation, Redmond, USA) was used to calculate frequencies and proportions of trials reporting on each of the specified domains mentioned above.

In order to manage the substantial volume of data presented in this study, the following decisions were made regarding how to best report the data:: i) for studies reporting forces measured in 3-dimensions (3D) and including the resultant forces (i.e. the total forces applied), only the resultant forces are reported; ii) for studies measuring forces applied in 3D but not including the resultant forces, only the forces measured in the primary direction of the applied force are reported in the tables (e.g. for prone PA thoracic SMob, the vertical forces are reported). Regarding the reporting of metrological data, a consensus was reached by two authors (LG and LN; LG and MP). The final decision as to whether adequate information was provided was reached by consensus by LG and MD. In cases where metrological details were discussed (e.g. it was stated that measurement equipment accuracy was good) but it was not clear if this statement was based on data (or what data), this was recorded as metrological details were not provided. No assessment of study quality was performed.

Cervical spine

Thirteen studies (36.1%) reported on SMob delivered to the cervical spine. Eleven (84.6%) of these studies reported on SMob delivered to the cervical spine of humans and the following force-time characteristics were reported: i) peak force: 0-128N; ii) duration: 60s; iii) frequency: 0.28–2.4Hz; and iv) force amplitude: 14.4–52.5N (detailed information provided in: Tables 2 & 3; S3 Appendix, Table A in S3 File). Two (15.4%) of these studies reported on SMob delivered to inanimate objects (i.e. human analogue manikin:1; instrumented tool:1) and peak forces 42-181N were reported.

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Table 2. Summary of force-time characteristics reported by region for studies reporting on spinal mobilization (SMob) (n = 36).

https://doi.org/10.1371/journal.pone.0289462.t002

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Table 3. Summary of studies reporting on the force-time characteristics of spinal mobilization (SMob) delivered to the cervical spine of humans (n = 11) and inanimate objects (i.e. human analogue manikins, instrumented tools) (n = 2).

https://doi.org/10.1371/journal.pone.0289462.t003

Thoracic spine

Three studies (8.3%) reported on SMob delivered to the thoracic spine. Two (66.7%) of these studies reported on SMob delivered to the thoracic spine of humans and the following force-time characteristics were reported: i) peak force: 297-323N; ii) duration: 3x60s; and iii) frequency: 0.47–0.53Hz (detailed information provided in: Tables 2 & 4; S3 Appendix, Table B in S3 File). In the one (33.3%) study that reported on SMob delivered to 12 T5-8 sections of human cadavers: i) peak force: 106-223N; and ii) frequency: 0.5Hz were reported.

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Table 4. Summary of studies reporting on the force-time characteristics of spinal mobilization (SMob) delivered to the thoracic spine of humans (n = 2) and inanimate objects (i.e. partial cadaveric sections) (n = 1).

https://doi.org/10.1371/journal.pone.0289462.t004

Lumbopelvic spine

Eighteen studies (50.0%) reported on SMob delivered to the lumbar spine. Seventeen (94.4%) of these studies reported on SMob delivered to the lumbar spine of humans and the following force-time characteristics were reported: i) peak force: 3-430N; ii) duration: 10-120s; iii) frequency: 0-5Hz; and iv) force amplitude: 1-102N (detailed information provided in: Tables 2 & 5; S3 Appendix, Table C in S3 File). In the one (33.3%) study that reported on SMob delivered to an instrumented tool: i) peak force: 36-119N; and ii) duration: 30s were reported.

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Table 5. Summary of studies reporting on the force-time characteristics of spinal mobilization (SMob) delivered to the lumbopelvic spine of humans (n = 17) and inanimate objects (i.e. instrumented tools) (n = 1).

https://doi.org/10.1371/journal.pone.0289462.t005

No region specified

Two (5.6%) studies reported on SMob delivered to an unspecified region and the following force-time characteristics were reported: i) peak force: 2-361N; ii) duration: 20s; and iii) frequency: 28–32 cycles/min (detailed information provided in: Tables 2 & 6; S3 Appendix, Table D in S3 File).

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Table 6. Summary of studies reporting on the force-time characteristics of spinal mobilization (SMob) delivered to inanimate objects (i.e. instrumented tools) with no region specified (n = 2).

https://doi.org/10.1371/journal.pone.0289462.t006

Detailed reporting of the intervention is necessary

It is unknown if meaningful clinical differences between different types of SMob ’technique’ (e.g. PA vs. oscillatory distraction) and/or grades of SMob exist [34]. To assess this in future studies, there should be a detailed description of the applied SMob, including an explicit explanation of how the grade of SMob was defined. In the current review, detailed descriptions of SMob were not consistently provided. Specifically, there was large variation in the detail of SMob reporting and in many studies, replication of the intervention would be impossible based on the (lack of) details provided. Despite the existence of established reporting guidelines for health-related interventions [41], this lack of detail is not unique to SMob, existing also for interventions including SM [37] and dry needling [42]. It is recommended that a specific guideline for the standardized reporting of SMob, similar to that for the standardized reporting of SM [43], is developed to improve the reporting of SMob interventions. Indeed the (development and) use of such a guideline would go some way to improving the generally poor level of manual therapy trials reporting [44].

It is not yet fully understood why forces measured at the clinician-patient and patient-table interfaces differ [45,46]. However, as differences do exist, it is important for authors to report where and how force-time characteristics were measured as results from studies measuring forces at different interfaces are not directly comparable. Furthermore, the terminology used should enable replication of study methodologies in future investigations and would be facilitated by the detailed reporting of force-time characteristic definitions and calculations (e.g. was the peak force measured directly by the equipment or, was it mathematically estimated?). Without such details, it is impossible to compare force-time characteristics between studies. Furthermore, it has been reported that demographics (e.g. sex, height and BMI) influence the application of manual therapy treatments (i.e. SMob and SM) [47–50]. Specifically, these interventions are delivered more forcefully to males with a higher BMI. In summary, the interface of measurement, equipment used, terminology and SMob recipients should be systematically reported in detail to allow for both replication in future studies and reader judgement of the clinical relevance of reported results.

Do force-time characteristics matter clinically?

It has been reported that force-time characteristics of SMob (e.g. peak force and force amplitude) differ between students and experienced clinicians [48]. Specifically, students apply less force, more slowly. Similar differences are also reported in the SM literature [51], further suggesting that the detailed reporting of the clinical experience of the individual delivering the intervention is necessary. Furthermore, Gorgos and colleagues reported on the reliability of inter-clinician and intra-clinician forces applied during joint mobilisation in a systematic review [52]. The authors concluded that while there is variability in the application of force between different clinicians, individual clinicians apply forces consistently. Despite such between-clinician differences in SMob force application, the literature shows that recipients experience beneficial clinical outcomes from various forms of manual therapy (including SMob) notwithstanding considerable differences in the force-time characteristics of the applied interventions (e.g. low velocity, variable amplitude SMob vs. high velocity, low amplitude SM) [34,53,54]. Additionally, some authors suggest that there is a threshold of ’dosage’ (in terms of force-time characteristics such as force and/or rate of force application), rather than an optimal intervention approach (i.e. SMob vs SM), required to elicit beneficial clinical outcomes [55,56]. However, to our best knowledge, this subject has not been systematically investigated and no reference standards (i.e. ranges of force-time characteristics) for SMob application have been published [52]. Furthermore, the lack of detailed description of force-time characteristics limits the generalizability of results reported by both individual studies, and their subsequent syntheses, to clinical practice as it remains unclear exactly what ’dosage’ was applied [42]. By exhaustively collating the existing literature, the current review provides a first step towards the development of such reference standards. However, the systematic biomechanical quantification of SMob is required to first establish if ’dosage’ is related to physiological responses (e.g. changes in the autonomic and somatic nervous systems) and/or clinical outcomes (e.g. hypoalgesia).

Strengths and limitations

This review is the first to synthesise SMob force-time characteristic data beyond peak force (including also duration, frequency and force amplitude) and includes 21 additional studies since the publication of the most recent 2006 review [22]. The review was conducted by an international and interprofessional team and reported according to the (PRISMA-ScR) statement [36]. The study provides a first step towards the systematic and detailed reporting of SMob interventions, which is necessary to investigate the relationship between the application of SMob and its’ observed clinical outcomes.

It is possible that there was unintentional exclusion of studies reporting on the parameters of interest. However, it is unlikely that seminal studies were excluded for several reasons: i) a comprehensive search strategy was developed by an international, interprofessional team with relevant methodological and clinical expertise with the assistance of an experienced medical sciences librarian; ii) the search strategy was piloted and refined prior to being used; and iii) the review was conducted in a systematic fashion (i.e. using groups of two independent reviewers and data extractors). While it was intended that only original quantitative research data from studies utilizing SMob would be reported, it was not always clear as to whether reported data were previously published in part or fully. However, in instances where this was unclear, the decision was made to include the data. This decision ensured that the current review reported exhaustively on all studies reporting force-time characteristics of SMob. It is recommended that secondary analyses of data are transparently reported as such [57,58], with citation of the original publication, allowing readers to identify that the data has been previously published and to interpret for themselves the impact of the re-reported data. Finally, due to the considerable heterogeneity within studies reporting on force-time characteristics of SMob, no critical appraisal of the included studies was performed. However, it has been reported that there is currently no evidence to suggest that a lack of critical appraisal of the included studies impacts on the update and relevance of results reported in scoping reviews [59].