Currently, the development and application of the functional assessment scales are mainly aimed at specific clinical condition or pathology6, 9, 10 The evaluation process usually requires medical professionals to complete, and the scales are unable to provide clinical evidence of the disease prevention for the healthy population. Garcia et al. 11 developed a novel shoulder functional movement test for the long-term clinical follow-up of patients with shoulder pain and verified the reliability and validity of the self-assessment test. It provided new ideas for functional self-assessment, but also cannot be applied to injury prevention in the healthy population because of the limitations of its clinical application. In terms of spinal function evaluation, most of the currently existing questionnaire-based self-evaluation scales for cervical and lumbar spine disorders, such as Oswestry Disability Index (ODI) for low back pain and Neck Disability Index (NDI) for the cervical spine which are recommended by the American Physical Therapy Association 12, 13 are only applicable to clinical pathological conditions and lack a "holistic view" of the healthy spinal function. Gabel et al 14 developed a questionnaire-based clinical evaluation of the overall spinal function, however it did not contain an evaluation of functional positioning (e.g., cervical, thoracic, and lumbar segments of the spine) and the functional properties (e.g., posture, flexibility, stability, etc.). The Self-Reported Spine Functional Scale (SSFS) was developed based on modern medical rehabilitation concepts and the principles of "integrative, comprehensive, accurate and practical", in an attempt to provide the public with a simple, reliable, and user-friendly tool for the self-assessment of spinal function in the healthy population. This can effectively and timely prevent the occurrence of spine-related disorders and to promote the awareness of spinal function, and to motivate evidence-based actions to maintain healthy spinal functions.
Construction of the SSFS
Studies have shown that a large percentage of the postsecondary students suffer from spinal dysfunction; and regular exercise can improve the spine flexibility and stability 2. The assessment of spinal condition is composed of three major components: posture assessment, spinal muscle strength assessment and function 15, 16. Combining the concepts of scientific, comprehensive, simple and reproducible, this study adapted from the conventional assessment methods, improved and innovated to develop a novel Self-Reported Spine Functional Scale (SSFS) containing 8 assessment items.
In the postural assessment component, positioning of the cervical spine, thoracic spine and lumbar spine on the three-dimensional plane were evaluated. The spine is the central axial structure that maintains the human posture, therefore the evaluation of human posture with the body surface markers can simply and accurately reflect the functional state of the spine. Many studies have suggested that different body positions (such as in standing, in lying, or in long-sitting) will cause spine and different muscle groups to activate differently 17, 18, so this study incorporated the innovative qualitative assessment of lumbar spine posture in side-lying, with an aim to evaluate the position of spine with the effect of gravity in the coronary plane. It is also a reflection of flexibility and stability of the lumbar spine and bilateral symmetry. In addition, the standing posture assessment is a practical and relatively accurate method to evaluate the lumbar spine and pelvis positioning in the sagittal plane, which is widely used in modern rehabilitation practice. The remaining test items are common orthopedic measurement methods 19.
The muscle strength test includes assessment of the neck flexor muscle strength and abdominal core muscle strength. Paraspinal musculatures are the key structure which function to maintain the stability of the spine and help with the coordinated movement, therefore relevant tests are essential in the evaluation of spinal function. In view of the special anatomical characteristics of the cervical spine and the high incidence of related diseases 20, cervical flexion was selected to represent cervical motor function14. Planking was used to determine abdominal core muscle strength and evaluate the overall spinal muscle strength, stability and movement coordination21. In order to facilitate the self-evaluation procedure, the testing methods and grading standards of the two tests were reasonably adapted.
The global functional test is used to evaluate the functional movement of the spine. Prone press-up and supine knee-to-chest respectively evaluate the flexibility and coordination of spine extension and flexion, which are practical assessment methods adapted from the McKenzie Method of assessment and management for spinal disorders22. Wall roll-down is included to evaluate the flexibility and coordination of the overall movement of the spine, and especially the mobility, stability and flexibility of each segment of the spine, and is an innovative functional testing method adapted from the Slump Test 19. The wall angel was used to evaluate the posture and mobility of the thoracic spine in the sagittal plane, flexibility of the upper extremities, and upper spinal movement coordination15.
Therefore, the assessment items on the SSFS are all supported by literature and modern rehabilitation concept, and strictly define the evaluation methods and grading standards. SSFS in turn can accurately and comprehensively portray the overall functional mobility of the spine. The scale can also be easily interpreted, completed by subjects with high reproducibility and effective application. SSFS has undergone expert consultation and Spearman correlation analysis, where all of the above 8 items on the SSFS significantly correlated with the total score and had been retained 23.
Reliability and Validity Analysis of SSFS
The initial sample population of the study was composed of 917 healthy young adults. Compared with similar studies, the sample size of this study was larger, and the age range of the subject population was narrower (between 18-30 years old), with occupation mainly consisted of air force recruits and postsecondary students. The collection of relevant data has high statistical importance for the reliability and validity analysis of SSFS. The expert consultation evaluation signifies that the experts in this study have high academic and professional authority, motivated participation, and reasonable and reliable consultation results.
Validity refers to whether the scale can accurately and effectively measure the corresponding characteristics. The higher the validity, the more effective the scale is, and the more it reflects correct test results. Validity is generally analyzed based on three aspects: content validity, structure validity and discriminative validity.
Content validity refers to the conformity between the elements of the scale and what the scale is intended to measure, namely the applicability and representation of the items in the scale 24. In this study, domestic experts in the field of sports medicine and sports rehabilitation were consulted for the selection of scale assessment items. Spearman correlation analysis was used to evaluate the correlation between each item on the scale and the total score. All items of the scale were kept, and statistical analysis confirmed the reasonable design of the items.
Structural validity, also known as construct validity, indicates whether the structure of the scale is consistent with the theoretical assumption of the scale formulation and whether the components of the measurement results are consistent with the purpose the researcher intends to measure. The use of factor analysis to evaluate the structural validity of a scale is a relatively well-accepted method 6. In this study, exploratory factor analysis and confirmatory factor analysis were conducted. In exploratory factor analysis, the factor loading matrix divided the eight items in the scale into two factors with a cumulative contribution rate of 46.057%. The factors were named the postural assessment factor and overall spinal function factor, establishing the two factors of the scale 25. The neck flexor and abdominal core muscle strength tests and the spine functional test were categorized into the same factor, which is basically in line with the design of this scale.
Confirmatory factor analysis was conducted to explore the consistency of the factor structure of the scale with the collected data, and whether each item in the scale can be effectively used as the measurement variable of the latent variable (or the factor) 26. AMOS21.0 was performed to carry out confirmatory factor analysis on the fitting of two-factor model of SSFS. The expected value of c2/df in various indicators is generally 2-3. To discuss the model fit of confirmatory factory analysis，it has been suggested that RMSEA value less than 0.05 is good, value between 0.08 and 0.1 is marginal, and value greater than 0.1 are poor 27. Therefore, the RMSEA value of 0.04 in this sample indicates an acceptable fit. Indicators GFI and CFI > 0.90 represents acceptable fit of the model. Therefore, GFI of 0.945 and CFI of 0.967 indicate good fit. In summary, statistical analysis confirms SSFS to have high structural validity.
The discriminative validity indicates the degree to which a measured variable has weak or no correlation with other measured variables designed to measure other conceptual variables. This is usually performed by comparing the square root of the correlation coefficient and the square root of AVE between the variables28. In the scale, the postural assessment and the overall spinal function assessment had some degree of differentiation from each other, indicating that the scale had an ideal discriminative validity.
Reliability analysis evaluates the magnitude of variance due to random errors during the assessment process29. This study incorporated the Cronbach’s α coefficient to evaluate the overall internal consistency of the scale, in other words, to examine whether the items of the scale measure the same construct [8,11-12]. Streiner 30 suggested that the Cronbach α coefficient should not exceed 0.9 and that the acceptable standard is 0.4-0.5 31. The results of this study show that the SSFS has an alpha coefficient of 0.727. which indicates that the scale has high reliability. After removing each individual item, the α coefficients maintained between 0.692-0.717, implying high internal consistency between items of the scale.
Since this assessment scale was completed through self-assessment, inter-rater reliability analysis was not conducted. Instead, the Spearman correlation coefficient was used to evaluate the test-retest reliability, and the time internal for repeated administration of the scale was selected to be two weeks after the initial data collection 32. Some researchers have also chosen a time interval of 24 hours in functional evaluation studies of patients with severe muscle weakness 9, perhaps taken into account the influence of clinical progression on test-retest reliability. The results of this study showed an excellent test-retest reliability of the total score (r=0.914, P<0.01), as well as good test-retest reliability of the postural assessment factor (r=0.800, P<0.01) and of the functional assessment factor (r=0.889, P<0.01). For the individual items of the SSFS, other than supine postural assessment (r=0.567, P<0.01), prone press-up (r=0.544, P<0.01) and wall angel (r=0.505, P<0.01), other items resulted in good test-retest reliability (r>0.6, P<0.01). The reliability analysis determines the SSFS to have high reliability.
Effect of BMI Levels on the Spinal Function in Healthy Population
This study analyzed the effect of BMI levels on the SSFS score and examined 819 samples with complete information. These subjects include military recruits, postsecondary students, and regular workers. The rate of spinal dysfunction in the underweight group as well as overweight and obese group in both male and female subjects were significantly higher than that in the normal weight group (P<0.05), indicating that excessive weight may increase the burden on the spine during movement, thus resulting in spinal dysfunction. For the purpose of the study, it has been subjectively determined that that a total score below 14 points indicates spinal dysfunction.
The results implied that the overall spinal function in the overweight and obese male subjects was significantly worse than that in the normal group (P<0.05), suggesting that excessive weight in young men may increase the burden on the spine during exercise, resulting in dysfunction. The neck flexor strength test was significantly lower in the male underweight group than in the male normal group, indicating that the muscle mass of young men with lower BMI may be relatively lower, leading to poorer performance of muscle strength testing. In female subjects, the spinal function in the overweight and obese groups was lower than that in the normal group, with no significant difference (P>0.05), which may be related to the smaller female sample size. In this study, the evaluation of lying postural assessment, neck flexor muscle strength, supine knee-to-chest, wall roll-down and the total SSFS score of the overweight and obese female subjects were significantly lower than those of the underweight group (P<0.05). The neck flexor muscle strength, prone press-back, wall angel and total SSFS score of the normal group were significantly lower than those of the underweight group (P<0.05). This suggests higher neck flexor muscle strength and better spinal posture and function in the underweight female subjects than in the overweight and normal weight female subjects. This finding may be related to the different exercise habits, physiological characteristics, and occupation in young healthy female population. Further investigation with larger sample size and in-depth objective analysis may be suggested.
Inadequacies and the way forward
Firstly, the subjects of this study were all healthy young adults with less than 18% of the subjects being female. Therefore, the results of this study are only applicable to this sample population, and further randomized controlled trials are recommended to extend its applicability to the general population, and to investigate the effects of sex, age, and occupation on the overall score.
The items in SSFS were selected by combining quantitative assessment and empirical evaluation, which can achieve more accurate "self-assessment". However, the reliability of various scale items in the evaluation of spinal function, such as the reliability, validity, sensitivity and specificity of wall roll-down, still need to be further reviewed. The functional scores of this scale are subjectively divided into three levels: good, satisfactory, and poor. Its rationality and accuracy need to be further studied in combination with relevant functional evaluation methods.
In addition, the current SSFS evaluation process requires a good level of comprehension and compliance from the subjects in order to be completed. Therefore, to improve the evaluation efficiency and enhance the participant experience, the SSFS can be administered by supplementary diagrams, animations or videos of the standardized test items in addition to the current text or verbal instructions. By integrating the engineering and material science methodology, the intelligent and accurate spine functional assessment and rehabilitation tools can be developed to further improve the scale and the assessment process, which can then be applicable to a wider range of sample population in the general public.