Table 1.

Self-reported independent variables, the form in which they were included in regression analysis, procedures for retrieving the data and rationale for categorisation

Independent variable	Reference	Exposure	Measurement procedure and variable management
Physical characteristics
Body weight	Continuous		Body weight (in kg) was self-reported and analysed as a continuous variable in the models.
Body height	>1.80 m	≤1.80 m	Body height was self-reported. Based on the hypothesis that being either ‘too tall or too short’ may be negative for musculoskeletal health in this environment, as previously identified for this population,4 body height was initially categorised as≤1.80 m, 1.81–1.85 m (reference) and≥1.86 m (representing body height tertiles of the SwAF marine population,4 15 but was reduced to a dichotomised variable due to no difference between the upper and the reference category being identified.
Rated health/health history
Back pain; within 6 months prior to course start	No	Yes	Self-reported musculoskeletal pain in the lower and/or thoracic back, defined as ‘Pain a couple of days per month or less’ or ‘Pain a couple of days per week or more’ within the past 6 months, analysed dichotomised as yes or no as previously for this population.4 15
Hip/knee pain; within 6 months prior to course start	No	Yes	Self-reported occurrence of musculoskeletal pain in the hip and/or knee, defined as ‘Pain a couple of days per month or less’ or ‘Pain a couple of days per week or more’ within the past 6 months, analysed dichotomised as yes or no, as previously for this population.
Neck/shoulder pain; within 6 months prior to course start	No	Yes	Self-reported musculoskeletal pain in the neck and/or shoulder, defined as ‘Pain a couple of days per month or less’ or ‘Pain a couple of days per week or more’ within the past 6 months, analysed dichotomised as yes or no, as previously in this population.
Mental distress (GHQ-12 score)	<4	≥4	The level of mental distress was captured by the GHQ-12,64 a widely used screening instrument developed to detect ‘cases’ of mental distress. It is a 12-question tool, summed up to give an overall score, ranging from 0 to 12, and a cut-off of 4 points or more is considered an indication of clinically relevant mental distress.65 As such, ‘Mental distress’ was categorised as ≥4 on the summary GHQ-12 scale.
Work related
Current work ability with regard to best ever	≥9	<9	Self-rated work ability captured with the single item question from the work ability index.16 Current work ability was rated, with regard to ever best, on a 10-point ordinal scale. Based on the hypothesis that ‘less-than-optimal’ work ability could constitute a risk in this environment, the responses were dichotomised as high (≥9) (reference) and moderate (<9).
Direct from basic military training (within 3 months)	No	Yes	Finishing basic military training within 3 months of the course start was considered a risk, due to the assumption that these soldiers had had less time to adapt to load carriage within the military. Therefore dichotomised as yes or no (reference).
Physical training habits
Physical training; sessions per week	>2 sessions/week	≤2 sessions/week	Average number of training sessions per week, exceeding 20 min, were rated on a five point ordinal scale as ≤1 day/week, 2 days/week, 3–4 days/week and ≥5 day/week. This item was derived (in addition to an increased number of maximum sessions) from items previously used in several public health cohorts in Sweden.66 67 A U-shaped relationship with LBP was hypothesised for number of physical training sessions per week, i.e. too little and too much training may both be risks for LBP. Consequently, the training sessions per week variable was categorised as ≤2 session/week, 3–4 sessions/week (reference) and ≥5 sessions/week, but reduced to a dichotomised variable for LBP limiting work ability as no significant difference between the upper and reference category was found.
Muscular strength training; session per week	2-4 sessions/week	≤1 sessions/week ≥5 sessions/week	A U-shaped relationship with LBP was hypothesised for number of strength training sessions per week, that is, too little and too much training may both be risks for LBP. Consequently Weekly strength training was categorised as ≤1 session/week, 2–4 sessions/week (reference) and ≥5 sessions/week.
Aerobic fitness training; sessions per week	>1 session/week	≤1 sessions/week	Weekly aerobic training was dichotomised as ≤1 session/week or >1 (reference), given two session per week a priori considered to be a realistic minimal amount of cardio vascular training necessary to maintain sufficient aerobic capacity during the physically demanding basic military training course.

GHQ, general health questionnaire; LBP, low back pain; SwAF, Swedish Armed Forces.