Table 2.

Physical test; 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
Strength tests
Kettlebells lift; kg x repetitions	>760	≤760	Pairs of kettlebells weighing 32, 24 or 16 kg each were used. The intended test weights were 2×32 kg, but subjects unable to perform the test safely with these loads could choose the lighter kettlebells. To make sure that the correct and safe lifting technique was used, all participants performed two test-lifts using a lower weight while being supervised by the test leader. The test measured the number of (correct) lifts of the weights performed in 1 min. Based on the assumption that marines with the lowest lifting capacity are at greater risk of LBP, the lower tertile of the product of ‘numbers of lifts x weight lifted’ was compared with the upper two tertiles (reference).
Pull-up; number of repetitions	≥4	≤3	Hanging from a pull-up bar, using an overhand grip with hands placed shoulder-width apart, the participants lifted their body until their chin was level with the bar. The number of (correct) lifts performed in 1 min was recorded in the test protocol. The number of correct ‘chins’ is dichotomised as ≤3 or ≥4 (reference). Internationally, the cut-off for passing a pull-up test during yearly physical assessments for marines ranges from 3 (US marines) to 5 (Royal Marines) and as such, assuming that marines with the lowest pull-up capacity are at greater risk of LBP, the cut-off for the reference category was set at the median, ≥4 pull-ups (reference).
Movement control tests			To make sure failure of any of the movement control tests was due to a ‘real’ inability to control direction and not unfamiliarity with the test movement, all participants performed the test three to six times with feedback from the tester to ensure familiarisation. To monitor the movement of the lumbar spine, an air-filled pressure sensor (Pressure Biofeedback Unit, Chattanooga Group, Hixson, TN) was placed under the lower back.
Double Leg Lift & Lower	pass	fail	The test assesses the ability to prevent extension and flexion of the lumbar spine.22 The subject, from a supine position, lifts both feet off the bench to a 90° hip flexion, and then lowers them back to the bench. Any uncontrolled movements in flexion or extension, defined as an ≥5 mm Hg change (from the starting pressure of 40 mm Hg), were recorded on the test protocol. Test performance on flexion and extension assessed in the tests was analysed as pass or fail.
Double Leg Lift & Alternate Leg Extension:	pass	fail	The test assesses the ability to prevent extension, flexion and rotation of the lumbar spine, and leg abduction, lateral rotation and hip forward glide.22 The subject, from a supine position lifts both feet off the bench to a 90° hip flexion, then lowers and straightens one leg to a fully extended position and then back to a 90° hip flexion, before repeating the test on the other side. The direction of any uncontrolled movements, defined as ≥5 mm Hg change (from the starting pressure of 40 mm Hg), was recorded on the test protocol. Test performance for flexion and extension assessed in the tests was analysed as pass or fail.

LBP, low back pain.