Figure 1 illustrates the study search process. The search yielded 3543 records, and after the screening process, 45 full-text reports of 39 studies were deemed eligible for inclusion. Ineligible reports with reasons for exclusion are documented in Supplement G of the ESM. Ten author groups [56,57,58,59,60,61,62,63,64,65] were followed up with data requests for missing data. Four author groups replied [57, 59, 62, 63] and two provided data for three reports (Supplement D of the ESM) [62, 63, 66].

Preferred reporting items for systematic reviews and meta-analyses (PRISMA) flowchart of the study selection process

In total, there were 4152 participants (mean: n = 101, median: n = 46, minimum: n = 17, maximum: 778, female: 20%), which included 2219 participants in the exposure groups (mean: n = 35, median: n = 22, minimum: n = 8, maximum: n = 219, female: 23%), and 1942 control participants (mean: n = 47, median: n = 20, minimum: n = 4, maximum: n = 716, female 17%). For participants with physical loading exposure, the mean age ranged from 12 to 68 years (studies reporting: n = 38, 97%), and the percentage with LBP was 37% (studies reporting: n = 31, 79%). For the control groups, mean age ranged from 11 to 56 years (studies reporting: n = 35, 90%) and the percentage with LBP was 14% (studies reporting: n = 30, 79%).

The physical loading exposure population groups had varied inclusion criteria, including national ranking [16, 60, 67, 68], elite [41, 58, 65, 69,70,71,72,73,74,75,76,77,78,79], greater than 12 weeks [80], 12 months [81], 2 years [82], 5 years [56, 59, 62, 83, 84], or 8 years [64, 85], regular power or endurance sports over the lifetime [61], running greater than 50 km per week [86] and/or 20–40 km per week [63, 87], running greater than 100 km per month [88], cycling greater than 150 km per week for greater than 5 years, [63, 66] yoga instructors [89] and individuals engaging in physical activity in the past 14 days [90], 20.1–28.9 metabolic equivalent hours per week [91], greater than 2 h per week [92], and vigorous physical activity [93, 94]. Physical loading exposures varied, including rowing [58, 65, 81], yoga [80, 89], skiing [41, 61, 69, 75, 76, 85], weightlifting [16, 56, 61, 67, 82], wrestling [16, 67, 68], orienteering (i.e. running to find locations on a map) [67], ice hockey [67], baseball [83, 84], basketball [83, 84], Chinese martial arts [78], kendo (i.e. Japanese martial art) [83, 84], running [61, 63, 83, 84, 86,87,88], soccer [68, 83, 84], swimming [70, 71, 83, 84], cycling [63, 66], horseback riding [72], dance [64], gymnastics [60, 68, 73, 95], cricket [74, 96], tennis [68], climbing [79], cross-fit [77], any regular sport [59, 62], high physical activity volume [90,91,92, 94], moderate physical activity volume [90,91,92,93] and vigorous physical activity intensity [93, 94].

The comparator groups consisted of non-athletes not participating in the same sport as the exposure [41, 56, 64, 72, 74, 75, 79, 80, 88, 89] or not participating in any regular sport [58,59,60,