Ground condition as a risk factor in sports injury aetiology studies: the level of concordance between objective and subjective measures

BACKGROUND: It is well known that the condition and type of sporting ground influences the risk of sports injury. However, the lack of evidence on the relationship between subjective and objective sporting ground condition assessments in sports injury aetiology studies has implications for the development of effective injury prevention strategies. This paper aims to examine concordance between subjectively rated and objective ground hardness and moisture measurements to inform data collection methods for future sports injury aetiology studies. METHODS: Subjective, observational assessments of ground hardness and soil moisture were recorded on 36 occasions during an Australian football season using two four-point scales of ‘very soft’ to ‘very hard’ and ‘very wet’ to ‘very dry’, respectively. Independent, objectively measured hardness and soil moisture were also undertaken at nine locations on the same grounds. The maximum and minimum ground values and the computed average of ground hardness and soil moisture were analysed. Somer’s d statistic was calculated to measure the level of concordance between the subjective and objective measures. RESULTS: A significant, moderate to substantial level of agreement was found between the subjective ratings and the average objective hardness values (d = 0.467, p <0.001), but there was perfect agreement on just less than half of the occasions. The level of concordance between the subjective and objective moisture ratings was low to moderate or trivial for all moisture measures (0.002 < d <0.264, p >0.05). CONCLUSIONS: Compared to objective measures, the subjective assessments were more accurate for ground hardness than for soil moisture levels and raters were just as likely to underestimate or overestimate the condition under review. This has implications for future sports injury aetiology studies that include ground condition assessments and particularly the use of subjective measures to underpin the development of future injury prevention strategies. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s40621-014-0027-y) contains supplementary material, which is available to authorized users.