Comparison of Clegg and TruFirm surface hardness for predicting tennis ball bounce standards on grass courts under match play

Participants playing tennis have increased by 33% since 2020. Vertical ball bounce (BB) ≥ 80% relative to concrete is the standard for all courts. The 0.5-kg Clegg impact soil tester (CIST) measures surface hardness (gravities, g) and is the standard for predicting tennis BB. No published research is available for critical CIST values to satisfy BB standards. TruFirm, used in golf, measures deformation depth (mm) and shows potential to relate to tennis BB as an alternative to CIST. To that end, tennis BB and 0.5-kg CIST were investigated over several years on cool-season turf species at the UMass Troll Turf Research Center under tennis match play. Published paired CIST and BB (n = 454) were combined with 3 years of current paired data (n = 1870) to assess BB and CIST. TruFirm and BB (n = 1276) were also compared. Regression models were developed for CIST and TruFirm to establish statistically derived BB standards. Field validation (model predicted vs. observed BB) for TruFirm and CIST was tested (n = 1254). The CIST device was more effective in predicting tennis BB based on r²-values (=0.45), validation r² (=0.53), and with a smaller statistical bias (<1 SE) compared to TruFirm (r²-values ranging from 0.27 to 0.41), validation r² (=0.40) with a larger statistical bias (>1 SE). Hardness of 150–170 g using CIST and TruFirm values of 5.8–9.6 mm were statistically derived (95% confidence interval) values for BB standards. TruFirm was not as effective as CIST with two to five times the bias in predicting tennis BB. Only 13.7% of all BB impacts (318 of 2326) satisfied ≥80% concrete standard. Similarly, 14.5% of all CIST impacts (≥150 g) closely approximated BB standards.