External work and peak power are reliable measures of ergometer grinding performance when tested under load, deck heel, and grinding direction conditions.

The reliability of grinding performance was assessed in 18 current Emirates Team New Zealand America's Cup sailors in two test sessions separated by 5 h using a custom-built ergometer. Sixteen different grinding conditions that varied by load (Light 39 N x m, Moderate 48 N x m, Heavy 68 N x m), deck heel (Flat 0 degrees control, Downhill 25 degrees, Uphill 25 degrees, Right 25 degrees, Left 25 degrees), and grinding direction (forwards, backwards) were assessed using peak power and external work over 5 s during maximal-effort 8-s grinds. Reliability statistics included the difference in mean (M(diff)), standard error of measurement (SEM), and intraclass correlation coefficients (ICC). External work (SEM = 1.6-6.9%; ICC = 0.91-0.99) was a more consistent performance measure than peak power (SEM = 1.3-9.6%; ICC = 0.84-0.99) across all test conditions. Testing under different load conditions resulted in external work SEMs of 1.6-3.9% with performance more reliable in lighter load conditions. Grinding performance during different heel conditions was less reliable (external work SEMs = 4.6-6.9%). Grinding direction (forward or backward) did not appear to affect performance reliability, although external work was 10-15% higher in forward grinding. Reliability is acceptable across various loads, but testing under different heel conditions may need some protocol development to allow the detection of smaller differences in performance.