Parsonage, J, Secomb, JL, Sheppard, JM, Ferrier, BK, Dowse, RA, and Nimphius, S. Upper-body strength measures and pop-up performance of stronger and weaker surfers. J Strength Cond Res 34(10): 2982-2989, 2020-The primary purpose of this study was to investigate the reliability of the isometric push-up (IPU), dynamic push-up (DPU), and force plate pop-up (FP POP) as measures of upper-body isometric and dynamic strength qualities in surfing athletes. Furthermore, the study aimed to compare pop-up performance between stronger and weaker surfers. Eighteen female (n = 9) and male (n = 9) surfers (age = 28.1 ± 6.4 years, mass = 69.6 ± 10.4 kg, and height = 172.5 ± 6.7 cm) completed a battery of upper-body strength assessments, of which exhibited high between-day reliability: IPU, (coefficient of variation [CV%] = 4.7, intraclass correlation coefficient [ICC] = 0.96), DPU (CV% = 5.0, ICC = 0.90), and FP POP (CV% = 4.4, ICC = 0.90). Participants were subsequently split into stronger (n = 9) and weaker (n = 9) surfers based on normalized peak force (PF) attained in the IPU. Pop-up performance was measured both in the water and during the FP POP and was referred to as time to pop-up (TTP). Significant between-group differences were observed for normalized PF during IPU (d = 1.59, p < 0.01) and DPU (d = 0.94 p = 0.04). Although not significant, there was a large magnitude difference in FP POP (d = 0.80, p = 0.08) and FP TTP (d = 0.85, p = 0.07). Significant correlations were identified between normalized IPU PF and normalized DPU FP (r = 0.69, p = 0.03) and FP TTP (r = 0.73, p = 0.02) in the stronger group. The weaker group exhibited a significant inverse correlation between normalized IPU PF and in-water TTP (r = -0.77, p < 0.01). The results suggest improvements in pop-up performance may be elicited by improving dynamic strength for stronger surfers, whereas pop-up performance in weaker surfers may be elicited by improving maximum strength. The upper-body strength assessments provided a novel insight into strength qualities that are associated with in-water performance of surfers (TTP).