Exercise-induced muscle damage (EIMD) is known to decrease muscle strength and power, but its effects on endurance performance and running economy are unclear (1-4). The main aim of this study was to determine whether EIMD impairs running performance in a 30 min time trial. Thirty runners (24 men and 6 women, age 31 ± 9 years, [vdot]O_2max 54.1 ± 6.0 ml/min/kg) were matched for gender and randomly assigned to an EIMD or control group. Subjects in the EIMD group jumped 100 times from a 35 cm bench while controls did not perform any muscle-damaging exercise. Before and 48 hours after experimental treatment, subjects were tested on (1) markers of muscle damage, (2) steady-state cardiorespiratory, metabolic and perceptual responses during a constant speed run at 70% of [vdot]O_2max, and (3) distance ran in 30 min on a treadmill. Two-way (group x test) mixed ANOVAs were used for group analyses. Individual changes in performance were analysed using the Reliable Change Index (5). All markers of EIMD were significantly affected by experimental treatment: delayed-onset muscle soreness increased 4 points on a 0-6 scale, serum CK increased from 159 to 332 IU/l, knee extensors strength was reduced by 12% (all P < 0.01). EIMD significantly reduced 30 min time trial performance by 4% (P < 0.01) with 7 subjects in the EIMD group showing a reliable decrement in performance compared to only 1 in the control group (Fig. 1). Running economy and other physiological responses to submaximal running were not affected by EIMD (Table 1). However, a trend for increased perceived exertion was found. EIMD has a negative impact on endurance running performance in humans despite no changes in running economy. Further studies on the central and peripheral factors mediating fatigue in subjects with EIMD are warranted.