This study investigated the effects of reduced quarter time due to COVID-19 pandemic rule changes, on running performance and injuries in Australian Football. Microsensor data for eight matches performed by the same 17 players were compared between the 2019 (standard) and 2020 (COVID-19) seasons using linear and generalised mixed models. Injury rates were assessed in 34 players across the full 2019 season, and 32 players across the full 2020 season. The total distance (ES = 1.28 [0.55 to 2.02]), high-speed (>18 km/h) (ES = 0.44 [-0.24 to 1.12]) and very highspeed (>24 km/h) (ES = 0.27 [-0.41 to 0.94]) distances, PlayerLoad™ (ES = 0.96 [0.25 to 1.67]), high-intensity efforts (ES = 0.48 [-0.20 to 1.16]), and accelerations (ES = 0.33 [-0.34 to 1.01]) were smaller (p ≤ 0.01) for the 2020 than the 2019 season. Expressed relative to playing time, distance (ES=-0.38 [-1.06 to 0.30]), PlayerLoad™ (ES = -0.27 [-0.94 to 0.41]), and acceleration efforts (ES = -0.50 [-1.18 to 0.18]) were greater (p < 0.05) for the 2020 than the 2019 season. No significant differences in maximum ball-in-play periods nor the difference between the 1^st and 4^th quarters were evident. Injury rates remained similar between 2019 (3.36 per game) and 2020 (3.55 per game). However, the proportion of injuries that led to lost time (missed games) was greater for the 2020 (38%) than 2019 season (24%). The changes in the rules had a profound impact on player performance and increased the likelihood of time loss injuries.

• Rule changes had predominantly a small effect on running performance

• Rule changes had no effect upon the decrement of running performance

• Rule changes had no effect on injury rates

• Rule changes increased the severity of injuries