PURPOSE:To investigate the acute effects of heavy bilateral and unilateral back squats on straight-line sprint running performance in a group of resistance-trained (RT) and sprint-trained (ST) athletes. METHODS: In a randomized cross-over design, a group of RT men (n=5; age: 19.8 ± 1.5 years; height: 1.83 ± 0.03 m; mass: 100.0 ± 9.5 kg) and a group of ST men (n=7; age: 18.9 ± 1.1 years; height: 1.77 ± 0.08 m; mass: 74.4 ± 5.5 kg) attended four testing sessions during a two week period. Each subject had their 1-repetition maximum (1-RM) back squat assessed both bilaterally and unilaterally during the first two sessions. During the final two sessions each subject performed three baseline 55 m sprints separated by two minutes of recovery. Following a further five minutes of recovery, each subject completed a series of heavy back squats performed with loads equivalent to 55, 75, and 90% of their bilateral (HBS) or unilateral (HUS) 1-RM. A further three 55 m sprints were completed four minutes after the heavy squat protocols. Analysis of variance was used to determine the differences between the sprint times following HBS and HUS and the differences in 1-RM loads between RT and ST. Alpha was set at ≤ 0.05. RESULTS: The bilateral 1-RM loads were greater than the unilateral 1-RM loads (mean difference: 53.4 kg, p < 0.001) while the bilateral deficit (bilateral 1-RM minus unilateral 1-RM) in the ST was less than RT (ST mean difference: 41.9 kg; RT mean difference: 65.0 kg; p = 0.12). The ST group were significantly faster than the RT group (mean difference: -0.64 s; p = 0.006). However, the HBS resulted in an increase in sprint time (mean difference: 0.09 s; p = 0.038) as did the HUS (mean difference: 0.12 s; p = 0.008) with no differences between the RT and ST groups. CONCLUSIONS: Neither HBS nor HUS were effective in eliciting acute improvements in short sprint performance in resistance-trained or sprint-trained men.