This study quantified the physiological and biomechanical effects of the 20 lb (9.07 kg, males) and 14 lb (6.35 kg, females) weighted vest used in CrossFit, and whether they were predisposed to injury. Twenty subjects (10 males, 10 females) undertook walking (0%, 5% and 10% gradient) and running trials in two randomised study visits (weighted vest/no weighted vest). Physiological demand during walking was increased with the vest at 10% but not 5% or 0% with no change in gait variables. In the running trial, the weighted vest increased oxygen uptake (males; females) (þ0.22L/min, p < 0.01; þ0.07 L/min, p < 0.05), heart rate (þ11bpm, p < 0.01; þ11bpm, p < 0.05), carbohydrate oxidation (þ0.6 g/min, p < 0.001; þ0.2 g/min, p < 0.01), and energy expenditure (þ3.8 kJ/min, p < 0.001; þ1.5 kJ/min, p < 0.05) whilst blood lactate was increased only in males (þ0.6 mmol/L, p < 0.05). There was no change in stride length or frequency. Weighted vest training increases physiological stress and carbohydrate oxidation without affecting measured gait parameters.Practitioner summary: We examined the effect of weighted vest training prescribed in CrossFit (20 lb/9.07 kg, males and 14 lb/6.35 kg, females) in a randomised controlled trial. We found that physiological stress is increased in both sexes, although three-fold greater in males, but with no change in biomechanical gait that predisposes to lower-limb injury.