The determination of energy expenditure is of major interest in training load and performance assessment. Small, wireless accelerometer units have the potential to characterise energy expenditure during swimming. The correlation between absorbed oxygen versus flume swimming speed and absorbed oxygen versus the three axis acceleration recorded on the sacrum, wrist and ankle for swimmers of varying abilities was calculated using Bland-Altman analysis of variance through parallel regression lines fitted for 60 participants, who swam at three different speeds for 6 min duration with 2 min rest times. Swimmers showed a strong positive relationship between VO₂ and RMS acceleration on the wrist (r = 0.77) and ankle (r = 0.73) sensors but not on the sacrum (r = 0.46). The sacrum data was split into elite and novice swimmers, resulting in a strong correlation for elite swimmers and a poor correlation for novice swimmers. A robust biomechanical technique for the determination of the energy expenditure of swimmers of different categories and genders from acceleration data has been developed.