Design: The study was retrospective. The subjects were selected randomly in the two groups and studied on three occasions (rounds) separated by approximately three months.

Subjects: There were 109, 101, and 80 NPNL women and 45, 31 and 16 lactating women in rounds 1, 2 and 3 respectively, 19-43 y of age, living under economically deprived conditions in Call, Colombia, who participated in the study.

Methods: Muscular efficiency was measured as delta efficiency on a cycle ergometer.

Results: Muscular efficiency was significantly higher in lactating women in all three rounds compared to NPNL women. In six women it was possible to measure efficiency at variable times prior to their pregnancies, and again during lactation about three months post partum. There was a statistically significant (P = 0.03) increase in muscular efficiency during lactation.

Conclusion: Lactation results in about a 5% increase in muscular efficiency which may contribute to the adaptation of the mother to the increased energy demands associated with lactation.