The present study investigates which physiological sources support the increase in mechanical power output (W out) that can be obtained using klapskates in speed skating. It was hypothesized that the increase in W out could be achieved through an increase in gross efficiency or an increase in aerobic power (W aer). Six speed skaters performed a submaximal and maximal 1,600-m skating test with both klapskates and conventional skates, to measure gross efficiency and maximal W aer during speed skating. The rate of oxygen uptake (VO2) and post-exercise blood lactate concentrations ([La]) were measured and video recordings were made. W aer was calculated from VO2. W out was derived from the power needed to overcome air and ice friction. Gross efficiency was calculated as the ratio of W out and W aer. In the maximal tests, the subjects skated faster with klapskates compared to conventional skates (10.0 vs 9.6 m x s(-1)). They sustained the resulting higher W out with klapskates with an equal VO2. [La] was, however, 1.7 mmol x l(-1) higher when klapskates were used, which might reflect an increase in anaerobic power. During the submaximal tests the skaters generated equal W out with both types of skate. Although not statistically significant, VO2 and W aer were, on average, lower when klapskates were used compared to conventional skates [mean (SD) 0.3 (0.43) l x min(-1), 105 (143) W]. Despite the lack of a statistically significant difference in W aer, gross efficiency was shown to be significantly higher with klapskates compared to conventional skates (16.3% vs 14.8%, P = 0.02). We conclude that the increase in W out when the subjects were using klapskates could be explained by an increase in gross efficiency rather than an increase in W aer.