Energy cost of running and Achilles tendon stiffness in man and woman trained runners

The energy cost of running (E(run)), a key determinant of distance running performance, is influenced by several factors. Although it is important to express E(run) as energy cost, no study has used this approach to compare similarly trained men and women. Furthermore, the relationship between Achilles tendon (AT) stiffness and E(run) has not been compared between men and women. Therefore, our purpose was to determine if sex‐specific differences in E(run) and/or AT stiffness existed. E(run) (kcal kg(−1) km(−1)) was determined by indirect calorimetry at 75%, 85%, and 95% of the speed at lactate threshold (sLT) on 11 man (mean ± SEM, 35 ± 1 years, 177 ± 1 cm, 78 ± 1 kg, [Image: see text] 1 = 56 ± 1 mL kg(−1) min(−1)) and 18 woman (33 ± 1 years, 165 ± 1 cm, 58 ± 1 kg, [Image: see text] 2 = 50 ± 0.3 mL kg(−1) min(−1)) runners. AT stiffness was measured using ultrasound with dynamometry. Man E(run) was 1.01 ± 0.06, 1.04 ± 0.07, and 1.07 ± 0.07 kcal kg(−1) km(−1). Woman E(run) was 1.05 ± 0.10, 1.07 ± 0.09, and 1.09 ± 0.10 kcal kg(−1) km(−1). There was no significant sex effect for E(run) or RER, but both increased with speed (P < 0.01) expressed relative to sLT. High‐range AT stiffness was 191 ± 5.1 N mm(−1) for men and 125 ± 5.5 N mm(−1), for women (P < 0.001). The relationship between low‐range AT stiffness and E(run) was significant at all measured speeds for women (r(2) = 0.198, P < 0.05), but not for the men. These results indicate that when E(run) is measured at the same relative intensity, there are no sex‐specific differences in E(run) or substrate use. Furthermore, differences in E(run) cannot be explained solely by differences in AT stiffness.