Effects of the Menstrual Cycle on Running Economy: Oxygen Cost Versus Caloric Cost

Purpose: This study investigated the effects of the menstrual cycle on running economy (RE). Method: Eleven eumenorrheic female athletes (mean age: 21.18 ± 3.65 years, height: 170.2 ± 6.6 cm, VO₂max: 49.25 ± 9.15 mL·kg^-1·min^-1, and menstrual cycle: 29.8 ± 0.98 days) were tested for anthropometric variables, physiological responses (oxygen consumption [VO₂], blood lactate [LA], heart rate [HR], and respiratory exchange ratio [RER]) at rest and while running. The RE was measured at speeds of 75%, 85%, and 95% of the lactate threshold at 3.5 mmol·L^-1 during the follicular (FP) and luteal phases (LP) of the menstrual cycle. The RE was evaluated as oxygen consumption (mL·kg·min^-1 [O_{2C_}min], mL·kg^-1·km^-1 [O_{2C_}km]) and caloric unit cost (kcal·kg^-1·km^-1 [E_C]) during both phases. Results: There were no significant differences in body composition or resting physiological measurements between the LP and FP (p > .05). Physiological responses measured during RE tests were similar in both phases (p > .05). The RE measured as O_{2C_}min, O_{2C_}km, and E_C was significantly lower during the LP than during the FP (p < .05). The RE defined as O_{2C_} min significantly increased with speed (p < .05), but RE defined as O_{2C_}km and E_C was unaffected by speed increment (p > .05). Conclusions: The RE is better in the LP than the FP and is independent of running speed when RE is evaluated as O_{2C_}km and E_C. The menstrual cycle had no effect on body composition and physiological variables measured at rest.