Hip muscular strength balance is associated with running economy in recreationally-trained endurance runners

BACKGROUND: The percentage of sustained maximal oxygen uptake and the running economy are important factors that determine the running success of endurance athletes. Running economy is defined as the oxygen uptake required to run at a given speed and depends on metabolic, cardiorespiratory, biomechanical, neuromuscular, and anthropometric factors. With regard to anthropometric characteristics, total body mass seems to be a crucial factor for the running economy. Moreover, neuromuscular components, especially knee muscular strength and the strength balance ratio, also seem to be critical for the running economy. In addition to knee muscle strength, hip muscle strength is also an important contributor to running performance on level or hilly ground. However, the relationship between running economy and the hip muscles is unknown. Thus the aim of the present study was to verify whether hip flexor and extensor isokinetic peak torque, the isokinetic strength balance ratio, total body mass and fat free mass were associated with running economy in both sexes and to compare sex differences in physical fitness and isokinetic strength characteristics. METHODS: A total of 24 male (31.0 ± 7.7 years, 176.2 ± 7.3 cm, and 70.4 ± 8.4 kg) and 15 female (31.3 ± 6.7 years, 162.9 ± 3.9 cm, and 56.0 ± 5.3 kg) recreationally-trained endurance runners were recruited. Maximal oxygen uptake, running economy, conventional (concentric flexors-to-concentric extensors) and functional (concentric flexors-to-eccentric extensors) hip isokinetic strength balance ratios, peak torque of the hip flexor and extensor muscles, total body mass, and fat-free mass were measured. Running economy was assessed on two separate days by means of the energy running cost (E(c)) using a motorized treadmill at 10.0 and 12.0 km h(−1) (3% gradient) and 11.0 and 14.0 km h(−1) (1% gradient). RESULTS: The functional balance ratio was significantly and negatively associated with E(c) at 11.0 (r = − 0.43, P = 0.04) and 12.0 km h(−1) (r = − 0.65, P = 0.04) when using a 3% gradient in male runners. Considering muscular strength, male runners only showed a significant relationship between E(c) (assessed at 12 km h(−1) and a 3% gradient) and peak torque for extensor muscle eccentric action (r = 0.72, P = 0.04). For female runners, only peak torque relative to total body mass for extensor muscles (180° s(−1)) was positively associated with E(c) when assessed at 10 km h(−1) using a 3% gradient (r = 0.59, P = 0.03). No significant relationships were found between E(c) and total body mass or fat-free mass. DISCUSSION: Given that the functional balance ratio was associated with a better E(c), coaches and athletes should consider implementing a specific strengthening program for hip flexor muscles to increase the functional ratio.