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Connecting the legs with a spring improves human running economy.

ABSTRACT: Human running is inefficient. For every 10 calories burned, less than 1 is needed to maintain a constant forward velocity - the remaining energy is, in a sense, wasted. The majority of this wasted energy is expended to support the bodyweight and redirect the center of mass during the stance phase of gait. An order of magnitude less energy is expended to brake and accelerate the swinging leg. Accordingly, most devices designed to increase running efficiency have targeted the costlier stance phase of gait. An alternative approach is seen in nature: spring-like tissues in some animals and humans are believed to assist leg swing. While it has been assumed that such a spring simply offloads the muscles that swing the legs, thus saving energy, this mechanism has not been experimentally investigated. Here, we show that a spring, or 'exotendon', connecting the legs of a human reduces the energy required for running by 6.4±2.8%, and does so through a complex mechanism that produces savings beyond those associated with leg swing. The exotendon applies assistive forces to the swinging legs, increasing the energy optimal stride frequency. Runners then adopt this frequency, taking faster and shorter strides, and reduce the joint mechanical work to redirect their center of mass. Our study shows how a simple spring improves running economy through a complex interaction between the changing dynamics of the body and the adaptive strategies of the runner, highlighting the importance of considering each when designing systems that couple human and machine.

SUBMITTER: Simpson CS

PROVIDER: S-EPMC6765174 | biostudies-literature | 2019 Sep

REPOSITORIES: biostudies-literature

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Connecting the legs with a spring improves human running economy.

Simpson Cole S CS Welker Cara G CG Uhlrich Scott D SD Sketch Sean M SM Jackson Rachel W RW Delp Scott L SL Collins Steve H SH Selinger Jessica C JC Hawkes Elliot W EW

The Journal of experimental biology 20190903 Pt 17

Human running is inefficient. For every 10 calories burned, less than 1 is needed to maintain a constant forward velocity - the remaining energy is, in a sense, wasted. The majority of this wasted energy is expended to support the bodyweight and redirect the center of mass during the stance phase of gait. An order of magnitude less energy is expended to brake and accelerate the swinging leg. Accordingly, most devices designed to increase running efficiency have targeted the costlier stance phase ...[more]

PMID: 31395676

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Project description:BackgroundThe 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.MethodsA 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 (Ec) 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).ResultsThe functional balance ratio was significantly and negatively associated with Ec 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 Ec (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 Ec when assessed at 10 km h-1 using a 3% gradient (r = 0.59, P = 0.03). No significant relationships were found between Ec and total body mass or fat-free mass.DiscussionGiven that the functional balance ratio was associated with a better Ec, coaches and athletes should consider implementing a specific strengthening program for hip flexor muscles to increase the functional ratio.

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Connecting the legs with a spring improves human running economy.

Publications

Connecting the legs with a spring improves human running economy.

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