Reduced prosthetic stiffness lowers the metabolic cost of running for athletes with bilateral transtibial amputations

Abstract

Inspired by the springlike action of biological legs, running-specific prostheses are designed to enable athletes with lower-limb amputations to run. However, manufacturer's recommendations for prosthetic stiffness and height may not optimize running performance. Therefore, we investigated the effects of using different prosthetic configurations on the metabolic cost and biomechanics of running. Five athletes with bilateral transtibial amputations each performed 15 trials on a force-measuring treadmill at 2.5 or 3.0 m/s. Athletes ran using each of 3 different prosthetic models (Freedom Innovations Catapult FX6, Össur Flex-Run, and Ottobock 1E90 Sprinter) with 5 combinations of stiffness categories (manufacturer's recommended and ± 1) and heights (International Paralympic Committee's maximum competition height and ± 2 cm) while we measured metabolic rates and ground reaction forces. Overall, prosthetic stiffness [fixed effect (β) = 0.036; = 0.008] but not height ( ≥ 0.089) affected the net metabolic cost of transport; less stiff prostheses reduced metabolic cost. While controlling for prosthetic stiffness (in kilonewtons per meter), using the Flex-Run (β = -0.139; = 0.044) and 1E90 Sprinter prostheses (β = -0.176; = 0.009) reduced net metabolic costs by 4.3-4.9% compared with using the Catapult prostheses. The metabolic cost of running improved when athletes used prosthetic configurations that decreased peak horizontal braking ground reaction forces (β = 2.786; = 0.001), stride frequencies (β = 0.911; < 0.001), and leg stiffness values (β = 0.053; = 0.009). Remarkably, athletes did not maintain overall leg stiffness across prosthetic stiffness conditions. Rather, the in-series prosthetic stiffness governed overall leg stiffness. The metabolic cost of running in athletes with bilateral transtibial amputations is influenced by prosthetic model and stiffness but not height. We measured the metabolic rates and biomechanics of five athletes with bilateral transtibial amputations while running with different prosthetic configurations. The metabolic cost of running for these athletes is minimized by using an optimal prosthetic model and reducing prosthetic stiffness. The metabolic cost of running was independent of prosthetic height, suggesting that longer legs are not advantageous for distance running. Moreover, the in-series prosthetic stiffness governs the leg stiffness of athletes with bilateral leg amputations.

Prosthetic shape, but not stiffness or height, affects the maximum speed of sprinters with bilateral transtibial amputations

“…The location of the RSP attachment to the sockets, posterior for J-shaped versus distal for C-shaped models [5], and their design (Fig 1) could have introduced discrepancies such as different masses and moments of inertia that may have altered running biomechanics and maximum speed. We acknowledge that our sample size (n = 5) limits our statistical power, but along with previous data from our group [5,24], to our knowledge this is the largest to date published dataset of athletes with bilateral TTAs. Furthermore, the magnitude of RSP stiffness and height changes used in this study may have reduced our ability to detect statistically significant effects of RSP configuration on maximum speed.…”

“…Five male athletes with bilateral transtibial amputations (age: 24.8 ± 4.8 years) participated and were the same subjects as in Beck et al [5,24]. All participants had at least one year of experience running using their own RSPs and competed in sanctioned track and field races ( Table 1).…”

“…RSPs mimic the spring-like behavior of tendons and ligaments during running by storing elastic energy during the first half of the contact phase and returning most of this energy during the second half of the contact phase [6,20,24]. In non-amputees, the storage and release of elastic energy in the Achilles tendon is described by Mero et al [10] as a "force performance potentiation" that allows non-amputee sprinters to apply greater forces on the ground at progressively faster speeds.…”

“…All of these hysteresis values suggest that a non-trivial portion of energy is lost in each step during running in tendons and in the RSPs used by athletes with TTAs. Different hysteresis values among RSP models [20] affect the relative amount of energy returned in each step [24] and may therefore influence sprinting performance in athletes with TTAs.…”

Adding carbon fiber to shoe soles does not improve running economy: a muscle-level explanation

“…38 Additionally, compared to the Adidas footwear, the respective Nike soles are 8 mm taller (35-62% taller depending on midsole location), the midsole foam is roughly half as stiff (in-series linear stiffness, not bending), and its hysteresis is 11.1% less during vertical loading and unloading. 38 Because both decreased midsole foam linear stiffness 39–41 and relative mechanical energy dissipation 42 in-series to the stance-limb are associated with more economical running, Nike footwear may elicit superior running economy values than Adidas footwear due to their relatively compliant and resilient midsole foam – not increased bending stiffness.…”