How do prosthetic stiffness, height and running speed affect the biomechanics of athletes with bilateral transtibial amputations?

Abstract

Limited available information describes how running-specific prostheses and running speed affect the biomechanics of athletes with bilateral transtibial amputations. Accordingly, we quantified the effects of prosthetic stiffness, height and speed on the biomechanics of five athletes with bilateral transtibial amputations during treadmill running. Each athlete performed a set of running trials with 15 different prosthetic model, stiffness and height combinations. Each set of trials began with the athlete running on a force-measuring treadmill at 3 m s^-1, subsequent trials incremented by 1 m s^-1 until they achieved their fastest attainable speed. We collected ground reaction forces (GRFs) during each trial. Prosthetic stiffness, height and running speed each affected biomechanics. Specifically, with stiffer prostheses, athletes exhibited greater peak and stance average vertical GRFs (β = 0.03; p < 0.001), increased overall leg stiffness (β = 0.21; p < 0.001), decreased ground contact time (β = -0.07; p < 0.001) and increased step frequency (β = 0.042; p < 0.001). Prosthetic height inversely associated with step frequency (β = -0.021; p < 0.001). Running speed inversely associated with leg stiffness (β = -0.58; p < 0.001). Moreover, at faster running speeds, the effect of prosthetic stiffness and height on biomechanics was mitigated and unchanged, respectively. Thus, prosthetic stiffness, but not height, likely influences distance running performance more than sprinting performance for athletes with bilateral transtibial amputations.

Keywords: amputee; prosthesis; sprinting.

Figure 1.

Illustration of a (a) spring–mass model and (b) spring–mass model with two in-series leg springs. Body mass is represented as a point mass (circle) and the touch-down angle is indicated by θ. (a) The stance leg is represented by a massless linear spring for non-amputees, and (b) two in-series massless linear springs for athletes with bilateral amputations. The initial leg length (L₀) shortens (ΔL), and vertical height (Δy) decreases during the stance phase of running. Modelled residual limb length (Res₀) and prosthetic height (RSP₀) compress and extend (ΔRes and ΔRSP) during the stance phase of running.

Figure 2.

From left to right, (a) the Össur Cheetah Xtend prosthesis (J-shaped) at a representative recommended height, (b) the Freedom Catapult FX6 prosthesis (C-shaped) at a representative height of +2 cm and (c) the Ottobock 1E90 Sprinter prosthesis (J-shaped) at a representative height of −2 cm. The C-shaped prostheses are connected to sockets via aluminium pylons, and the J-shaped prostheses are connected to sockets via custom aluminium brackets. (Online version in colour.)

Figure 3.

The average (±s.e.) stiffness of the overall leg (k_leg), the prosthesis (RSP; k_RSP) and the residual limb (k_res) across 3 through 7 m s⁻¹ and across all prosthetic configurations. Across all conditions, simple linear regression equations follow as: k_leg = −0.30 Speed + 16.4, R² = 0.05, p < 0.001; k_res = −4.0 Speed + 56.0, R² = 0.16, p < 0.001; k_RSP (kN m⁻¹) = 0.82 Speed + 22.9, R² = 0.07, p < 0.001. Error bars indicate inter-subject variability and may be hidden behind the symbols.

Figure 4.

(a) Contact length (L_c), (b) stance average vertical GRF, (c) contact time (t_c) and (d) step frequency, while using RSPs averaged from three different models at one stiffness category below recommended (−1 Cat), at recommended (Rec Cat) and at one stiffness category greater than recommended (+1 Cat) across running speeds. Prosthetic stiffness categories correspond to a 70 kg athlete. See table 2 for prosthetic stiffness values (k_RSP in kN m⁻¹) used at each speed (v) and stiffness category recommendation. Biomechanical data are derived from statistical linear mixed models. The linear mixed model regression equations follow as: (a) L_c = 0.08 v − 0.02 k_RSP + 0.001 v · k_RSP + 0.76; (b) avg vertical GRF = 0.11 v + 0.03 k_RSP − 0.003 v · k_RSP + 0.75; (c) t_c = −0.038 v − 0.007 k_RSP + 0.001 v · k_RSP + 0.446; (d) step frequency = 0.315 v + 0.042 k_RSP − 0.005 v · k_RSP + 1.258.

Figure 5.

(a) Touch-down angle (θ), (b) leg spring compression (ΔL) and (c) peak vertical GRF averaged from three different running-specific prosthetic models at one stiffness category below recommended (−1 Cat), at recommended (Rec Cat) and at one stiffness category greater than recommended (+1 Cat) across running speeds. Prosthetic stiffness categories correspond to a 70 kg athlete. See table 2 for prosthetic stiffness values (k_RSP in kN m⁻¹) used at each speed (v) and stiffness category recommendation. Biomechanical data are derived from statistical linear mixed models. The regression equations follow as: (a) θ = 0.055 v + 0.004 k_RSP − 0.001 Speed · k_RSP + 0.145; (b) ΔL = 0.010 v − 0.001 k_RSP + 0.109; (c) peak vertical GRF = 0.10 v + 0.03 k_RSP + 1.59.

Figure 6.

Representative vertical GRF traces (left axis), and CoM vertical displacements (ΔY; right axis) as a function of time for a participant using Össur Cheetah Xtend prostheses at (a) −1 prosthetic stiffness category, (b) recommended prosthetic stiffness category and (c) + 1 prosthetic stiffness category. Light grey lines represent running at 3 m s⁻¹, medium grey lines represent running at 6 m s⁻¹ and black lines represent running at 9 m s⁻¹. On average, peak and stance average vertical GRFs increased with speed (v) (p < 0.001). Linear regressions for all participant and prosthetic combinations that achieved speeds 3 through 9 m s⁻¹ were peak vertical GRF = 0.115 v + 2.439, R² = 0.48, p < 0.001; stance average GRF = 0.047 v + 1.513, R² = 0.38, p < 0.001. However, some participant and prosthetic combinations (e.g. this figure) increased running speed despite decreasing vertical GRFs from 6 to 9 m s⁻¹.

Figure 7.

The average (±s.e.) elicited step frequency (StF) and step length (StL) at each running speed (v) across all prosthetic configurations. Linear regression equations follow as: StF = 0.20 v + 2.35, R² = 0.67, p < 0.001; StL = 0.20 v + 0.49, R² = 0.88, p < 0.001.