Effects of prosthetic stiffness and added mass on metabolic power and asymmetry in female runners with a leg amputation

Abstract

Similar to nonamputees, female athletes with unilateral transtibial amputation (TTA) using running-specific leg prostheses (RSPs) may have worse running economy and higher rates of running-related injury than male athletes. Optimizing RSP configuration for female athletes could improve running economy and minimize biomechanical asymmetry, which has been associated with running-related injury. Nine females with a TTA ran at 2.5 m/s while we measured metabolic rates and ground reaction forces. Subjects used an RSP with a manufacturer-recommended stiffness category, one category less stiff and two categories less stiff than recommended. Use of an RSP two categories less stiff resulted in 3.0% lower net metabolic power (P = 0.04), 7.8% lower affected leg stiffness (P = 6.01 × 10^-4), increased contact time asymmetry (P = 0.04), and decreased stance average vertical ground reaction force asymmetry (P = 0.04) compared with a recommended stiffness category RSP. Lower RSP stiffness (kN/m) values were associated with lower net metabolic power (P = 0.02), lower affected leg stiffness (P = 1.36 × 10^-4), longer affected leg contact time (P = 1.46 × 10^-4), and similar affected leg peak and stance-average vertical ground reaction force compared with higher RSP stiffness values. Subjects then used the RSP stiffness category that elicited the lowest net metabolic power with 100 g, 200 g, and 300 g added distally. We found no significant effects of added mass on net metabolic power, biomechanics, or asymmetry. These results suggest that female runners with a TTA could decrease metabolic power during running while minimizing biomechanical asymmetries, which have been associated with running-related injury, by using an RSP two categories less stiff than manufacturer recommended.NEW & NOTEWORTHY Females with unilateral transtibial amputation can improve running performance through reductions in net metabolic power by using a running-specific prosthesis (RSP) that is less stiff than manufacturer-recommended. Lower RSP stiffness values are associated with greater leg stiffness and contact time asymmetry, and lower stance-average vertical ground reaction force asymmetry. However, we found that adding mass to the RSP did not affect net metabolic power and stance-phase biomechanical asymmetries during running.

Keywords: amputee; biomechanics; energetics; running; symmetry.

Graphical abstract

Figure 1.

Study protocol. A: we measured the effects of using a running-specific prosthesis (RSP) with different stiffness categories while female athletes ran at 2.5 m/s using the RSP stiffness category recommended by the manufacturer, which is based on the user’s body mass and a high activity level. Athletes also used an RSP one and two categories less stiff than recommended. B: after determining the RSP stiffness category that elicited the lowest net metabolic power, we added mass 10 cm from the distal end of the athlete’s RSP. RSPs were either C-shaped or J-shaped. We measured net metabolic power, individual leg stiffness, as well as contact time, peak vertical ground reaction force, and stance average vertical ground reaction force symmetry for all conditions.

Figure 2.

A: Box and whisker plot of net metabolic power (W/kg) for running at 2.5 m/s using running-specific prosthesis (RSP) stiffness categories (Cat) that were two Cat less stiff (−2 Cat; n = 9) and one Cat less stiff (−1 Cat; n = 9) than the manufacturer-recommended Cat (Rec Cat) and the Rec Cat (n = 9). Colored dots represent subject-specific data, with group median shown as a horizontal black line. The median is derived using the lower and upper quartile values. The maximum and minimum values are displayed with vertical lines (“whiskers”) connecting the points to the center box. Data points lying outside of the whiskers represent outliers. Colored dots are offset for clarity. *Denotes a statistically significant difference from the −2 Cat using linear mixed-effects models. B: net metabolic power (W/kg) across RSP stiffness (kN/m) values. Colored dots represent subject-specific data (n = 9), and the black line represents the linear mixed model overall predictions across stiffness values [P = 0.02; Net Metabolic Power (W/kg) = 9.84 + 0.10 × RSP stiffness (kN/m)].

Figure 3.

A: Box and whisker plot of individual leg stiffness (kN/m) for running at 2.5 m/s for the affected leg and unaffected leg using running-specific prosthesis (RSP) stiffness categories (Cat) that were two Cat less stiff (−2 Cat; n = 9) and one Cat less stiff (−1 Cat; n = 9) than manufacturer-recommended (Rec Cat) and Rec Cat (n = 9). Colored dots represent subject-specific data, with group median shown as a horizontal black line. The median is derived using the lower and upper quartile values. The maximum and minimum values are displayed with vertical lines (“whiskers”) connecting the points to the center box. Data points lying outside of the whiskers represent outliers. Colored dots are offset for clarity. * Denotes a statistically significant difference from the −2 Cat using linear mixed-effects models. B: affected leg stiffness (kN/m) across RSP stiffness (kN/m) values for running at 2.5 m/s. Colored dots represent subject-specific data (n = 9), and the black line represents the linear mixed model overall predictions across stiffness values [P = 1.36 × 10⁻⁴; R² = 0.95; affected leg stiffness (kN/m) = 6.36 + 0.26 × RSP stiffness (kN/m)].

Figure 4.

Box and whisker plot of individual affected leg and unaffected leg values of A: contact time, B: peak vertical ground reaction force (vGRF), and C: stance-average vertical ground reaction force (vGRF) for running at 2.5 m/s using running-specific prosthesis (RSP) stiffness categories (Cat) that were two Cat less stiff (−2 Cat; n = 9) and one Cat less stiff (−1 Cat; n = 9) than manufacturer-recommended (Rec) and Rec Cat (n = 9). Force data are presented in units of body weight (BW). Colored dots represent subject-specific data, with group median shown as a horizontal black line. The median is derived using the lower and upper quartile values. The maximum and minimum values are displayed with vertical lines (“whiskers”) connecting the points to the center box. Data points lying outside of the whiskers represent outliers. Colored dots are offset for clarity. *Denotes a statistically significant difference from the −2 Cat using linear mixed effects models.

Figure 5.

Box and whisker plot of net metabolic power for running at 2.5 m/s with no mass (0 g; n = 9), and 100 g (n = 9), 200 g (n = 9), and 300 g (n = 8) added to the RSP. Colored dots represent subject-specific data, with group median shown as a horizontal black line. The median is derived using the lower and upper quartile values. The maximum and minimum values are displayed as vertical lines (“whiskers”) connecting the points to the center box. Data points lying outside of the whiskers represent outliers. Colored dots are offset for clarity. We found no statistically significant differences in net metabolic power between added mass conditions using linear mixed effects models. RSP, running-specific prosthesis.

Figure 6.

Box and whisker plots for individual affected leg and unaffected leg values for running at 2.5 m/s for A: contact time, B: peak vertical ground reaction force (vGRF), and C: stance-average vertical ground reaction force (vGRF) with no mass (0 g; n = 9), and 100 g (n = 9), 200 g (n = 9), and 300 g (n = 8) added to the RSP. Forces are presented in units of body weight (BW). Colored dots represent subject-specific data, with group median shown as a horizontal black line. The median is derived using the lower and upper quartile values. The maximum and minimum values are displayed with vertical lines (“whiskers”) connecting the points to the center box. Data points lying outside of the whiskers represent outliers. Colored dots are offset for clarity. We found no statistically significant differences between added mass conditions for any of the variables using linear mixed effects models. RSP, running-specific prosthesis.

Figure 7.

A: affected leg (dashed lines) and B: unaffected leg (solid lines) average vertical ground reaction force vs. percentage of leg length for a representative subject running at 2.5 m/s using a running-specific prosthesis with stiffness categories (Cat) that were two Cat less stiff (−2 Ca) and one Cat less stiff (−1 Cat) than manufacturer-recommended (Rec) and Rec Cat. Leg stiffness (filled circle) is the ratio of the peak vertical ground reaction force [N] and maximum percentage of initial leg length during ground contact. C: affected leg (dashed lines) and D: unaffected leg (solid lines) average vertical ground reaction force vs. percentage of leg length for the same representative subject running at 2.5 m/s using the RSP that elicited the lowest metabolic power with no added mass, and 100 g, 200 g, and 300 g added to the RSP.