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Understanding the degree of leg stiffness during human movement would provide important information that may be used for injury prevention. In the current study, we investigated bilateral differences in leg stiffness during one-legged hopping. Ten male participants performed one-legged hopping in place, matching metronome beats at 1.5, 2.2, and 3.0 Hz. Based on a spring-mass model, we calculated leg stiffness, which is defined as the ratio of maximal ground reaction force to maximum center of mass displacement at the middle of the stance phase, measured from vertical ground reaction force. In all hopping frequency settings, there was no significant difference in leg stiffness between legs. Although not statistically significant, asymmetry was the greatest at 1.5 Hz, followed by 2.2 and 3.0 Hz for all dependent variables. Furthermore, the number of subjects with an asymmetry greater than the 10% criterion was larger at 1.5 Hz than those at 2.2 and 3.0 Hz. These results will assist in the formulation of treatment-specific training regimes and rehabilitation programs for lower extremity injuries.

Despite the presence of several different calculations of leg stiffness during hopping, little is known about how the methodologies produce differences in the leg stiffness. The purpose of this study was to directly compare K _leg during hopping as calculated from three previously published computation methods. Ten male subjects hopped in place on two legs, at four frequencies (2.2, 2.6, 3.0, and 3.4 Hz). In this article, leg stiffness was calculated from the natural frequency of oscillation (method A), the ratio of maximal ground reaction force (GRF) to peak center of mass displacement at the middle of the stance phase (method B), and an approximation based on sine-wave GRF modeling (method C). We found that leg stiffness in all methods increased with an increase in hopping frequency, but K _leg values using methods A and B were significantly higher than when using method C at all hopping frequencies. Therefore, care should be taken when comparing leg stiffness obtained by method C with those calculated by other methods.

Although many athletic activities and plyometric training methods involve both unilateral and bilateral movement, little is known about differences in the leg stiffness (K _leg) experienced during one-legged hopping (OLH) and two-legged hopping (TLH) in place. The purpose of this study was to investigate the effect of hopping frequencies on differences in K _leg during OLH and TLH. Using a spring-mass model and data collected from 17 participants during OLH and TLH at frequencies of 2.0, 2.5, and 3.0 Hz, K _leg was calculated as the ratio of maximal ground reaction force (F _peak) to the maximum center of mass displacement (ΔCOM) at the middle of the stance phase measured from vertical ground reaction force. Both K _leg and F _peak were found to be significantly greater during TLH than OLH at all frequencies, but type of hopping was not found to have a significant effect on ΔCOM. These results suggest that K _leg is different between OLH and TLH at a given hopping frequency and differences in K _leg during OLH and TLH are mainly associated with differences in F _peak but not ΔCOM.

To understand the step characteristics during sprinting in lower-extremity amputees using running-specific prosthesis, each athlete should be investigated individually. Theoretically, sprint performance in a 100-m sprint is determined by both step frequency and step length. The aim of the present study was to investigate how step frequency and step length correlate with sprinting performance in elite unilateral transtibial amputees. By using publicly-available Internet broadcasts, the authors analyzed 88 races from 7 unilateral transtibial amputees. For each sprinter’s run, the average step frequency and step length were calculated using the number of steps and official race time. Based on Pearson’s correlation coefficients between step frequency, step length, and official race time for each individual, the authors classified each individual into 3 groups: step-frequency reliant, step-length reliant, and hybrid. It was found that 2, 2, and 3 sprinters were classified into step-frequency reliant, step-length reliant, and hybrid, respectively. These results suggest that the step frequency or step length reliance during a 100-m sprint is an individual occurrence in elite unilateral transtibial amputees using running-specific prosthesis.

The aim of this study was to develop a normative sample of step frequency and step length during maximal sprinting in amputee sprinters. We analyzed elite-level 100-m races of 255 amputees and 93 able-bodied sprinters, both men and women, from publicly-available Internet broadcasts. For each sprinter’s run, the average forward velocity, step frequency, and step length over the 100-m distance were analyzed by using the official record and number of steps in each race. The average forward velocity was greatest in able-bodied sprinters (10.04 ± 0.17 m/s), followed by bilateral transtibial (8.77 ± 0.27 m/s), unilateral transtibial (8.65 ± 0.30 m/s), and transfemoral amputee sprinters (7.65 ± 0.38 m/s) in men. Differences in velocity among 4 groups were associated with step length (able-bodied vs transtibial amputees) or both step frequency and step length (able-bodied vs transfemoral amputees). Although we also found that the velocity was greatest in able-bodied sprinters (9.10 ± 0.14 m/s), followed by unilateral transtibial (7.08 ± 0.26 m/s), bilateral transtibial (7.06 ± 0.48 m/s), and transfemoral amputee sprinters (5.92 ± 0.33 m/s) in women, the differences in the velocity among the groups were associated with both step frequency and step length. Current results suggest that spatiotemporal parameters during a 100-m race of amputee sprinters is varied by amputation levels and sex.

Individuals with lower extremity amputation must adapt the mechanical interactions between the feet and ground to account for musculoskeletal function loss. However, it is currently unknown how individuals with amputation modulate three-dimensional ground reaction forces (GRFs) when running. This study aimed to understand how running with running-specific prostheses influences three-dimensional support forces from the ground. Eight individuals with unilateral transtibial amputations and 8 control subjects ran overground at 2.5, 3.0, and 3.5 m/s. Ten force plates measured GRFs at 1000 Hz. Peak and average GRFs and impulses in each plane were compared between limbs and groups. Prosthetic limbs generated reduced vertical impulses, braking forces and impulses, and mediolateral forces while generating similar propulsive impulses compared with intact and control limbs. Intact limbs generated greater peak and average vertical forces and average braking forces than control subjects’ limbs. These data indicate that the nonamputated limb experiences elevated mechanical loading compared with prosthetic and control limbs. This may place individuals with amputation at greater risk of acute injury or joint degeneration in their intact limb. Individuals with amputation adapted to running-specific prosthesis force production limitations by generating longer periods of positive impulse thus producing propulsive impulses equivalent to intact and control limbs.

Understanding the characteristics of ground reaction forces (GRFs) on both limbs during sprinting in unilateral amputees wearing running-specific prostheses would provide important information that could be utilized in the evaluation of athletic performance and development of training methods in this population. The purpose of this study was to compare GRFs between intact and prosthetic limbs during sprinting in unilateral transfemoral amputees wearing running-specific prostheses. Nine sprinters with unilateral transfemoral amputation wearing the same type of prosthesis performed maximal sprinting on a 40-m runway. GRFs were recorded from 7 force plates placed in the center of the runway. Peak forces and impulses of the GRFs in each direction were compared between limbs. Peak forces in vertical, braking, propulsive, and medial directions were significantly greater in intact limbs than those in prosthetic limbs, whereas there were no significant differences in peak lateral force between limbs. Further, significantly less braking impulses were observed in prosthetic limbs than in intact limbs; however, the other measured impulses were not different between limbs. Therefore, the results of the present study suggest that limb-specific rehabilitation and training strategies should be developed for transfemoral amputees wearing running-specific prostheses.

This study assessed the agreement between K_vert calculated from 4 different methods of estimating vertical displacement of the center of mass (COM) during single-leg hopping. Healthy participants (N = 38) completed a 10-s single-leg hopping effort on a force plate, with 3D motion of the lower limb, pelvis, and trunk captured. Derived variables were calculated for a total of 753 hop cycles using 4 methods, including: double integration of the vertical ground reaction force, law of falling bodies, a marker cluster on the sacrum, and a segmental analysis method. Bland-Altman plots demonstrated that K_vert calculated using segmental analysis and double integration methods have a relatively small bias (0.93 kN⋅m^–1) and 95% limits of agreement (–1.89 to 3.75 kN⋅m^–1). In contrast, a greater bias was revealed between sacral marker cluster and segmental analysis (–2.32 kN⋅m^–1), sacral marker cluster and double integration (–3.25 kN⋅m^–1), and the law of falling bodies compared with all methods (17.26–20.52 kN⋅m^–1). These findings suggest the segmental analysis and double integration methods can be used interchangeably for the calculation of K_vert during single-leg hopping. The authors propose the segmental analysis method to be considered the gold standard for the calculation of K_vert during single-leg, on-the-spot hopping.