Biological or Prosthetic Limb—Which Is More Advantageous for Running Performance? A Narrative Review

Abstract

As the field of prosthetic engineering advances, questions around whether these new prosthetics hold the ability to outperform biological limbs become more relevant. To further clarify such a debate and discover gaps in our understanding, a narrative review of the present literature on this topic is needed. The purpose of the present review was to explore whether prosthetic legs grant amputee athletes an unfair advantage over traditional athletes by reviewing 11 articles pertaining to the running performance and potential among athletes with transtibial amputations. The findings of the included articles were categorized into three domains of running performance, chosen due to their precedence in the current literature: propulsion forward, limb repositioning, and physiological limitations. Our review indicated that the present literature alludes to transtibial amputee runners having a potential competitive advantage over able-bodied runners, with the caveat that some performance domains appear not to be differentiated. The present findings offer a unique perspective on understanding the impact of prosthetics on the running performance among para-athletes and suggest future research directions. As the depth of this area of literature increases, future systematic reviews and meta-analyses may be able to answer with greater certainty whether transtibial prosthetics allow for supra-biological running performances.

1. Introduction

Athletic activities help participants maintain proper physical health and provide therapeutic benefits, such as improving self-esteem, increasing the perceived quality of life, and enhancing important domains of mental health [1]. However, if an athletic competition is viewed as being rigged to favor one party over another, commonly referred to as cheating, it can induce various adverse consequences among the competitors, their fans, and the industry [2]. To prevent cheating in sports, overarching organizations such as the International Association of Athletics Federations (IAAF), more recently known as the World Athletics Association (WAA), have developed regulations to ensure no foul plays by any athlete. However, with the advancement of performance-related technologies (such as specialized shoes, orthotic inserts, and/or other wearables), it may sometimes be challenging to identify the unfairness provided implicitly by a specific technology to the competitors. Therefore, sports scientists must better understand new technological developments through stringent research methodologies to provide further insurance for a fair playing field.

Over the past few decades, one topic that received attention from researchers in sports science has been whether or not prosthetic legs, commonly referred to as running-specific prosthetics in the context of athletic use, may grant amputees heightened performance abilities over those with biological legs. During the past century, improvements to the functionality of prosthetics have grown exponentially. One of the first significant improvements came in the 1950s: the Solid Ankle and Cushioned Heel (SACH) for below-the-ankle amputees [3]. The SACH provided greater force absorption, water resistance, and durability with comparable affordability to previous prosthetics. The next great leap in this technology was not seen until the 1980s, when carbon fiber was used to create prosthetics. The inclusion of carbon fiber continued the trend established by the SACH, allowing prosthetics to become lighter, more aerodynamic, and more durable on both the distal and proximal ends [4]. This progression of carbon fiber soon led to one of the first blade-like running prosthetics, called the “Flex-Foot”, which took advantage of the attributes of carbon fiber to achieve stronger running abilities and officially hit the consumer market in 1996. This model was further enhanced into the Flex-Sprint I, which multiple para-Olympic athletes (specifically those with transtibial amputations) used to win several medals and set new para-athlete world records [5]. Since these advancements, further improvements to force absorption, weight, durability, and overall efficiency have been delivered to prosthetic limb users using titanium and graphite [6].

As prosthetics continue to advance due to the ever-growing understanding of biomechanics, some in both amputee and non-amputee running sports communities are concerned that different models or brands of prosthetics will offer users unfair advantages over other para-athletes or even able-bodied athletes. This topic has undergone many rounds of discussion, with each reemergence corresponding to exceptional performances by amputee athletes. For example, in 2012, Oscar Pistorius, a double transtibial amputee, became the first amputee to compete in the Olympics and placed 8th in a 4 × 400 relay (group time of 3:03:46 min). Pistorius also placed second in his preliminary 400 m race (45.44 s) at the 2012 Olympics; however, he later put up a subpar performance (46.54 s) compared to his initial debut in the semifinal heat and placed last.

While this performance inspired many individuals with disabilities, it also began an in-depth conversation on whether these mechanically engineered carbon fiber prosthetics can yield a performance beyond typical biological abilities. In response, the WAA ruled that, to compete alongside typical biological runners, those using prosthetics must prove to the WAA-designated board that their prosthetics do not grant a significant advantage through an appeal (please see [7] for an in-depth overview). This discussion was then reawakened when Blake Leeper, another double transtibial amputee, attempted to follow in Pistorius’s steps and compete in the Tokyo 2020 Olympics, only to be barred from competing due to a ruling stating that Leeper’s prosthetic blades were too long (15 cm above Paralympic standards) and would give him an unfair competitive advantage due the additional length shortening the swing time and lengthening the stride length, both of which are related to increased gait speed [8]. This prompted a comparison of the running performance between Leeper and his typical biological counterparts [9]. Researchers found that Leeper’s prosthetics provided no inherent advantage due to their height when compared to the chosen biological runners. However, this immediately caught the attention of Weyand et al. [10], who wrote a rebuttal commentary paper citing misleading methodology and background information that could not accurately substantiate the pro-Leeper research team’s findings [9], thus stating that the initial ruling was worthy of being upheld. Ultimately, Leeper was not allowed to compete in the Tokyo Olympics, as the committee abided by their previous ruling [11]. There is a clear need for sporting events to remain fair for all parties to ensure that athletes, fans, and sporting organizations can fully benefit from the events.

Over the years, considerable advances in the prosthetics field have posed the threat of giving those with amputations an advantage when performing against typical biological norms. However, the unjust segregation of able-bodied athletes and para-athletes could also be detrimental to the spirit of competition. These decisions regarding competitive events should be based on clear and robust scientific experimental evidence. With this in mind, the topic of running prosthetics is an understudied domain in the biomechanics and prosthetics field. An extensive review of the present literature is highly desired to properly determine what direction future research should take and whether clear evidence is available to support a difference in running performance between amputee athletes and typical runners. Furthermore, recent publications on this topic make this review more feasible than in the past. Therefore, the primary purpose of the present review was to explore whether prosthetic legs provide amputee athletes with an unfair advantage over traditional athletes during sports competitions, particularly during running. Secondarily, the present study sought to provide insight into prospective paths for research within this underserved population. The findings from our review could provide more information and insight to help the sports, research, and clinical communities understand the possible benefits of prosthetics relative to biological legs.

2. Materials and Methods

This review was organized based on the assumption that there are three key components to successful sustained running performance: (1) the propulsion of the body forward, (2) the ability to properly and effectively reposition limbs, and (3) the relatively finite nature of physiological systems in their ability to consistently perform and the limitations they may carry specific to different chronic conditions. This assumption was based on our observation of the literature pertaining to the running performance in both amputee and non-amputee populations. Furthermore, these domains have been frequently used within the ongoing debates on whether prosthetics may grant transtibial amputees an inherent advantage [9,10]. An extensive literature search was performed in three major databases or search engines: PubMed, Google Scholar, and Galileo Academic Search Complete. The keywords used for the literature search consisted of a group of basic categories and additional terms for each key component of the running performance (

Table 1. Keywords for the initial literature search regarding the three key components to successfully sustained running performance: (1) the forward propulsion of the body, (2) lower-extremity repositioning, and (3) physiological systems.

Category	Terms
Basic Search Terms	Able-Bodied Amputees, Bilateral Transtibial Amputee, Unilateral Transtibial Amputee, Double Transtibial Amputee, Single Transtibial Amputee, Para-Athletes, Runners, Running Gait, Transtibial Amputee, Typical Runners
Propulsion Forward	Ground Reaction Forces, Kinematics, Kinetics, Propulsion, Running Blades
Anatomical Repositioning	Gait Analysis, Stiffness, Stride Frequency, Stride Length, Swing Time
Physiological Limitations	Cardiorespiratory Health, Endurance, Fatigue, Lung Capacity, Metabolic Efficiency, Metabolic Threshold, Musculoskeletal Limitations, Physiological Limitations, Oxygen Transport, Running Economy

).

The literature search was conducted between October 2023 and July 2024. To gain a broad understanding of the literature, no restrictions were applied to the publication date, study design, or region of the studies. To be eligible for review, the articles needed to include data related to one or more of the three key components in running performance among those with unilateral or bilateral transtibial amputations (congenital and non-congenital) who use a model of blade prosthetic to run. Furthermore, only studies that collected the data on a flat surface, such as a gym floor, concrete, asphalt, polyurethane, or a treadmill belt, were included due to the wildly non-disputed disadvantage amputees have on rugged or sloped surfaces [12,13]. Articles with only transfemoral amputees of any nature were excluded due to their unique limitations and variance associated with running gait and musculoskeletal function, which are typically different than observations of transtibial runners and outside the scope of this review [14]. Articles relating to pediatric or geriatric populations were excluded (<18 or >50 years of age). Only articles in English and published in peer-reviewed journals were considered.

The literature search comprised three steps: reviewing titles, reading abstracts, and selecting articles for a full-text analysis. The initial articles selected from the database search were sorted and examined independently by two authors (D.W.E. and M.M.), and any disagreement in the selection was resolved by the third author (F.Y.) to reach a consensus. Articles that did not meet the predetermined inclusion criteria were removed upon the authors’ agreement. The same reviewers independently evaluated the abstracts of the remaining articles, and ineligible articles were excluded in the second step. Next, articles selected for the final analysis were obtained in full text, which was further examined and analyzed.

3. Results

The initial search yielded a total of 55 articles. After the screening based on reading the titles and abstract, 44 articles were excluded (n = 25 due to inappropriate populations (e.g., those with transfemoral amputations and/or populations of certain ages) and n = 19 due to data collection taking place on excluded surfaces (e.g., grass and/or indoor/outdoor inclines)). Therefore, 11 articles were included in our review (Figure 1 and Table 2). The participants in the studies ranged in age between 18 and 33 years of age, with an approximate average age of 26.8 years of age. As indicated, the review of these 11 articles was organized based on the three key components required for successful sustained running performance.

3.1. Forward Propulsion

The ability of the body to move in different planes is often taken for granted. However, these basic actions contain a myriad of complex principles of biomechanics, such as adjustments for the center of mass, the coordination of muscles and body segments, the production of joint moments, maintaining an upright posture, and continuous interaction with the environment. In this array of concepts lies the ability to move the body forward. Forward locomotion is defined as walking at slower speeds and transforming to running once the body accelerates to higher speeds, during which the gait cycle develops a float phase [15]. To properly accelerate the body into a running movement, the individual must produce adequate levels of both vertical and horizontal ground reaction forces, with the greater contribution being attributed to vertical ground reaction forces post-acceleration phase. Based on Newton’s Second Law of Motion, a large ground reaction force would create a great acceleration and, thus, a fast velocity, potentially enhancing the athletes’ performance.

These concepts become essential, especially in the present conversation, as some have speculated that prosthetics may enhance an amputee’s ability to produce ground reaction forces. For example, it has been reported that some prosthetics might compensate for these limitations better than others, although the deficiency in ground force reactions could be a limiting factor for transtibial athlete running performance [16]. One way in which prosthetics may yield a better performance could stem from their customizable structure. For instance, a prior study indicated that J-shaped blades were able to reduce the ground contact time with higher forces at the point of contact than C-shaped blades, and the values for the J-shaped blades were reported as neither greater nor lesser compared to biological leg performance as previously published [17], implying that the shape of the blade prosthetic significantly alters the ground reaction forces between amputee runners. The findings relating to prosthetic shape were reinforced by a 2016 study detailing that J-shaped running prosthetics provide more elastic energy and mechanical power return than C-shaped prosthetics [18]. It should be noted that the publication of the Taboga et al. [17] paper has since been retracted due to concerns about the low sample size and the misleading interpretation of statistical evidence [19]. As the investigation of this study [17] proceeded, it became evident that not all the authors’ conclusions were adequately supported by their results. While the retracting editor did not discredit their findings of differences between prosthetic shapes, they mainly found fault in the authors’ interpretation that stiffness and height did not affect the maximum speed. When put into a wholistic context, it should be recognized that the Taboga et al. [17] paper, while conclusively inaccurate, contained a methodologically sound means of collecting their data, which was not called into question. Further, when viewing the results in light of the retracted conclusions, the overall data fit well with the present literature, which will be discussed later in the present review. Therefore, the authors believe that this past research is vital for adequately communicating the present dialog in an understudied field.

Another hypothesis made on how prosthetics may achieve more optimal ground reaction forces than typical biological legs is due to their variety of stiffness. The most prevalent running blade models for transtibial amputees are primarily made of carbon fiber, but different models can vary in the stiffness of the prosthetic. One non-participant laboratory-based study found that stiffer models of carbon fiber prosthetics yielded higher ground reaction forces and shorter ground contact times [20]. This suggests that stiffer models may, therefore, lead to faster running times. Similar results reporting prosthetic stiffness being positively associated with running speed have also been found [20]. Both findings are mirrored by another study [21] that observed that a higher prosthetic stiffness in bilateral transtibial amputee runners can yield faster speeds, greater ground reaction forces, and shorter ground contact periods. Another study of elite long jumpers found that unilateral transtibial amputee jumpers could jump farther than non-amputee jumpers due to their ability to produce higher propulsive forces at the point of takeoff [22]. While past research suggests that stiffer prosthetic models produce higher running speeds, it was suggested that the increased running velocity comes at a cost, as stiffer prosthetics may also result in higher metabolic costs [23]. Unfortunately, no studies have compared these prosthetics and their corresponding stiffness to biological legs/bones. This is likely due to measurements being challenging to collect and the well-documented tendency of bones to become more resilient (stiffer) to ground reaction forces as the running individual adapts to training [24]. This gap in the literature could shadow doubts on whether prosthetics can be made to be superhuman due to the nature of their stiffness. On the other hand, the ability/potential of amputees to gain high levels of prosthetic stiffness is present without having to endure the same style of training as their non-amputee counterparts.

Besides the ground reaction force, another perspective from which to examine the effects of running prosthetics on running performance is their performance efficacy. Directly illustrating the overall performance of prosthetics is a difficult feat. However, one alternative way is to analyze the overall efficiency of the forward-moving body. In terms of the total mechanical work by the lower limbs, Brüggemann et al. reported that an athlete with a bilateral transtibial amputation was able to run by producing less mechanical work in their lower legs compared to runners with intact limbs [25]. Furthermore, the authors suggested that this difference in the ability to perform with significantly less work expenditure was due to prosthetics weighing less than the biological lower leg. While this study seems to be the only one of its kind to compare amputee and non-amputee runners, another one reported similar levels of mechanical work in their sample of non-amputee runners regarding how much mechanical work was required for running performance [26].

3.2. Anatomical Repositioning

The dimensions of the prosthetic leg also have significant implications for amputee runners’ repositioning of their legs. The ability to properly and effectively control one’s lower limbs can play a vital role in optimizing running performance [27]. There are two possible ways for running prosthetics users to improve their running performance by adjusting anatomical repositioning. One is to decrease the time it takes for the push-off leg to swing forward and be ready for another push-off motion (swing time). This movement could increase the step frequency or cadence. The other is to increase the stride length. The product of the cadence and stride lengths defines the running speed, given the intercorrelation between these three metrics. It is widely accepted that stride frequency is negatively associated with running time [28]. Therefore, as either the cadence or stride length increases, it becomes harder for runners to simultaneously prevent a decrease in the other, causing runners to need optimal stride frequencies for different paces [29].

The struggle to fully optimize the stride frequency may limit the running performance more in able-bodied runners than in amputee runners. For example, it was documented that an elite double transtibial runner had a 15.8% shorter swing time than the control of four able-bodied elite runners and a 9.8% reduced swing time than the average non-amputee elite runner from a previously published database of the International Running Foundation [30]. Furthermore, this amputee runner had a 9.6% longer stride length than the other runners and a 16.2% extended stride length when the body height was controlled. Both of these advantages in performance could be related to the prosthetic running limbs being lighter than typical biological limbs.

Another critical aspect of anatomical repositioning could also result from the stiffness of the leg/prosthetic. While examining the running mechanics of typical non-amputee runners, Farley and González found that higher stride frequencies require larger leg stiffness values, suggesting that athletes with larger leg stiffness values may have the ability to achieve higher stride frequencies and, therefore, better running performances [31]. This aligns with previously reported findings that double transtibial amputee runners with stiffer prosthetics could run at faster speeds due to an advantageous stride frequency and ground reaction forces [17,18,21].

Three modifiable aspects of prosthetic legs (length, weight, and stiffness) have been used as manipulation targets to optimize the running performance of prosthetics users. This may lead to some concern as engineers work to increase the leg length to improve its ability to reach farther, decrease the weight of the prosthetic to reduce the moment of inertia of the leg and minimize swing time, and increase prosthetic stiffness to increase the stride frequency as well as ground reaction forces. One pilot study, which was dedicated to inspecting this topic, illustrated that increases in height and decreases in weight yielded an improved performance in running [32]. While this pilot study did not compare this customizable nature to biological legs, it further highlights the ability to customize the physical attributes of these below-the-knee prosthetics and their impacts on running performance. Along this line of thought and paired with previously discussed findings of inequalities found at the elite competitive level [30], there may be a concern about prosthetics providing a competitive advantage. As will be discussed later, this is a dire area of investigation that future research must address.

3.3. Physiological Limitations

A physiological limit is typically defined as the point at which an individual’s performance reaches its plateau, and they are unable to sustain a steady state of work. Physiological limitations include age, height, weight, and body composition in the anthropometric aspect. Other limitations that one may experience during exercise are VO₂max, running economy (RE), and the lactate threshold (LT), which can be influenced by environmental factors, genetic makeup, and overall fitness [33,34]. Understanding these variables comes from observing an individual in different modes and intensities of exercise to test how their body responds and sets its physiological limit. When comparing endurance athletes to power athletes, the recruitment of muscle fiber types and the point of ultimate fatigue are different between the two groups. Ultimate fatigue is influenced by the main variables (LT, RE, and VO₂max) that determine an individual’s physiological limit, regardless of their training level [34]. Understanding how these variables affect physiological processes such as O₂ consumption, O₂ delivery, and lactate utilization augments our understanding of how these physiological metrics affect the performance between endurance athletes and power athletes.

Transtibial amputees face the loss of one or both legs below the knee joint, lowering the amount of metabolic tissue utilized while running. This means that the prosthetic limbs of the amputee athletes are essentially passive bodies that cannot inject extra energy into the system, such as their traditional non-amputee limb counterparts via musculature contractions. However, reduced metabolic tissue may also fundamentally alter the perception of fatigue experienced by the amputee athlete, as fewer biomarkers of physical fatigue are transmitted throughout the body. Within the amputee athlete population, the metabolic cost of running is most efficient when transtibial amputee athletes use prosthetic configurations that decrease their peak horizontal braking ground reaction forces, stride frequency, and leg stiffness [35]. If these variables are optimal, then the metabolic cost will be efficient and similar to that of able-bodied individuals because the overall structure and stiffness of the prosthetic configuration play a vital role in the impact on the physiological processes [23].

RE is the ability to maintain sufficient oxygen utilization at a set submaximal pace. It is affected by five determinants: training, environment, physiology, anthropometry, and running biomechanics [36], and it can differ between traditional biological runners and transtibial amputee runners. These five determinants can be modified or improved to make an economical runner who can maintain an efficient and sustained level of cardiorespiratory, neuromuscular, and metabolic function while running. Typically, economical runners are in long-distance settings where the need for maintaining pace and cadence is paramount. Based on the data from 32 participants, a past review discovered that the average gross metabolic cost of transport for unilateral transtibial runners was 205.9 O₂·kg⁻¹·km⁻¹, the cost for bilateral transtibial runners was 188.9 O₂·kg⁻¹·km⁻¹, and that for non-amputee runners was 202.2 O₂·kg⁻¹·km⁻¹ [37]. This study provided insight into the aerobic capacity and VO₂max among the amputee participants, showing that they do not significantly vary compared to non-amputees.

Table 2. Summary of the sample size, participant characteristics, primary findings, and design of all the studies included in this review. The age in years is expressed as the mean ± standard deviation.

Article	Sample Size and Age	Type of Participants	Primary Findings	Study Design
Grabowski et al. [16]	6 (29.2 ± 5.3)	Male (n = 4) and female (n = 2) unilateral transtibial amputee runners	-Type of running prosthetic significantly affects force generation.	Repeated-measures experimental design
Taboga et al. [17] * Retracted paper, please see Section 3.1. for full context.	5 (24.8 ± 4.8)	All male bilateral transtibial amputee runners	-J-shaped running prosthetics significantly reduce ground contact time compared to C-shaped prosthetics. -Double transtibial amputee runners with stiffer running prosthetics were able to run at faster speeds due to advantageous stride frequency and ground reaction forces than those amputees with less stiff models.	Repeated-measures quasi-experimental design
Beck et al. [18]	11 (27.8 ± 5.7)	Male (n = 5) and female (n = 6) unilateral transtibial amputee runners	-J-shaped running prosthetics provide greater elastic energy and mechanical power return than C-shaped running prosthetics. -Double transtibial amputee runners with stiffer running prosthetics were able to run at faster speeds due to advantageous stride frequency and ground reaction forces than those amputees with less stiff models.	Repeated-measures quasi-experimental design
Beck et al. [21]	5 (24.8 ± 4.8)	All male bilateral transtibial amputee runners	-Higher running prosthetic stiffness yields faster speeds, greater ground reaction forces, and shorter ground contact periods compared to less stiff models. -Double transtibial amputee runners with stiffer running prosthetics were able to run at faster speeds due to advantageous stride frequency and ground reaction forces than those amputees with less stiff models.	Repeated-measures experimental design
Willwacher et al. [22]	10 (26.0 ± 1.7)	All male unilateral transtibial amputees (n = 3) and non-amputee (n = 7) long jumpers	-Unilateral transtibial amputee jumpers were able to jump farther than non-amputee jumpers due to their ability to produce higher propulsive forces at point of takeoff.	Comparative observational design
Beck et al. [23]	10 (33.4 ± 6.1)	Male (n = 7) and female (n = 3) unilateral transtibial amputee runners	-Stiffer running prosthetics result in higher metabolic costs.	Repeated-measures experimental design
Brüggemann et al. [25]	6 (Not Specified)	All male bilateral transtibial amputee (n = 1) and non-amputee (n = 5) runners	-Bilateral transtibial amputees were able to run by producing less mechanical work in their lower legs compared to non-amputee runners.	Comparative observational design
Weyand et al. [30]	5 (Not Specified)	All male bilateral transtibial amputee (n = 1) and non-amputee (n = 4) runners	-Double transtibial runner had a 15.8% faster swing than present non-amputee runners and a 9.8% faster swing time than average non-amputee elite runner from a previously published database. -Amputee runners had 9.6% longer stride lengths than non-amputee runners and a 16.2% longer stride length when height was controlled.	Comparative observational design
Wilson et al. [32]	2 (18.5 ± 0.7)	Male (n = 1) and female (n = 1) unilateral transtibial amputees	-Increases in height and decreases in weight of prosthetics yielded improved performance in running.	Repeated-measures experimental design
Beck et al. [35]	5 (24.8 ± 4.8)	All male bilateral transtibial amputee runners	-Metabolic cost of running improved when athletes used prosthetic configurations that decreased peak horizontal braking ground reaction forces, stride frequencies, and leg stiffness values.	Repeated-measures experimental design
Beck and Grabowski [37]	32 (29.3 ± 3.5)	Male (n = 25) and female (n = 7) bilateral transtibial amputee (n = 15), unilateral transtibial amputee (n = 7), and non-amputee (n = 10) runners	-Running economy is similar among amputee runners and non-amputee runners.	Literature review

Table 2. Summary of the sample size, participant characteristics, primary findings, and design of all the studies included in this review. The age in years is expressed as the mean ± standard deviation.

Article	Sample Size and Age	Type of Participants	Primary Findings	Study Design
Grabowski et al. [16]	6 (29.2 ± 5.3)	Male (n = 4) and female (n = 2) unilateral transtibial amputee runners	-Type of running prosthetic significantly affects force generation.	Repeated-measures experimental design
Taboga et al. [17] * Retracted paper, please see Section 3.1. for full context.	5 (24.8 ± 4.8)	All male bilateral transtibial amputee runners	-J-shaped running prosthetics significantly reduce ground contact time compared to C-shaped prosthetics. -Double transtibial amputee runners with stiffer running prosthetics were able to run at faster speeds due to advantageous stride frequency and ground reaction forces than those amputees with less stiff models.	Repeated-measures quasi-experimental design
Beck et al. [18]	11 (27.8 ± 5.7)	Male (n = 5) and female (n = 6) unilateral transtibial amputee runners	-J-shaped running prosthetics provide greater elastic energy and mechanical power return than C-shaped running prosthetics. -Double transtibial amputee runners with stiffer running prosthetics were able to run at faster speeds due to advantageous stride frequency and ground reaction forces than those amputees with less stiff models.	Repeated-measures quasi-experimental design
Beck et al. [21]	5 (24.8 ± 4.8)	All male bilateral transtibial amputee runners	-Higher running prosthetic stiffness yields faster speeds, greater ground reaction forces, and shorter ground contact periods compared to less stiff models. -Double transtibial amputee runners with stiffer running prosthetics were able to run at faster speeds due to advantageous stride frequency and ground reaction forces than those amputees with less stiff models.	Repeated-measures experimental design
Willwacher et al. [22]	10 (26.0 ± 1.7)	All male unilateral transtibial amputees (n = 3) and non-amputee (n = 7) long jumpers	-Unilateral transtibial amputee jumpers were able to jump farther than non-amputee jumpers due to their ability to produce higher propulsive forces at point of takeoff.	Comparative observational design
Beck et al. [23]	10 (33.4 ± 6.1)	Male (n = 7) and female (n = 3) unilateral transtibial amputee runners	-Stiffer running prosthetics result in higher metabolic costs.	Repeated-measures experimental design
Brüggemann et al. [25]	6 (Not Specified)	All male bilateral transtibial amputee (n = 1) and non-amputee (n = 5) runners	-Bilateral transtibial amputees were able to run by producing less mechanical work in their lower legs compared to non-amputee runners.	Comparative observational design
Weyand et al. [30]	5 (Not Specified)	All male bilateral transtibial amputee (n = 1) and non-amputee (n = 4) runners	-Double transtibial runner had a 15.8% faster swing than present non-amputee runners and a 9.8% faster swing time than average non-amputee elite runner from a previously published database. -Amputee runners had 9.6% longer stride lengths than non-amputee runners and a 16.2% longer stride length when height was controlled.	Comparative observational design
Wilson et al. [32]	2 (18.5 ± 0.7)	Male (n = 1) and female (n = 1) unilateral transtibial amputees	-Increases in height and decreases in weight of prosthetics yielded improved performance in running.	Repeated-measures experimental design
Beck et al. [35]	5 (24.8 ± 4.8)	All male bilateral transtibial amputee runners	-Metabolic cost of running improved when athletes used prosthetic configurations that decreased peak horizontal braking ground reaction forces, stride frequencies, and leg stiffness values.	Repeated-measures experimental design
Beck and Grabowski [37]	32 (29.3 ± 3.5)	Male (n = 25) and female (n = 7) bilateral transtibial amputee (n = 15), unilateral transtibial amputee (n = 7), and non-amputee (n = 10) runners	-Running economy is similar among amputee runners and non-amputee runners.	Literature review

4. Discussion

The present literature review sought to explore the modern research landscape by comparing the performance potential of transtibial amputee and non-amputee athletes. In this timely review, propulsion techniques, anatomical repositioning, and physiological limitations were explored colloquially to analyze the overall athletic performance. These areas of study have been researched, considered, and presented as the most relevant and current evidence on this topic. This review examined recent research studies on the running biomechanics for the transtibial amputee population.

One reoccurring theme within this review was how different models of prosthetics vary in their performance value, in that they confer more advantageous levels of forward propulsion and anatomical repositioning. In other words, the ability of some models to produce higher ground reaction forces, maintain a higher degree of stiffness, and allow for a more optimal ratio between the stride frequency and stride length could allow for improved performances. This leads to the explicit recommendation that these running prosthetics should be normalized based on the anthropomorphic parameters of the athlete. Although the WAA presently requires a prosthetic’s length to be comparable to the amputee athlete, there is little consideration for the related materials’ flexibility, weight, or stiffness. This could be due to the overall scarcity associated with biomechanic-related research within transtibial amputee athletes. Finding methods of normalizing and prescribing based on these considerations could benefit both para-athletic and traditional athletic competition domains. Future research should explore this topic further to lessen the concerns regarding different models of prosthetics contributing to an unfair competitor environment.

The current research seems to allude to the notion that prosthetics may provide an advantage in running biomechanics for people with a transtibial amputation relative to their non-amputee peers. However, the present review also noted areas of performance that may not significantly differ between the two populations. Therefore, the full body of available research should be considered to pursue an equitable field of competition.

The ability to alter the structure and configuration of a lower-leg prosthetic on an individualized basis may be one of the most significant advantages that transtibial runners possess. Runners can change their blade shape depending on personal preference or the event being run, which can influence their mechanical power and optimal ground reaction forces. This may partially explain the previously noted observations of propulsive forces being proven to be greater in transtibial amputees during the takeoff phase, as observed in long jump events, or the lower amount of mechanical work needed for the running dynamics of an amputee athlete [22]. Given that these observations were only documented in a single study [22], there is still contention on whether prosthetics can truly confer advantages to the entirety of a running competition. While the takeoff phase of the long jump event may be instructive to advantages relating to the initiation of running motions, other studies have argued that prosthetics may be disadvantageous in force production throughout the race. Perhaps the greatest advantage displayed by prosthetics users is the ability to reposition their legs with their stride length and frequency more efficiently than non-amputees. Wholistically, these modifiable features may be advantageous to transtibial amputee runners, which non-amputees may find difficult or even impossible to replicate through rigorous training at some competitive levels, as the capability to simply transition to different models that produce changes in performance is an ability that non-amputee athletes are unlikely to have.

On the other end of this discussion, the present review illustrated that some performance domains may not be meaningfully different between amputee and non-amputee athletes. For instance, clear evidence exists that the running economies and metabolic costs of transtibial amputees (both unilateral and bilateral) are not significantly lower than those of non-amputee athletes. While this shows that the rate of oxygen delivery and utilization may not differ between amputees and non-amputee athletes, questions still exist regarding how the inherent lack of metabolic tissue within transtibial amputees may affect their performance. Are prosthetics that maximize ground reaction forces comparable to the provided energy from muscle contractions? Do amputee athletes experience fatigue to a different extent than traditional athletes? Future research should consider filling these gaps in the present literature. Furthermore, while evidence points to other models possessing a larger forward propulsion potential due to their ability to produce higher ground reaction forces, partly due to corresponding higher stiffness levels, these concepts have not been compared to their biological limb counterparts. Therefore, while the present review argues that the concern regarding prosthetics providing performance advantages is valid, more direct comparative studies between amputees with prosthetics and non-amputees are needed to reach definitive claims. Overall, to draw a more cohesive conclusion, future research is required to distinctly determine whether the forward propulsion provided by artificial limbs exceeds that of biological limbs.

As a whole, the literature on the present topic remains highly limited. A few factors could contribute to the shortage of research, such as the cost of equipment, a small target population, and an extended study duration. However, the current literature compiled in the present review should provide a solid foundation for future studies. For example, the technological advancements in prosthetic design, which could make devices more affordable, stiffer, and lighter in weight, could allow the research field to spend more effort and resources to further investigate the effects of prosthetics on the running performance among people with transtibial amputations. Additionally, the function, design, and weight of prosthetics have continued to evolve, inspiring the need for more research to help enhance the performance amongst the amputee population.

Future studies should inspect how prosthetics affect physiological limitations, forward propulsion, and anatomical repositing among non-athletic individuals, as well as compare them to similar non-amputee peers or at least previously published norms. Additional possible research areas could be the neurological response to movement and sensation, turning and maneuvering techniques, and biomechanical efficiency and efficacy among people with transtibial amputations. Furthermore, this field of study may benefit from a retrospective analysis of past transtibial amputee athletic performances compared to traditional athletic performances at similar competitive levels, which is an understudied research area. Due to limitations in data collection modalities, future detailed research is imperative to understanding the advantages that amputees have with current prosthetics.

While the present review helps further encompass the current understanding of the performance capabilities of transtibial athletes, certain limitations of this project should be considered when interpreting its findings and conclusions. First, the total population of amputees is relatively small across the studies included in this review. This population is further diminished when only transtibial amputees are selected for observation. Therefore, the small sample sizes may reduce the external validity of the findings and accuracy in comparisons among the included articles. In line with these concerns also lies the problem of a small sample of studies exploring the performance capabilities of amputee athletes. Further, while the present work found eleven studies that fit the narrative of the review, only four studies recruited non-amputees to be analyzed for direct comparison. Future discoveries in this field may benefit by creating a database containing potential amputee participants with para-athletic interests. Second, the present review only summarized peer-reviewed published works in English. Such a strategy could introduce selection biases, as some gray literature (such as conference papers, works in press, or other non-peer-reviewed findings) or non-English work may have been excluded. Lastly, it should be noted that this literature review contained limitations inherent to its methodological design. Specifically, the “snapshot” nature of the findings only represents a fixed perspective of the questions. Including studies with multiple types of study designs and utilizing different makes/models of prosthetics may reduce the ability to draw truly comprehensive and rigorous conclusions. The lack of a quantitative analysis or systematic methodology may also reduce the reliability of the findings. Future systematic reviews and meta-analyses may provide more insight into the effects of prosthetics on the running performance relative to able-bodied individuals. Therefore, more studies are needed to allow us to extend our review to those topics further.

5. Conclusions

In conclusion, in the past five decades, considerable progress has been made in prosthetic engineering, allowing amputees to participate in society and even perform movements at elite levels. However, as these abilities further develop, caution must be practiced when comparing amputee athletes to traditional athletes at all levels of competition.

The current literature seems insufficient to either entirely support or refute a performance advantage among amputee athletes compared to their non-amputee counterparts. Some studies have suggested that amputee athletes may have a unique running performance advantage over non-amputee athletes in certain performance domains, particularly regarding their stride length, their stride frequency, and an inherent ability to make alternations/customize the mechanical properties of their prosthetics. While some segments of running performance may elicit optimal ground reaction forces (i.e., at takeoff points relating to vertical force production), the present literature points to ground reaction forces potentially being disadvantageous during the totality of competition. Therefore, there is still a significant gap in the literature comparing the running performance between biological and prosthetic legs, and more work is needed to decipher if amputees may or may not enjoy advantages during certain aspects of a race compared to others. Additionally, it is crucial to point out the limited nature of the present field of literature and the relative lack of comparative studies between amputee and non-amputee athletes, which can help the reader derive their takeaways from this narrative review. Therefore, to protect the sanctity of competition and continue to allow all athletes to receive the full benefits of fair competition, caution should be practiced, meaning that traditional athletes and para-athletes should compete in separate categories in sanctioned sporting environments until further research can clarify and add to the present observations.