Sprint prostheses used at the Paralympics: a proposal for an assessment method to maintain fairness

Abstract

Background:

Unique to sport with a disability such as those performed at the Paralympics, the need to improve the performance of lower limb prostheses can conflict with the need to provide an equal opportunity to win whilst still needing to encourage and maximise participation.

Objectives:

This paper extends previous research by attempting to propose a method suitable for sports governing bodies to help any functional assessment of sprinting lower limb prosthesis technology in the future.

Study Design:

The study comprises two elements: 1) A historical review and evaluation of drop jump assessment techniques. 2) A pilot test of a candidate using two mechanically different lower limb regions.

Methods:

A unilateral drop jump technique is assessed historically as an evaluation technique for sprinters with a lower-limb amputation. Further, a unilateral drop jump using mechanically altered lower limbs is piloted.

Results:

The historical review provides no evidence to suggest that this technique is not suitable for athlete participants. The pilot trials show a statistically stable and repeatable method of demonstrating a mechanical deficiency of one limb to another. Six jumps are suitable to obtain stable results but the mechanical behaviour of one limb may eventually change based on accumulated fatigue.

Conclusions:

The unilateral drop jump is shown to be viable for application to an athlete population with a lower limb unilateral amputation.

Clinical relevance

This paper develops previous sports stakeholder research and pilots an assessment strategy to provide a functional limb-to-limb comparison of the same lower limb region. This would potentially be used by both prosthetists and the sport’s governing body to help maintain fairness in the sport as prosthesis technology develops.

Background

Disability sport, and notably Paralympic competition, has seen major changes in its use of technology from its inception midway through the 20th century. The development of specific prostheses for sport, which differed from clinical prostheses used for everyday lifestyle-based tasks, did not firmly take hold until the 1980s. ¹ Highlighted by the bilateral amputee Oscar Pistorius attempting to compete in the 2008 Olympics, such technology has seen increasing debate over its inclusion in athletics. From this, contention exists that lower limb prosthesis are either advantageous ² or merely restorative to an athlete using them. ³

The shortfall of many studies regarding running prosthesis behaviour is that the participants do not always perform the evaluated motion or activity at the events representative speed or effort. This is especially true of amputee sprinting. Concerns arise when it is assumed when evaluating these studies that differences in equipment do exist between prostheses designs but that it could take some significant time before their magnitude is known, if ever. ¹

An ongoing project is seeking to ascertain the perceptions of prosthesis within Paralympic sprinting and whether these are unfair. ⁴ Concerns over such technology have been raised with respect to the future. ⁵

The Dyer et al. study ⁵ was recently undertaken to ascertain the role and perception of lower limb sprinting prostheses and to determine a framework for any subsequent assessment. The Delphi technique is an iterative, qualitative method of assessment of an assembled expert panel ⁶ with the aim being to eventually obtain consensus on a key issue which has limited knowledge or expertise. ⁷ The key findings of this study were that:

1. The lower limb prosthesis is considered to be a form of equipment.

2. The lower limb running prosthesis is a restorative form of technology.

3. Increased legislative control should be imposed on sports lower limb prostheses technology to maintain a fair environment.

4. A lower limb prostheses contribution is defined by the individuals own physical abilities that they have obtained under naturally generated means.

5. Any proposed assessment should be relevant to the act of sprinting.

These findings will provide the basis and framework for any proposals for the proposed assessment of lower limb prostheses in elite sport.

The Delphi Study finding one suggested that prosthesis should be perceived as a form of equipment not unlike how one regards a tennis racket or a pair of running shoes. As a result, an equipment-centred approach was recommended. Any proposed method should allow the ability to test the prosthesis mechanically but still have the ability to use the biological limb as a point of reference. This could also be argued from existing research in that if the sprinting prosthesis does not perform the same way, ¹ look the same, ⁸ or act the same as a biological limb, ⁹ then it is effectively a device or piece of equipment and therefore should be judged accordingly.

The Delphi study proposed that the prosthesis should be considered restorative. When additionally applying the Delphi Study finding four to this, if a prosthesis is inferior in performance, any method chosen to assess this should be able to measure any limb-to-limb differences. Statistical methods such as the symmetry indices ¹⁰ or the symmetry angle ¹¹ have been shown to be reliable solutions to specify limb-to-limb imbalances.

Finding five proposes that any assessment should have some relevance to the act of sprinting. However, it would be unrealistic to expect any athlete to perform a full sprint of their opted race distance to monitor their prosthesis behaviour. Therefore, an alternative would be to select a method which provides sprinting key performance indicators (KPI). The KPIs of short distance sprinting have been proposed as leg cadence and stride length,^12,13 plus the impact-generated high leg stiffness ¹⁴ irrespective of track surface ¹⁵ and large applied ground reaction forces. ¹⁶ The Weyand study ¹⁶ also indicated that it is ground reaction force which directly governs the magnitude of both stride length and cadence. This, therefore, diminishes their value for assessment. As a result, both ground reaction force and lower limb stiffness remain as essential KPIs of an equipment-centred approach to prostheses assessment.

Amputee sprinting assessment: a unilateral drop jump

To measure the KPIs of amputee sprinting, a unilateral drop jump is explored and proposed as a solution. The drop jump test is typically undertaken by having a participant stand on a platform of a given height above the ground. Once commenced, the participant jumps down to the floor landing on either one or (more typically) two feet and then immediately executes a vertical or horizontal jump on the same foot/feet (depending on the technique method chosen). The achieved displacement is then measured as the magnitude of muscular power.

Drop jumps are typically used as a form of dynamic training to improve jumping performance, ¹⁷ as a form of plyometric training which involves eccentric muscular contraction, ¹⁸ or for sprint training to improve the horizontal impulse. ¹⁹ It has additionally been used to ascertain the stiffness in the limbs upon landing. ²⁰ The drop jump is used either as a training method to improve the physical ability of jumping or is used as a diagnostic assessment to measure jumping strength. In this case, a unilateral drop jump would allow the performance of a prosthesis to be compared and limited by the naturally generated performance on the athlete’s biological limb.

The impacts of drop height used for this method (and its effects) have also been investigated. Several studies with able-bodied participants have used comparatively low heights of 20 cm,^17,18 12 inches, ²¹ 30 cm, ²² 31 cm, ²³ medium-sized drops of 39 cm, ¹⁸ 40 cm ¹⁷ and large heights such as 45 cm, ²⁴ 60 cm ¹⁷ and 80 cm. ²⁵ Several comparative studies of varying heights have been investigated^17,25 to determine the optimum height but validity has been established for heights as low as 20 cm. ¹⁷

Technique when performing the drop jump has been suggested to be tightly controlled ²⁶ although prior familiarization or training of the technique has also been suggested as not being required to obtain test reliability. ²⁷

Within sport science, the drop jump has been used by physically active individuals,^17,28,29 adolescent athletes, ²² Norwegian national level triple jumpers, ²⁵ volleyball players ²⁶ and men from team sports. ¹⁸

Several studies have attempted to identify the correlation between sprinting ability and jump testing performance using able-bodied participants to ascertain its ability as a diagnostic or talent identification test. Some studies have shown a direct correlation with bilateral jump testing and short distance sprinting ³⁰ however; this has been disputed. ³¹ These two studies do agree that vertical jump displacement and sprinting performance itself do correlate significantly. Additionally further work using several dynamic jump methods have shown improved correlation within different phases of the 100 m sprint using different jump methods ³² or similarly different leg strength qualities. ³⁰ A single leg horizontal drop jump has been shown to be highly related to sprinting ¹⁹ with the observation that (while not often used), horizontally performed jumps are better predictors of sprint performance. ³⁰ However, it is not known at this time any issues regarding test subject safety when performing horizontal jumps within a clinical setting as opposed to vertical jumps which alternatively use a smaller footprint and less forward momentum of the amputee which still demonstrate qualities representative of sprinting.^30,31 As a result, dynamic leg power assessment methods such as the drop jump have been shown to be applicable to sprinting but their selection may relate to a particular phase within short distance sprinting with increasing/decreasing correlation.

Little evidence has been seen of drop jump use when performed directly on lower limb amputees. Lower limb amputees have, however, performed other jump test variants such as unilateral vertical jumping ³³ and a one-foot vertical jump with approach with unilateral trans-tibial amputees. ³⁴ In the 2004 study, the maximum height and flight time were reduced noticeably on the prosthetic side but this study’s findings should be taken with a degree of caution as it was only performed with two participants. No studies of athletes with an amputation performing unilateral or bilateral drop jumps are currently evident within the literature.

Drop jumping in practise does have several concerns as to its use. The magnitude of the height used has been cited as modifying the technique used to perform it, ¹⁷ likewise upper-limb motion has been shown to affect the generated impulse ²⁷ meaning arm motion to stabilize height or create comfort to the participant of the dropping height would have to be removed. If this is not controlled, the effect of the arms may influence the results affecting its correlation to sprinting. It has also been suggested that to increase the jump height up to 60 cm creates net joint reaction forces with sharp peaks, ¹⁷ as well as observations of an altered jump technique by the participants. ²⁵ These observations may be less desirable or raise ethical issues. However, in terms of test competence, it should be noted that athletes with a lower limb amputation already performed the long jump event at the 2008 Paralympic Games (www.paralympic.org/Sport/Results/) and empirical assessment of jumping events have taken place.^35,36 With this in mind, a pilot test has been conducted to investigate feasibility of the unilateral drop jump technique for further study.

Methods

Ethical approval for the pilot test was formally registered and approved by the author’s research institution. A single male able-bodied candidate with a background in international level sport undertook these pilot tests. The test candidate was male, 195 cm tall and had a mass of 90 kg.

The protocol of the drop jump was to stand on the intended landing leg on top of a 20-cm high platform. When ready, the drop down action were performed, landing in the centre of the pressure pad and then, using the same take-off leg, immediately execute a maximal effort single leg vertical jump. The jump procedure is concluded by landing on both feet. Any required ‘run off’ for the participant after the jump was performed was allowed. There was a 1-minute timed break between each jump being performed. The left and right feet were alternated and 12 jumps on each leg were performed. Any changes in performance over the course of the jump testing were assessed by comparing jumps 1 to 6 and 7 to 12.

A RS Footscan pressure pad, (RSscan Ltd, Ipswich, UK) sampling data at 253 Hz was used to record the mean ground reaction force data of the drop jumps first and second contact phases and the duration from the first to final landing. Force data were assessed since force was the key performance indicator of sprinting as promoted by Weyand ¹⁶ and both landing and takeoff could be compared directly. The Footscan’s accuracy was checked beforehand using known masses. In addition, the symmetry index (SI) of both limbs was used as described by Barber et al. ³⁷ as:

$$\text{Non}\text{dominant}\text{leg}/\text{dominant}\text{leg}\times 100$$

Due to the test subject not using a running prosthesis, the subject’s limb-to-limb performance difference was created through use of different footwear. The participants left foot was bare (dominant) whereas the right foot was shod (non-dominant) using new and unused sports footwear. This was undertaken to help provide a greater difference in the mechanical behaviour between the left and right feet that overcomes a typical limb asymmetry as shown in a review by Sadeghi et al. ³⁸ and limb-to-limb symmetry indices differences observed from several jump test experiments by Maulder and Cronin of a 4–11% range. ³⁰ It was conceded that use of the footwear on one side would increase the drop height by a small fraction but the magnitude of the force data was not being assessed in this pilot.

Results

The coefficient of variation (CV) in ground reaction force (GRF) of the full landing-to-landing phase across the 12 unilateral drop jumps was 6% (left side barefoot) and 8% (right side shod). The coefficient of variation of the duration of the first to second landing of the 12 unilateral drop jumps was 5% (left side barefoot) and 11% (right side shod). The Pearson Coefficient Correlation between the left and right side average force outputs was 0.12 indicating a low relationship of the two data sets.

The SI of the average GRF of the full landing to landing phase was a 29% impairment to the side that was shod.

To check how the jump performance changed over the series of drop jumps, the 12 jump set was also split into jumps 1 to 6 and jumps 7 to 12. In this case, the mean GRF created by the left foot (bare) saw a CV of 6% (jumps 1 to 6) and 7% (jumps 7 to 12). The mean GRF created by the right foot (shod) saw a CV of 3% (jumps 1 to 6) and 8% (jumps 7 to 12). The SI impairment on the shod side was 32% from jumps 1 to 6 and 26% from jumps 7 to 12.

The first landing to second landing time duration from the left foot (bare) saw a CV of 2% (jumps 1 to 6) to 7% (jumps 7 to 12). The first landing to second landing time duration from the right foot (shod) saw a CV of 11% (jumps 1 to 6) to 12% (jumps 7 to 12).

Discussion

A pilot test of the unilateral drop jump was undertaken to assess the statistical viability of the technique. From 12 jumps, there was a marked difference in mean jump GRF behaviour between jumps 1 to 6 and 7 to 12 in both feet. For the left bare foot, jumps 1 to 6 saw a relatively minor GRF CV increase of 6% to 7% from jumps 7 to 12. However, the right shod side saw a marginally larger change of CV from jumps 1 to 6 and 7 to 12 in GRF data of 3% to 8%. This increase still maintains a desirable level of CV but does indicate a marked increase in variance of the data as the test jumps increase. It is possible that use of the footwear created some degree of fatigue and/or jump technique degradation on the shod side as the series of jumps continues. Such an effect could be evident when assessing an amputee’s biological limb compared to their prosthesis.

The duration of the jump being performed from drop landing to final landing saw CVs from the left bare foot of 2% (jumps 1 to 6) and 7% (jumps 7 to 12) whereas the right shod side saw 11% (jumps 1 to 6) and 12% (jumps 7 to 12). This larger variability in the behaviour of the footwear continues the theme of either fatigue and/or jump technique degradation. However, the mean average of landing to landing jump duration over jumps 1 to 6 and 7 to 12 are virtually identical suggesting that limb fatigue per se may or may not be the cause but that some kind of technique change occurred over the length of the test. Like the GRF data, this shows greater variability after six jumps. On this basis, it is proposed that the number of jumps in future trials needs to be as few as possible to maintain good technique while maintaining a desirable level of statistical repeatability and reliability. It needs to be ascertained whether this degradation in an impaired limb is also seen when using an energy return prostheses. Motion capture assessment may go some way to identify this.

The Pearson Coefficient Correlation between the left and right side average force output data sets was 0.12 and is very low. It could be speculated that one side will fatigue at a different rate to the other and therefore potentially behave very differently. The fact that the CV of the right hand side was much greater than the left coupled with both the symmetry index indicating a 29% (across all 12 jumps) and the mean durations being similar across both halves of the trials suggests that the shoe acted as a major impairment and behaved much more erratically in continued use. While no SI data have been identified historically when performing the unilateral drop jump technique, the SI score of 29% in this study is considerably higher than an 11% range recorded when performing a vertical squat jump with able-bodied subjects. ³⁰ This demonstrates a consistent limb-to-limb performance-significant impairment.

The greater erratic performance of the shod side after six jumps suggests that while mean performances are stable, the number of jumps should be kept under six. This raises an interesting point about the evaluation of mixed limb performances. The SI between the two limbs when the limbs are fresh compared to when the limbs are fatiguing towards the end of a 100-m sprint could be very different. As a result, this pilot study raises the philosophical question of not only viability of this evaluative technique but also at which prior state of limb fatigue are compared as both are part of the nature of the 100-m sprint race. This fatigue profile and SI will vary when based upon Nolan’s observations that prostheses designs behave very differently. ¹

It should also be noted based upon these trials and observation of running amputees that the relative instability of composite running prosthesis may require a participant to have ‘jog off’ after the jump is performed rather than expecting an abrupt stop.

Concern could be raised that athletes could cheat by purposely underperforming the drop jump test. While the final vertical phase could be manipulated, the initial drop is governed by the athletes mass, gravity and the height of the drop and therefore could not be underperformed. As a result, the calculated limb stiffness could not be doctored. If limb power from the vertical phase were intentionally underperformed, it is plausible that several successive jumps would likely highlight an erratic behaviour and thus signify potential cheating. Ongoing competition-to-competition testing would likely build up a long-term performance profile of the athlete to aid this.

In summary, no concerns were raised in the pilot testing that suggests that this technique is not a viable solution for the evaluation of sprint athletes with unilateral lower limb amputations. This technique is recommended for use by assessing a larger test subject group of athletes with a lower limb amputation. The technique proposal satisfies the findings of the study that defined the framework for technique selection. ⁵

Conclusion

This paper has attempted to develop from a previous study what is a practical proposal to assess sprinting lower limb prostheses in disability sport. The Dyer et al. Delphi ⁵ study was designed to provide a framework in which a method of athlete with a unilateral amputation could be determined. From this, a review of the literature has indicated that a unilateral drop jump technique could be used for limb-to-limb comparison of an athlete. Drop jumping is shown to be a reliable, well-used technique that correlates with able-bodied sprinting performance. It is proposed that this technique is entirely feasible to be performed and is recommended for wider study in an athletic amputee population in the future.

A pilot test of the unilateral drop jump was undertaken and it is noted that the number of jumps executed should be less than six and that a means to detect any changes in technique should be put in place. The characteristics of limb-to-limb performance and the rate and type of fatigue require further study with athletes with an amputation. There were no observations that implied that the technique is not suitable for use as an assessment tool.

This research is part of an ongoing project to develop an appropriate holistic approach for the assessment of the fair use of lower limb sprinting prostheses by athletes in disability sport through both qualitative and quantitative methods.