Paralympics: how to optimise jumping blades for long-jumper amputees

The 2024 Paris Olympic and Par­a­lympic Games are fast approach­ing. As part of the SCIENCES2024 project, we are work­ing to improve the per­for­mance of the tib­ial pros­the­ses used by cer­tain Par­a­lympians for the long jump. There are sev­er­al cat­e­gories of amputees – low or high leg ampu­ta­tion, both limbs or one limb – but we are focus­ing on uni­lat­er­al amputees, i.e. those who have had one leg ampu­tat­ed below the knee.

The aim of our approach is to find solu­tions to opti­mise the resti­tu­tion of ener­gy at the crit­i­cal moment of the jump: it is the very moment of the impulse that allows the ath­lete to project him­self for­ward. In uni­lat­er­al amputees, the jump­ing blade – made of rigid but very slen­der car­bon – replaces the ampu­tat­ed limb and is like a large, almost per­fect spring.

The blade, which fits over the resid­ual limb, allows the ath­lete to take off by press­ing on it and thus com­press­ing it strong­ly at the moment of impulse. Dur­ing a long jump, an ath­lete makes a long run-up: the faster he or she runs, the more kinet­ic ener­gy he or she stores, which is then “trans­formed” into impulse dur­ing the final take-off. The main chal­lenge is to con­vert this kinet­ic ener­gy, linked to the speed of the run-up, into “impulse ener­gy” to enable the ath­lete to run as far as possible. 

The main chal­lenge is to con­vert the momen­tum into impulse ener­gy to allow the ath­lete to run as far as possible.

Cer­tain para­me­ters, such as an unsuit­able jump angle, a poor body posi­tion or the athlete’s own move­ments dur­ing the jump, can dis­si­pate this pre­cious ener­gy. In addi­tion, fric­tion in the sock­et or shock to the ath­lete’s body can lead to injury – even if the ges­ture or pros­the­sis is designed in the best pos­si­ble way for per­for­mance. We are there­fore look­ing at how to opti­mal­ly trans­fer this kinet­ic ener­gy to the impulse in a length-effi­cient man­ner with­out caus­ing injury that is detri­men­tal to performance. 

The sports ges­ture and the pros­the­sis are there­fore essen­tial for ener­gy con­ver­sion. To this end, we study the ener­gy accu­mu­lat­ed in the blade so that the ath­lete can project as far as pos­si­ble. We are more inter­est­ed in per­for­mance in this part of our work, and we work with the tech­ni­cal direc­tor of the French Han­d­is­port Fed­er­a­tion who puts us in con­tact with the ath­letes and their trainers.

So, in addi­tion to improv­ing per­for­mance, we also seek to improve the com­fort of the ath­letes, to lim­it their fatigue and, of course, their injuries. The jump­ing blade is an appendage that allows them to jump, but it also cre­ates vibra­tions and shocks when it hits the ground. These vibra­tions can cause fric­tion in the sock­et, lead­ing to dis­com­fort and even pain. We are look­ing to mea­sure these shocks through numer­i­cal mod­el­ling to analyse their trans­mis­sion to the athlete’s body.

We will be able to under­stand how the mate­ri­als mak­ing up the blade and the pros­the­sis can deform mechan­i­cal­ly and reduce the shock.

We have set up exper­i­men­tal pro­to­cols in the lab­o­ra­to­ry that will enable us to repro­duce the sport­ing ges­ture. With these, we will be able to under­stand the over­all defor­ma­tion of the blade, under the effect of com­pres­sion, dur­ing the impulse, but above all we will be able to under­stand how the mate­ri­als mak­ing up the blade and the pros­the­sis trans­mit the forces of the track to the ath­lete’s body. In addi­tion, we will iden­ti­fy their role both in the resti­tu­tion of ener­gy and in the reduc­tion of the risk of injury inher­ent in the practice.

To do this, we have devel­oped numer­ous lab­o­ra­to­ry exper­i­ments with var­i­ous instru­ments. High-speed cam­eras and sen­sors allow us to analyse dynam­ic phe­nom­e­na in detail and to mea­sure efforts and wave trans­fer. These mea­sure­ments allow us to move towards a glob­al opti­mi­sa­tion of the blade-pros­the­sis system.

We are also work­ing on the devel­op­ment of the mate­ri­als that make up the pros­the­sis and we are try­ing to find those that will give good over­all per­for­mance. These mate­ri­als are man­u­fac­tured by 3D print­ing, oth­er­wise known as addi­tive man­u­fac­tur­ing.

To best analyse the impact of mate­ri­als and ath­let­ic move­ment, we are look­ing at two sce­nar­ios. In the first, when the ath­lete press­es on the blade slow­ly enough, this caus­es a pro­gres­sive defor­ma­tion. In this case, the infor­ma­tion can be analysed in a fair­ly sim­ple way through a sta­t­ic lab­o­ra­to­ry test pro­to­col. Since the blade is made of a rigid but slen­der mate­r­i­al, it deforms glob­al­ly but also local­ly at the sole and at the point of con­tact with the resid­ual limb via the sock­et. We are there­fore study­ing the local sta­t­ic defor­ma­tions, which allow us to under­stand how ener­gy is dis­si­pat­ed, and the nature of the pres­sures on the ampu­tat­ed limb, which can lead to pos­si­ble injuries.

Sec­ond sce­nario: the speed of the load is pro­gres­sive­ly increased to repro­duce the con­di­tions of the athlete’s last impulse on the blade. Thanks to our obser­va­tions dur­ing the ath­lete’s run and at the moment of his jump, we are able to mea­sure the defor­ma­tion speed of the blade and the sole. The trans­mis­sion of vibra­tions and shocks from the ground to the blade and then to the athlete’s limb is thus char­ac­terised by the­o­ret­i­cal mod­els. In this sec­ond sce­nario, it is also inter­est­ing to see the dynam­ic effects and forces that inter­act between the track, the blade, and the ath­lete. These forces vary rapid­ly at the moment of impulse and are rep­re­sen­ta­tive of the impact phe­nom­e­na that often lead to injuries.

In the first sce­nario, sta­t­ic forces are at work; in the sec­ond, dynam­ic or impulse forces are at work. These con­cepts are com­mon­ly used to study struc­tured mate­ri­als or meta­ma­te­ri­als in indus­try. We trans­pose them to the ser­vice of humans, to advance the field of “aug­ment­ed human” or the future medal-win­ning athlete!

Isabelle Dumé

Références

• Inter­ac­tion ciné­ma­tique sol-struc­ture d’un mas­sif ren­for­cé par inclu­sions rigides sous séisme
• Exper­i­men­tal inves­ti­ga­tions about com­plex non-relax­ing fatigue loads for car­bon-black filled nat­ur­al rubber
• https://​www​.han​d​is​port​.org