How to help swimmers win their races 

Mak­ing it to the podi­um can be a mat­ter of mere hun­dredths of a second. To max­im­ise the chances of win­ning medals, a great num­ber of factors must be optim­ised. Rémi Car­migni­ani and his team’s research focuses on so-called “row­ing” water sports: swim­ming, row­ing, canoe­ing, and kayak­ing. In these dis­cip­lines, ath­letes use an oar to move at the water/air inter­face – in swim­ming, this oar cor­res­ponds to the hand and forearm.

To vary their aver­age speed, swim­mers rely mainly on the speed of exe­cu­tion of a peri­od­ic move­ment with their arms, called cadence. The high­er the cadence, the high­er the aver­age speed. To study this rela­tion­ship in swim­ming, a test is car­ried out in a pool: a pro­gress­ive 10 × 25 m with a start every three minutes. The cadence and speed are meas­ured at each run. In swim­ming, speed evolves as does the cadence at high speeds¹.

The aim is to improve under­stand­ing² of the evol­u­tion of speed as a func­tion of cadence and to relate it to the power delivered to the arms to move for­ward. For the time being, mac­ro­scop­ic meas­ure­ments (aver­age speed, aver­age cadence, and total force in the so-called tethered stroke) have been made, but these will be fol­lowed by meas­ure­ments of the dynam­ic forces at the level of the hands using strain gauges and of the kin­et­ics of the move­ment with iner­tial meas­ure­ments. A gen­er­al mod­el to explain the dif­fer­ent regimes observed will then be pro­posed. This pro­ject is sup­por­ted by the EDF Foundation. 

Dur­ing the so-called act­ive phases, the swimmer’s body is sub­jec­ted to addi­tion­al res­ist­ance forces that gen­er­ally increase the res­ist­ance in the water³. For example, dur­ing a wave, due to the deform­a­tion of the body, the swim­mer must fight – in addi­tion to the pass­ive res­ist­ance of the unde­formed body – against an addi­tion­al force that is also quad­rat­ic with speed and depends on the shape of the wave. In swim­ming, in con­trast to oth­er “row­ing” sports, the swim­mer is both the shell and the oar.

A clas­si­fic­a­tion and quan­ti­fic­a­tion of the dif­fer­ent forms of act­ive res­ist­ance for row­ing sports is underway.

This pro­ject star­ted in 2018 in the frame­work of the ANR NeP­TUNE (ANR-19-STHP-0004). The par­tic­u­lar­it­ies of swim­ming are the absence of equip­ment (no boat hull or oar, just the athlete’s body) and the pres­ence of the water/air inter­face mak­ing the use of equip­ment com­plex. This equip­ment can include iner­tial units (small water­proof boxes the size of a €2 coin con­tain­ing gyro­scopes and accel­er­o­met­ers that are attached dir­ectly to the swim­mer) and force sensors. They can inter­fere with the swim­mer­’s stroke and even affect the way they feel… 

One of the main sub­jects is the study of the swim­mer­’s start and the optim­isa­tion of his trajectory.

To sup­port these instru­ments, the INSEP pool has been equipped with twenty cam­er­as (which film at a fre­quency of 50 Hz). Ten are placed on the sur­face every five metres and ten under­wa­ter. The data cap­tured by these cam­er­as is used by machine learn­ing pro­grammes to mon­it­or the swim­mers. The force meas­ure­ments are used to explain the obser­va­tions by phys­ics and to find para­met­ers to char­ac­ter­ise the pos­i­tion of the swim­mer as well as his speed and acceleration.

One of the main top­ics is the study of the swimmer’s start, and the optim­isa­tion of its tra­ject­ory. The start can be quite com­plic­ated to optim­ise: the swim­mer starts out of the water on the pad, then breaks through the sur­face at a high speed rel­at­ive to the swim­ming speed, sinks under­wa­ter by about one metre before resur­fa­cing, and all this over a char­ac­ter­ist­ic time of five seconds and a hori­zont­al dis­tance of 15 metres. 

For the first part, try to under­stand the best take-off angle the swim­mer should have when leav­ing the pad. The aer­i­al tra­ject­ory that fol­lows until the swim­mer enters the water is less inter­est­ing from a phys­ics point of view, as it can be described by the clas­sic­al equa­tions of free fall (air fric­tion is neg­li­gible). What is import­ant in this phase is the ori­ent­a­tion that the swim­mer will man­age to give to his body to enter the water by turn­ing around his centre of mass. 

Remem­ber: by pro­pelling him­self, the swim­mer gen­er­ates addi­tion­al res­ist­ance! The speeds in this phase are high (nearly 4 m/s). You should not trig­ger undu­la­tions too early at the risk of pen­al­ising your­self with great­er res­ist­ance and finally brak­ing harder while try­ing to accel­er­ate. But be care­ful, wait­ing too long can also be prob­lem­at­ic as you risk sink­ing fur­ther and hav­ing to cov­er a great­er dis­tance to return to the surface. 

In gen­er­al, in the front crawl, swim­mers trig­ger undu­la­tions when they reach the low­est point at the apex of their tra­ject­ory. This gives them zero ver­tic­al velo­city. The situ­ation is dif­fer­ent in the back­stroke: they start in the des­cent phase, as they start with much lower speeds.

The final object­ive of this pro­ject, the thes­is work of Charlie Prétot, who is a mem­ber of Rémi Car­migni­an­i’s team, is to help swim­mers optim­ise their tra­ject­ory over the first 15 metres (the length over which swim­mers are allowed to be under water in com­pet­i­tion) at the start and at the turns. Should you go out a bit before the 15 metres? Maxime Grous­set, run­ner-up in the 100 m free­style at the last World Cham­pi­on­ships, who has been mon­itored for about a year as part of this pro­ject, has shown that his best start occurs when he comes out of the water between 11 and 12 m. This pro­ject aims to help him improve his turn tra­ject­ory as well. Since the start of the pro­ject, more than 300 starts and turns have been ana­lysed on about 20 swimmers. 

Since the begin­ning of the pro­ject, more than 300 starts and turns have been ana­lysed on about 20 swimmers.

For the video obser­va­tions, the swim­mer is tracked using sev­er­al mark­ers on the skel­et­on: head, hands, shoulders, sternum, elbows, wrists, knees and ankles. A neur­al net­work was developed to auto­mat­ic­ally track starts, turns and swim­ming for the first 15 metres.

In view of the open water swim­ming events that will take place in the Seine in 2024, sev­er­al pro­jects are focus­ing on this dis­cip­line. An ana­lys­is of the inter­ac­tion of swim­mers in an 80 m long canal in Chat­ou is under­way. The aim of this pro­ject is to under­stand using scale mod­els how, depend­ing on the pos­i­tion of the swim­mers, the wakes they cre­ate influ­ence oth­er swim­mers nearby. As in cyc­ling, in this type of race, swim­mers can take advant­age of the com­pet­it­ors’ suc­tion to save them­selves or even dis­turb anoth­er swim­mer. This work is being car­ried out by Bap­tiste Bolon, a research engin­eer in Rémi Car­migni­an­i’s team.

Anoth­er pro­ject also aims to provide inform­a­tion to the swim­mers of the French open water team on the expec­ted race con­di­tions: flow speed in the Seine and water tem­per­at­ure. For speed, this is the first time an Olympic race has been held in a river since the rein­tro­duc­tion of open water races in Beijing in 2008. For the tem­per­at­ure, it influ­ences the equip­ment (neo­prene or fab­ric wet­suit) and on the peri­od of train­ing two weeks before the race. This work is being car­ried out with Mari­on Cocusse, a stu­dent at École des Ponts and is sup­por­ted by the EDF Foundation.

Isabelle Dumé