Load cells work swimmingly in calculating drag (Updated 2023)

Back in 2017, we first covered how load cells were assisting in measuring drag for elite swimmers. Six years on, we’re updating this article with results of a new study, to coincide with the 2023 World Aquatics Championships in Fukuoka, Japan.

A team from Queen’s University Belfast and Ulster University have reported on a new method of measuring the active drag profile for swimmers doing front crawl. The difference in this research is that it looks at the whole stroke cycle, during which the swimmer is subject to both active and passive drag.

“Active drag is the resistive force that acts on a swimmer when swimming through the water and is constantly changing during a stroke cycle … Active drag is composed of frictional, pressure, and wave drag.

Passive drag is the resistance generated by a swimmer’s body whilst the swimmer moves through the water in a fixed position. Passive drag acts on the swimmer during the glide phases of the stroke … and is composed of frictional, pressure and wave drag.”

 

The team used “a stationary load cell set-up and a commercial resistance trainer to record the tension force in a rope caused by an athlete swimming.” Working out the fully-tethered force with no drag involved positioning a submersible load cell between the rope and a clamp that secured it to a diving block. A semi-tethered version involving a motion sensor and an extendable rope enabled the swimmer to swim up to the 25metre point in the pool, and in turn enabled the team to measure the active drag

The team compared mean active drag values with those from an established method (Velocity Perturbation Method (VPM)) to create an active drag profile across the entire stroke cycle, not just a part of it. For diagrams of the set-up, head to Nature Website.

Elite swimmers find water a drag. They know that the resistance that water exerts on their body is affected by several factors including their physical size, swimming speed and their swimming style stroke rate. However. calculating this resistance, known as drag, has always been difficult.

Now, a team from the University of Tsukuba in Japan have developed a method of accurately measuring drag for swimmers doing front crawl, using load cells. The method estimates the drag in swimming using measured values of residual thrust (MRT). And it’s causing more than just ripples of excitement.

The team first needed to overcome the variables created by swimming speed and stroke. They achieved this by placing the swimmer in a water flume tank, and connected them to load cells at the front and rear of the flume. They could then measure the force applied in both directions while the swimmer maintained a fixed position by swimming at the same speed as the flume. This gave a figure for residual thrust, which could be compared with the results from a inactive state, when the swimmer is simply towed through the water. The number of strokes taken was also regulated by swimmers timing their strokes to the beat of a metronome.

With this innovative set-up, the team could them vary both the flume speed and the metronome/stroke rate, as the report’s author Hideki Takagi*** explained:

“We set six swimmers up in the apparatus and applied different water flow rates to them… We were also able to compare the active drag they were subjected to with passive drag when they were pulled through the water while adopting streamlined positions.” 

 

The team were much encouraged by the low variability for active drag, showing that their method was both reliable and consistent. Not surprisingly perhaps, they found that the active drag (drag created by movement) exceeded passive drag (being towed), mainly because when a swimmer is making a stroke, their chest area is raised, creating more surface area in the direction of travel and therefore creating more drag.

The team hope the results will help swimmers reduce drag through minor adjustments to body position while swimming, and improve their times as a result. Watch out, Michael Phelps!

 

 

 

NOTES:

**** Kenzo Narita, Motomu Nakashima, Hideki Takagi. Developing a methodology for estimating the drag in front-crawl swimming at various velocities. Journal of Biomechanics, 2017; DOI: 10.1016/j.jbiomech.2017.01.037