The Impact of continuous Glucose Monitor Positioning on Readings
by Doug Stewart
The launch of continuous glucose monitoring products, such as Supersapiens, for athletes arrived with much fanfare, with discussions around how it could be used to help inform fuelling strategies and avoid bonking in races and training.
However, researchers exploring their usage have often been less than positive. For example, a paper from 2022 stated that they would not recommend using one during exercise and found that accuracy decreased when carbohydrates were eaten prior to exercise.
A paper published this year explored the impact of their readings based on the position of the glucose monitor on the body. Traditionally, it has been recommended to place the monitor on the back of the upper arm around the triceps area. However, the current research explored the difference in readings from the triceps vs the vastus medialis (the muscle above the knee).
A small sample of 3 women and 6 men participated in the study, undergoing 4 different scenarios, each 30 minutes in length, after consuming 100g of glucose. The 4 settings were: resting control, a graded cycle test, electrically-simulated quadriceps contractions, and passive whole body heating.
A finger capillary sensor was used to assess glucose as a control and to evaluate the continuous glucose monitor for accuracy. It was found that the triceps positioned monitor was reporting values closer to those from the finger sensor, suggesting that this is a better position for the sensor to be placed. There was still a variation of up to 9% between glucose reads though.
However, when exercising, the continuous glucose monitors varied even more than the finger sensors. Given that their primary purpose is to monitor glucose levels when exercising, this is not a great endorsement. Moreover, there was changing variance in the monitor reading, depending on whether the athletes were resting or exercising. For example, when at rest, the leg monitor recorded lower glucose than the arm positioned monitor, but when cycling it was only at maximal intensity that it showed a lower reading compared to the arm. There was no difference between monitors during the electrical quadricep stimulation, while the passive heating resulted in lower readings in the leg.
The authors conclude that positioning on the arm for rest may be best, but when exercising, or certainly cycling, placing a glucose monitor on a leg muscle may provide a greater level of insight on the muscle glucose levels. This is likely the result of the increased blood-flow to that muscle, rather than due to muscle contractions. But it is worth highlighting that, compared to the capillary sensor position on the finger, it was exercising that resulted in the largest variance from that control readings. So this supports the earlier paper around their potential limited application when exercising.
References:
Clavel, P., Tiollier, E., Leduc, C., Fabre, M., Lacome, M., & Buchheit, M. (2022). Concurrent validity of a continuous glucose-monitoring system at rest and during and following a high-intensity interval training session. International journal of sports physiology and performance, 17(4), 627-633.
Coates, A. M., Cohen, J. N., & Burr, J. F. (2023). Investigating sensor location on the effectiveness of continuous glucose monitoring during exercise in a non-diabetic population. European Journal of Sport Science, (just-accepted), 1-21.