THE RESEARCH BEHIND KYLA.

Walløe, L. (2015). Arterio-venous anastomoses in the human skin and their role in temperature control. Temperature, 3(1), 92-103.

Foundational review of arteriovenous anastomoses (AVAs), the dense vessel network in the palms, soles, and face that moves blood between core and surface to regulate temperature. AVA density is around 100 per cm² in the palm.

Taylor, N. A. S., Machado-Moreira, C. A., van den Heuvel, A. M. J., & Caldwell, J. N. (2014). Hands and feet: physiological insulators, radiators and evaporators. European Journal of Applied Physiology, 114(10), 2037-2060.

Review article on how the hands and feet take part in heat balance. Each hand has a surface area to mass ratio four to five times that of the body, and its vessels can switch between holding heat in and giving it off. During hypothermia the hands are effectively shut off, with heat flow under 0.1 W. The authors conclude that the hands and feet work as radiators, insulators and evaporators.

Lissoway, J. B., et al. (2015), Stanford University. Novel application of chemical cold packs for treatment of exercise-induced hyperthermia: a randomized controlled trial. Wilderness & Environmental Medicine, vol. 26, issue 2, 173-179.

Randomized controlled trial with 10 men walking in 40°C heat in insulated military overgarments until oesophageal temperature reached 39.2°C. Each man did all three conditions on separate days. Both cooling arms used three sites at once: the traditional arm covered neck, groin and axillae together, the glabrous arm covered cheeks, palms and soles together. Cooling applied to glabrous skin lowered core temperature by 0.30°C every 10 minutes, against 0.17°C for cold packs on the traditional neck, groin, and armpit sites, and 0.12°C with no cooling. Direct evidence that the palms and other glabrous surfaces are the body’s most effective route for moving heat out of the body.

Nybo, L., and Nielsen, B. (2001). Hyperthermia and central fatigue during prolonged exercise in humans. Journal of Applied Physiology, 91(3), 1055-1060.

Fourteen men cycled at 60% of maximal oxygen uptake in 40 °C and, on a separate day, in 18 °C. In the heat their core temperature reached 40.0 °C and they were exhausted after 50 minutes; in the cool trial it settled near 38.0 °C and they completed the hour. Immediately afterwards they held a two-minute maximal contraction while the researchers superimposed electrical stimulation on the nerve. If the brain were already calling up everything the muscle had, that extra pulse would add no force. It added force, and it added more when the men were hot, which is how the shortfall is measured. Voluntary activation was 54% ± 7 when hot against 82% ± 6 when cool, while total force of the muscle, voluntary plus stimulated, did not differ between the two trials. The same pattern appeared in a handgrip that had done none of the cycling. Not a palm cooling study. It establishes the cost that cooling is meant to address.

Tucker, R., Rauch, L., Harley, Y. X. R., and Noakes, T. D. (2004). Impaired exercise performance in the heat is associated with an anticipatory reduction in skeletal muscle recruitment. Pflügers Archiv, 448(4), 422-430.

Ten men rode two self-paced 20 km time trials, one at 35 °C and one at 15 °C. Thigh muscle activity, measured with EMG, was lower in the heat at 10 and 20 km, and it began to fall at a point when core temperature, heart rate and rated perceived exertion were still the same in both rides. Power output was significantly lower over the last fifth of the hot ride. Core temperature peaked at 39.2 °C in the heat and 38.8 °C in the cool. The authors read this as the body reducing muscle recruitment in advance to limit heat production. That reading is debated; the measurements are not. Not a palm cooling study.

Grahn, D. A., Cao, V. H., & Heller, H. C. (2005). Heat extraction through the palm of one hand improves aerobic exercise endurance in a hot environment. Journal of Applied Physiology, 99(3), 972 to 978. PMID 15879169.

Original Stanford research, and the study that established the pathway. Subjects walked uphill on a treadmill in 40 degrees C until they could not continue. With the device, exercise duration rose from 32.3 to 46.1 minutes, a gain of 43%, and core temperature climbed more slowly, 2.1 against 2.9 degrees C per hour. The device did two things at once. It held the palm at 18 to 22 degrees C, and it applied gentle suction, 35 to 45 mmHg, to the whole hand to draw more blood into it. This is a study of a pathway, not of a held object, and we do not claim its numbers for ours.

Grahn, D. A., et al. (2012). Work volume and strength training responses to resistive exercise improve with periodic heat extraction from the palm. Journal of Strength and Conditioning Research.

Stanford follow-up on palm cooling between sets of resistance training. Subjects gained 40% more bench press volume over 3 weeks (vs 13% in the control group), 144% more pull-up volume in trained subjects (vs 5% control), and 22% in 1-rep max bench press over 10 weeks of pyramid training. These two results had no parallel control group.

Stanford University (2012). Stanford researchers’ cooling glove 'better than steroids’ for athletes. Stanford Report.

Coverage of the Stanford palm cooling research and its adoption by Stanford athletics, the San Francisco 49ers, the Oakland Raiders, and Manchester United. Reports approximately 20% performance improvement during activity using palm cooling, alongside a significantly faster settle between efforts.

Caruso, J. F., et al. (2015). Intermittent Palm Cooling’s Impact on Resistive Exercise Performance. International Journal of Sports Medicine, vol. 36, issue 10, 814-821.

Randomized study on leg press, comparing no palm cooling, palm cooling between sets, and palm cooling both between sets and after exercise. Cooling between sets cleared blood lactate faster and slowed the drop in average power, with the cooled condition holding higher power through the fourth set. Independent support, from the University of Tulsa, for cooling between efforts to limit fatigue.

O’Brien, I. T., et al. (2021). Use of Gloves to Examine Intermittent Palm Cooling’s Impact on Rowing Ergometry. Journal of Strength and Conditioning Research, vol. 35, issue 4, 931-940.

Study of 34 subjects completing rowing ergometer workouts with and without intermittent palm cooling, applied through gloves. The cooled workouts produced greater distance rowed, alongside lower heart rate and lower blood lactate, which the authors attribute to reduced fatigue. Extends the evidence into an endurance, repeated-effort sport.

Kwon, Y. S., et al. (2010). Palm cooling delays fatigue during high-intensity bench press exercise. Medicine and Science in Sports and Exercise, vol. 42, issue 8, 1557-1565.

Sixteen resistance-trained men performed four sets of bench press to fatigue, with palm cooling, palm heating, or no treatment during the rest intervals. Total training volume reached 2,480 kg with palm cooling, against 1,972 kg with no cooling, a gain of about 26%. Muscle activation was higher, and core temperature and perceived exertion were lower. The device also applied 35 to 45 mmHg of suction to the whole hand, which draws more blood into the palm. This study supports the mechanism, not the object. An independent replication of the Stanford resistance findings, from a separate lab at the University of New Mexico.

Brown, M., Daniels, J., Crabtree, M., Thompson, K., Murphy, J., Pannell, W., & McGlawn, R. (2025). The Effects of Palmar Cooling on Repeated Sprinting Ability: A Randomized Controlled Clinical Trial. Sensors, 25(6), 1830. PMID 40292933.

Randomized controlled trial with 15 subjects on repeated 60-meter sprints with directional changes. Subjects using palm cooling completed the final sprint 2.76% faster, with heart rate roughly 14 bpm lower at the end of the rest period, and reported significantly less delayed-onset muscle soreness 48 hours after testing.

Iwahashi, M., Chaen, Y., Yanaoka, T., Kurokawa, Y., & Hasegawa, H. (2023). Cold water immersion of the hand and forearm during half-time improves intermittent exercise performance in the heat. Frontiers in Physiology, 14, 1143447. PMID 37362443.

Randomized crossover study with 11 active men cycling in 33°C heat, with hands and forearms immersed in 15 to 17°C water during a 15-minute break. Average power output in the second half was significantly greater than control. Rectal temperature dropped by 0.54°C, heart rate dropped by 16 bpm, and skin blood flow decreased by approximately 40%.

Hu, X., Aihara, K., & Hasegawa, H. (2026). Pre-exercise and inter-bout palm cooling enhances intermittent sprint performance despite no thermoregulatory advantage during exercise in the heat. Temperature. Published online 20 September 2026.

Wang, A., Fu, W., Shen, B., and Hurr, C. (2026). Ergogenic effects of intermittent hand cooling on high-intensity resistance exercise performance: a placebo-controlled crossover study. Journal of Strength and Conditioning Research, 40(6), e562-e570

Fourteen recreationally trained men did four sets of biceps curls to failure at 70% of one-rep max under three randomised conditions: hands immersed in cold water (10 degrees Celsius) for 60 seconds of each 90-second rest, a thermoneutral immersion presented to them as a treatment, and no cooling at all. Cooling beat the control condition (p = 0.002) and beat the placebo (p = 0.013). The placebo also beat the control condition (p = 0.003).

We include it because it is the first study in this field to ask that question directly, and because the honest reading is that both things are true: the cooling did more than the placebo, and the placebo did more than nothing. Worth noting that this was whole-hand immersion in water rather than a held object, on a single-joint exercise, with 14 men.

Kwon, Y. S., Robergs, R. A., Mermier, C. M., Schneider, S. M., and Gurney, A. B. (2015). Palm cooling and heating delays fatigue during resistance exercise in women. Journal of Strength and Conditioning Research, 29(8), 2261-2269.

Eight resistance-trained women did four sets of bench press to failure at 85% of one-rep max, with three-minute rests, on three separate days: palms cooled to 10 °C, palms heated to 45 °C, or nothing. Both cooling and heating raised the total volume lifted, cooling by about 17% over the thermoneutral day. The authors read it as a central effect: a strong signal from the palms changes how the brain paces fatigue. Small sample, one exercise, same group that published the 2010 study in men.

Wrabley, E. R., Lagerquist, B. L., Smith, G. A., Jones, R. A., McNeilly, M. M., Buxton, J. D., Gerhart, H. D., and Prins, P. J. (2025). The effects of palm cooling on repeat sprint ability following a fatigue inducing protocol in collegiate female athletes. International Journal of Exercise Science.

Twenty female collegiate athletes ran a modified Loughborough Intermittent Shuttle Test and then a 20-metre repeat sprint test, in a randomised crossover. During the rest intervals they held a portable palm-cooling device at 7 to 15 °C, or nothing. With cooling they completed significantly more sprints at 90% or more of their maximum velocity. Heart rate, blood lactate and perceived exertion did not differ between the conditions. A small handheld device, no vacuum, no water bath.

Bongers, C. C. W. G., et al. (2014). Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. British Journal of Sports Medicine, 49(6), 377-384.

Meta-analysis of 28 studies of men exercising in the heat (above 30°C), 20 cooling before exercise and 8 cooling during it. Both approaches improved performance by a similar amount, and among the cooling-during-exercise methods an ice vest was the most effective. These are whole-body or torso cooling methods applied before or throughout exercise, not palm cooling taken between efforts, so the results support the general case for cooling rather than KYLA’s specific method. The review also found no correlation between finishing core temperature and performance.

Drust, B., et al. (2000). Investigation of the effects of the pre-cooling on the physiological responses to soccer-specific intermittent exercise. European Journal of Applied Physiology, 81(1-2), 11-17.

Six male university soccer players completed a 90-minute intermittent treadmill protocol with and without a pre-cooling cold shower. The pre-cooling lowered rectal temperature before exercise, but the authors found no evidence of benefit to oxygen consumption, heart rate, perceived exertion, or blood markers under normal laboratory conditions. This is whole-body pre-cooling before intermittent exercise, not palm cooling between efforts. It is a useful counterpoint, showing that cooling is not automatically ergogenic and that the effect depends on the method and the heat load.

van de Kerkhof, T. M., et al. (2024). Performance benefits of pre- and per-cooling on self-paced versus constant workload exercise: a systematic review and meta-analysis. Sports Medicine, 54(2), 447-471.

This review pooled 59 studies (563 athletes) of men exercising in heat above 30°C, comparing cooling before exercise and during it across self-paced and fixed-intensity protocols. Cooling consistently reduced the performance decline, but the benefit was larger for constant-workload exercise and for pre-cooling specifically. As with the other entries here, these are whole-body and pre-exercise cooling methods rather than palm cooling between efforts. It is a current summary of how much the effect depends on context.

Arngrímsson, S. A., et al. (2004). Cooling vest worn during active warm-up improves 5-km run performance in the heat. Journal of Applied Physiology, 96(5), 1867-1874.

Seventeen competitive runners completed two simulated 5-km treadmill runs in hot, humid conditions (32°C, 50% humidity), warming up either in a T-shirt or in a vest filled with ice. The vest lowered body temperature, heart rate, and thermal discomfort during the warm-up, and the runners were about 13 seconds faster over 5 km, with most of the advantage appearing later in the run. This is torso cooling applied before the run, not palm cooling between efforts, and the sample is small, but it is a clean example of pre-cooling producing a measurable performance gain.

Duffield, R., & Marino, F. E. (2007). Effects of pre-cooling procedures on intermittent-sprint exercise performance in warm conditions. European Journal of Applied Physiology, 100(6), 727-735.

Nine male rugby players performed a 2 x 30-minute intermittent-sprint protocol in warm conditions (32°C) under three conditions: control, ice vest, and ice bath plus ice vest, with cooling applied before exercise and at half-time. Maximal sprint times did not differ between conditions. This is whole-body pre-cooling for team-sport exercise, not palm cooling between efforts.

Scheadler, C. M., Saunders, N. W., Hanson, N. J., & Devor, S. T. (2013). Palm cooling does not improve running performance. International Journal of Sports Medicine, 34(8), 732-735. PMID 23444094.

Twelve subjects, continuous treadmill running to exhaustion at 30°C. A different device, the BEX Runner, worn during the run rather than held in the rest between efforts. Time to exhaustion was shorter with cooling, 41.3 minutes against 46.7, and core temperature rose at the same rate either way.

Kenville, R., Clauß, M., Arup, A., Ragert, P., & Maudrich, T. (2024). No Effect of Intermittent Palm or Sole Cooling on Acute Training Volume during Resistance Exercise in Physically Active Adults: A Summary of Protocols. Sports, 12(10), 281. PMID 39453247.

The largest palm cooling study in resistance training. 41 physically active adults, three randomised crossover protocols, no external funding. Cooling ran the full three-minute rest between sets, at a temperature close to the one KYLA holds. Pull-ups fell from 38.2 repetitions to 32.2. Push-ups showed nothing either way. Leg extensions were tested with the soles cooled and showed nothing either way. Core temperature never moved across six sets, and the authors put the pull-up drop down to grip rather than temperature.