Cold operational readiness in the military: from science to practice

Cold weather operations are logistically difficult to orchestrate and extremely challenging for soldiers. Decades of research and empirical evidence indicate that humans are extremely vulnerable to cold and that individual responses are highly variable. In this context, it may be necessary to develop personalised strategies to sustain soldiers' performance and ensure overall mission success in the cold. Systematic cold weather training is essential for soldiers to best prepare to operate during, and recover from, cold weather operations. The purpose of this review is to highlight key aspects of cold weather training, including (1) human responses to cold, (2) nutrition, (3) sleep and (4) protective equipment requirements. Bringing science to practice to improve training principles can facilitate soldiers performing safely and effectively in the cold. Cold weather training prepares soldiers for operations in cold, harsh environments. However, decreases in physical, psychological and thermoregulatory performance have been reported following such training, which influences operational ability and increases the overall risk of injuries. When optimising the planning of field training exercises or operational missions, it is important to understand the soldiers' physical and cognitive performance capacity, as well as their capacity to cope and recover during and after the exercise or mission. Even though the body is fully recovered in terms of body composition or hormonal concentrations, physical or cognitive performance can still be unrecovered. When overlooked, symptoms of overtraining and risk of injury may increase, decreasing operational readiness.

Influence of acute beetroot juice supplementation on cold-induced vasodilation and fingertip rewarming

PURPOSE

Vasoactive ingredients in beetroot (BR) such as nitrate are known to induce vasodilation in temperate conditions. This study investigated the effect of BR ingestion on cold induced vasodilation (CIVD) and rewarming of finger skin temperature (T_fing) during and after hand immersion in cold water.

METHODS

Twenty healthy males (mean ± SD; age 22.2 ± 0.7 years, height 172.6 ± 6.0 cm, body mass 61.3 ± 11.7 kg) repeated a hand cold water immersion test twice with prior BR or water beverage ingestion (randomised order). They rested for 2 h in thermoneutral conditions (27 °C, 40% relative humidity) after consuming the beverage, then immersed their non-dominant hand in 8 °C water for 30 min. They then rewarmed their hand in the ambient air for 20 min. Skin temperature at seven body sites, T_fing, finger skin blood flow (SkBF_fing), and blood pressure were measured.

RESULTS

During hand immersion parameters of CIVD (T_fing and SkBF_fing) were not different between BR and water conditions although skin temperature gradient from proximal to distal body sites was significantly smaller with BR (P < 0.05). During rewarming, SkBF_fing and cutaneous vascular conductance were significantly higher with BR than with water (P < 0.05). The rewarming speed in T_fing and SkBF_fing was significantly faster with BR at 15- (BR 1.24 ± 0.22 vs water 1.11 ± 0.26 °C/min) and 20-min rewarming (P < 0.05). Additionally, individuals with slower rewarming speed with water demonstrated accelerated rewarming with BR supplementation.

CONCLUSION

BR accelerated rewarming in T_fing and SkBF_fing after local cold stimulus, whereas, CIVD response during hand cold immersion was not affected by BR ingestion.

Effect of oral administration of GABA on thermoregulation in athletes during exercise in cold environments: A preliminary study

Background

γ-aminobutyric acid (GABA), a common ingredient in sports supplements and other health products, regulates body temperature in the preoptic area and anterior hypothalamus (PO/AH). To date, no study has examined the effect of GABA on thermoregulation during exercise in humans in a cold temperature environment (11 ± 0.3°C, 45% ± 2% relative humidity).

Methods

We performed a randomized, double-blind study. Ten trained male athletes consumed either a drink (3 ml/kg weight) containing GABA (1,000 mg, trial G) or an equivalent amount of placebo drink (trial C) before exercise. They rested for 20 min and then cycled at 60% of maximum output power for 40 min, pedaling at 60 rpm, and recovered for 20 min. Core temperature (T_c), skin temperature (upper arm, chest, thigh, calf), and heart rate (HR) were monitored at rest (T₀), exercise begins (T₂₀), 20 min of exercise (T₄₀), the exercise ends (T₆₀), and at recovery (T₈₀).

Results

Compared to T₀, T_c decreased significantly at T₂₀ and increased significantly at T₄₀, T₆₀ and T₈₀ (p < 0.01). From 35–80 min, the T_c was higher in trial G (peaked at 37.96 ± 0.25°C) than in trial C (37.89 ± 0.37°C), but it failed to reach significant difference (p > 0.05); T_sk continued to increase during exercise and was significantly higher than T₀ at T₄₀ (p < 0.05), T₆₀ and T₈₀ (p < 0.01). There was no significant difference in T_sk between the two trials (p > 0.05).

Conclusion

Our findings provide initial evidence that oral administration of GABA does not affect thermoregulation and has no adverse effects on the body as an ergogenic exercise supplement during exercise in cold environments.

Direct Effect of Local Cryotherapy on Muscle Stimulation, Pain and Strength in Male Office Workers with Lateral Epicondylitis, Non-Randomized Clinical Trial Study

Background: Local cryotherapy (LC) is one of the physiotherapeutic methods used in the conservative treatment of lateral epicondylitis (LE). The aim of the study was to verify the direct effect of a single LC procedure on the clinical symptoms of lateral epicondylitis enthesopathy (pain, pain free grip, PFG) and its effect on the bioelectrical properties of the wrist extensor muscles at rest, on maximal contraction and isometric contraction during fatigue. Methods: The study group was 28 men (35.4 ± 6.13 years) with confirmed unilateral epicondylitis. The performed procedures included the assessment of pain (visual analogue scale, VAS), PFG and ARMS (root-mean-square amplitude) and mean frequencies (MNF) of the sEMG signal before (T0) and after (T1) LC on the side with enthesopathy (ECRE) and without enthesopathy (ECRN/E). Results: There was an increase in the ARMS values of the signals recorded during rest and MVC from the ECR muscles both with and without enthesopathy (p = 0.0001, p = 0.006), an increased PFG after LC only on the side with LE (p < 0.0001) and decreased pain (p < 0.0001). During isometric fatigue contraction, a higher ARMS on both the ECRE side (p < 0.0001) and the ECRN/E side (p < 0.0001) was observed after LC treatment, and a lower MNF was observed on both the ECRN/E side (p < 0.0001) and the ECRE side (p < 0.0001) after LC. Conclusions: LC reduces the pain and increases PFG and muscle excitation expressed by ARMS and seems to delay muscle fatigue.

Practicing Sport in Cold Environments: Practical Recommendations to Improve Sport Performance and Reduce Negative Health Outcomes

Although not a barrier to perform sport, cold weather environments (low ambient temperature, high wind speeds, and increased precipitation, i.e., rain/water/snow) may influence sport performance. Despite the obvious requirement for practical recommendations and guidelines to better facilitate training and competition in such cold environments, the current scientific evidence-base is lacking. Nonetheless, this review summarizes the current available knowledge specifically related to the physiological impact of cold exposure, in an attempt to provide practitioners and coaches alike with practical recommendations to minimize any potential negative performance effects, mitigate health issues, and best optimize athlete preparation across various sporting disciplines. Herein, the review is split into sections which explore some of the key physiological effects of cold exposure on performance (i.e., endurance exercise capacity and explosive athletic power), potential health issues (short-term and long-term), and what is currently known with regard to best preparation or mitigation strategies considered to negate the potential negative effects of cold on performance. Specific focus is given to "winter" sports that are usually completed in cold environments and practical recommendations for physical preparation.

The effect of mild whole-body cold stress on isometric force control during hand grip and key pinch tasks

Prolonged exposure to cold can impair manual performance, which in turn can affect work task performance. We investigated whether mild whole-body cold stress would affect isometric force control during submaximal hand grip and key pinch tasks. Twelve male participants performed isometric hand grip and key pinch tasks at 10% and 30% of maximal voluntary contraction (MVC) for 30 and 10 s respectively, in cold (8 °C) and control (25 °C) conditions. Finger temperature decreased significantly by 18.7 ± 2.1 °C and continuous low-intensity shivering in the upper trunk increased significantly in intensity and duration during cold exposure. Rectal temperature decreased similarly for the 8 °C and 25 °C exposures. Force variability (FCv) was <2% for the hand grip tasks, and <3% for the key pinch tasks. No significant changes in FCv or force accuracy were found between the ambient temperatures. In conclusion, isometric force control during hand grip and key pinch tasks was maintained when participants experienced mild whole-body cold stress compared with when they were thermally comfortable.

Exercise Reverses the Alterations in Gut Microbiota Upon Cold Exposure and Promotes Cold-Induced Weight Loss

Gut microbiota has been reported to contribute to reduced diet-induced obesity upon cold exposure. Furthermore, gut microbiome fermentation determines the efficacy of exercise for diabetes prevention and enhances exercise performance. However, there have been no systematic examinations of changes in gut microbiome composition in relation to exercise performed under low-temperature conditions. In this study, we investigated the effects of exercise performed under different conditions (room temperature, acute cold, intermittent cold, and sustained cold) in obese rats maintained on a high-fat diet at four time points during experimental trials (days 0, 1, 3, and 35), including observations on white fat browning, weight loss, cardiovascular effects, and changes in gut microbiota among treatment groups. We found that exercise under sustained cold conditions produced a remarkable shift in microbiota composition. Unexpectedly, exercise was found to reverse the alterations in gut microbiota alpha-diversity and the abundance of certain bacterial phyla observed in response to cold exposure (e.g., Proteobacteria decreased upon cold exposure but increased in response to exercise under cold conditions). Moreover, exercise under cold conditions (hereafter referred to “cold exercise”) promoted a considerably higher level of white fat browning and greater weight loss and protected against the negative cardiovascular effects of cold exposure. Correlation analysis revealed that cold exercise-related changes in gut microbial communities were significantly correlated with white fat browning and cardiovascular phenotypes. These results could reveal novel mechanisms whereby additional health benefits attributable to both cold and exercise are mediated via altered gut microbes differently compared with either of them alone.

Productivity and physiological responses during exposure to varying air temperatures and clothing conditions

This study assessed the effects of clothing and air temperature combinations on workplace productivity and physiological response. Ten male Japanese subjects were exposed to six combinations of clothing (0.3 clo and 0.9 clo) and air temperature (16°C, 26°C, and 36°C) during which cognitive performance (Bourdon and calculation tests), manual motor performance (finger-tapping test), and physiological responses (heart rate, blood pressure, and skin and oral temperatures) were measured. Both cold exposure and lower clothing levels likely increased the Bourdon test performance. Calculation test performance tended to be affected by exposure to cold or neutral temperatures at the beginning of the test. Cold exposure undermined manual motor performance (especially when combined with fewer clothing items) while heat exposure significantly increased heart rate. Both cold exposure and higher clothing level during heat exposure increased blood pressure. Body temperature, particularly mean skin temperature, increased with higher air temperature and was significantly influenced by clothing insulation during cold exposure. These results provide novel evidence for the effects of clothing and air temperature (particularly cold) on human productivity and physiological responses in humans.

Effects of muscle cooling on kinetics of pulmonary oxygen uptake and muscle deoxygenation at the onset of exercise

This study investigated effects of skeletal muscle cooling on the metabolic response and kinetics of pulmonary oxygen uptake (V˙O₂) and skeletal muscle deoxygenation during submaximal exercise. In the cooling condition (C), after immersion of the lower body into 12°C water for 30 min, eight healthy males performed 30‐min cycling exercise at the lactate threshold while undergoing thigh cooling by a water‐circulating pad. In the normal condition (N) as control, they conducted the same exercise protocol without cooling. Blood lactate concentration was significantly higher in C than N at 10 min after onset of exercise (4.0 ± 1.7 and 2.4 ± 1.2 mmol/L in C and N, P < 0.05). The percent change in the tissue oxygen saturation of the vastus lateralis, measured by a near‐infrared spectroscopy, was significantly lower in C at 2, 8, 10, and 20 min after the exercise onset compared with N (P < 0.05). The percent change in deoxy hemoglobin+myoglobin concentration (Deoxy[Hb+Mb]) showed a transient peak at the onset of exercise and significantly higher value in C at 10, 20, and 30 min after the exercise onset (P < 0.05). Compared to N, slower V˙O₂ kinetics (mean response time) was observed in C (45.6 ± 7.8 and 36.1 ± 7.7 sec in C and N, P < 0.05). The mean response time in C relative to N was significantly correlated with the transient peak of Deoxy[Hb+Mb] in C (r = 0.84, P < 0.05). These results suggest that lower oxygen delivery to the hypothermic skeletal muscle might induce greater glycolytic metabolism during exercise and slower V˙O₂ kinetics at the onset of exercise.

Effect of Cold-Water Immersion on Handgrip Performance in Rock Climbers

PURPOSE

To determine the effect of 2 cold-water-immersion (CWI) temperatures (15°C and 8°C) on repeat handgrip performance to failure.

METHODS

A total of 32 participants completed 3 intermittent trials to failure on a climbing-specific handgrip dynamometer on 3 laboratory visits. For each visit, a different recovery strategy was employed: passive (PAS) recovery, CWI at 8°C (CW8), or CWI at 15°C (CW15). The force time integral (FTI: time of contraction multiplied by the force of contraction) was determined to assess handgrip performance.

RESULTS

There was no significant difference between recovery strategies at the end of trial 1. In response to the PAS recovery strategy, there were 10% and 22% decreases in FTI in the second and third trials, respectively. The PAS recovery-strategy FTI values were lower than both CWI strategies for trials 2 and 3 (P < .05). FTI increased in the second trial (↑32% and ↑38%; P < .05) for both immersion strategies (CW8 and CW15, respectively) compared with trial 1. During the third trial, FTI was significantly higher for CW15 than CW8 (↑27% and ↓4% with respect to baseline trial; P < .05).

CONCLUSIONS

The results suggest that CWI has potential performance advantages over PAS recovery for rock climbing. The data show that in events where multiple recoveries are required, 15°C CWI may be more beneficial for climbers than 8°C CWI. Future research should focus on the optimization of protocols for sport performance.

Localized and systemic variations in central motor drive at different local skin and muscle temperatures

This study investigated the ability to sustain quadriceps central motor drive while subjected to localized heat and metaboreceptive feedback from the contralateral leg. Eight active males each completed two counter-balanced trials, in which muscle temperature (T_m) of a single-leg (TEMP-LEG) was altered to 29.4°C (COOL) or 37.6°C (WARM), while the contralateral leg (CL-LEG) remained thermoneutral: 35.3°C and 35.2°C T_m in COOL and WARM, respectively. To activate metaboreceptive feedback, participants first performed one 120-s isometric maximal voluntary contraction (MVC) of the knee extensors in the TEMP-LEG, immediately followed by postexercise muscle ischemia (PEMI) via femoral blood flow occlusion. To assess central motor drive of a remote muscle group immediately following PEMI, another 120-s MVC was subsequently performed in the CL-LEG. Voluntary muscle activation (VA) was assessed using the twitch interpolation method. Perceived mental effort and limb discomfort were also recorded. In a cooled muscle, a significant increase in mean force output and mean VA (force, P < 0.001; VA, P < 0.05), as well as a significant decrease in limb discomfort (P < 0.05) occurred during the sustained MVC in the TEMP-LEG. However, no differences between T_m were observed in mean force output, mean VA, or limb discomfort during the sustained MVC in the CL-LEG (force, P = 0.33; VA, P > 0.68; and limb discomfort, P = 0.73). The present findings suggest that elevated local skin temperature and T_m can increase limb discomfort and decrease central motor drive, but this does not limit systemic motor activation of a thermoneutral muscle group.

Effect of repeated forearm muscle cooling on the adaptation of skeletal muscle metabolism in humans

This study aimed to investigate the effect of repeated cooling of forearm muscle on adaptation in skeletal muscle metabolism. It is hypothesized that repeated decreases of muscle temperature would increase the oxygen consumption in hypothermic skeletal muscle. Sixteen healthy males participated in this study. Their right forearm muscles were locally cooled to 25 °C by cooling pads attached to the skin. This local cooling was repeated eight times on separate days for eight participants (experimental group), whereas eight controls received no cold exposure. To evaluate adaptation in skeletal muscle metabolism, a local cooling test was conducted before and after the repeated cooling period. Change in oxy-hemoglobin content in the flexor digitorum at rest and during a 25-s isometric handgrip (10% maximal voluntary construction) was measured using near-infrared spectroscopy at every 2 °C reduction in forearm muscle temperature. The arterial blood flow was occluded for 15 s by upper arm cuff inflation at rest and during the isometric handgrip. The oxygen consumption in the flexor digitorum muscle was evaluated by a slope of the oxy-hemoglobin change during the arterial occlusion. In the experimental group, resting oxygen consumption in skeletal muscle did not show any difference between pre- and post-intervention, whereas muscle oxygen consumption during the isometric handgrip was significantly higher in post-intervention than in pre-test from thermoneutral baseline to 31 °C muscle temperature (P < 0.05). This result indicated that repeated local muscle cooling might facilitate oxidative metabolism in the skeletal muscle. In summary, skeletal muscle metabolism during submaximal isometric handgrip was facilitated after repeated local muscle cooling.

Neuromuscular function during knee extension exercise after cold water immersion

Background

Human adaptability to cold environment has been focused on in the physiological anthropology and related research area. Concerning the human acclimatization process in the natural climate, it is necessary to conduct a research assessing comprehensive effect of cold environment and physical activities in cold. This study investigated the effect of cold water immersion on the exercise performance and neuromuscular function during maximal and submaximal isometric knee extension.

Methods

Nine healthy males participated in this study. They performed maximal and submaximal (20, 40, and 60% maximal load) isometric knee extension pre- and post-immersion in 23, 26, and 34 °C water. The muscle activity of the rectus femoris (RF) and vastus lateralis (VL) was measured using surface electromyography (EMG). The percentages of the maximum voluntary contraction (%MVC) and mean power frequency (MPF) of EMG data were analyzed.

Results

The post-immersion maximal force was significantly lower in 23 °C than in 26 and 34 °C conditions (P < 0.05). The post-immersion %MVC of RF was significantly higher than pre-immersion during 60% maximal exercise in 23 and 26 °C conditions (P < 0.05). In the VL, the post-immersion %MVC was significantly higher than pre-immersion in 23 and 26 °C conditions during 20% maximal exercise and in 26 °C at 40 and 60% maximal intensities (P < 0.05). The post-immersion %MVC of VL was significantly higher in 26 °C than in 34 °C at 20 and 60% maximal load (P < 0.05). The post-immersion MPF of RF during 20% maximal intensity was significantly lower in 23 °C than in 26 and 34 °C conditions (P < 0.05), and significantly different between three water temperature conditions at 40 and 60% maximal intensities (P < 0.05). The post-immersion MPF of VL during three submaximal trials were significantly lower in 23 and 26 °C than in 34 °C conditions (P < 0.05).

Conclusions

The lower shift of EMG frequency would be connected with the decrease in the nerve and muscle fibers conduction velocity. To compensate for the impairment of each muscle fibers function, more muscle fibers might be recruited to maintain the working load. This might result in the greater amplitude of EMG after the cold immersion.

The effect of cold ambient temperatures on climbing-specific finger flexor performance

Different ambient temperatures are known to affect muscular performance based on the type of contraction. The effect of cold (10°C) and thermoneutral (TN) (24°C) ambient temperatures on finger flexor performance was examined in 12 rock climbers. After 30 min of seated rest in the designated temperature condition, participants completed maximal voluntary contractions (MVC) on a climbing-specific finger flexor assessment device equipped with a crimp grip hold. Participants then completed an intermittent fatiguing task until failure. The fatiguing task consisted of 10-s contractions at 40% MVC followed by a 3-s of rest. MVC recovery was assessed immediately, 5, 10, and 15 min post-task failure. Estimated muscle temperature and subjective thermal ratings were significantly lower throughout testing in the cold condition (P < .001). Finger flexor MVC strength was similar between conditions at baseline and throughout recovery. Time to task failure was significantly longer (364 ± 135 vs. 251 ± 97 s, P = .003) and force time integral was greater (53,715 ± 19,988 vs. 40,243 ± 15,360 Ns, P = .001) during the cold condition. No significant differences were found between conditions for force variability or electromyography (EMG) at the start and end of the fatiguing task. However, the rate of increase in EMG for the TN condition was significantly faster (P = .03). These results suggest important implications for researchers when examining climbing performance, especially in outdoor settings where temperatures may vary from day to day. Inconsistencies in testing temperatures might significantly affect muscular endurance.

Interaction between environmental temperature and hypoxia on central and peripheral fatigue during high-intensity dynamic knee extension

This study investigated causative factors behind the expression of different interaction types during exposure to multistressor environments. Neuromuscular fatigue rates and time to exhaustion (TTE) were investigated in active men (n = 9) exposed to three climates [5 °C, 50% relative humidity (rh); 23 °C, 50% rh; and 42 °C, 70% rh] at two inspired oxygen fractions (0.209 and 0.125 FiO2; equivalent attitude = 4,100 m). After a 40-min rest in the three climatic conditions, participants performed constant-workload (high intensity) knee extension exercise until exhaustion, with brief assessments of neuromuscular function every 110 s. Independent exposure to cold, heat, and hypoxia significantly (P < 0.01) reduced TTE from thermoneutral normoxia (reductions of 190, 405, and 505 s from 915 s, respectively). The TTE decrease was consistent with a faster rate of peripheral fatigue development (P < 0.01) compared with thermoneutral normoxia (increase of 1.6, 3.1, and 4.9%/min from 4.1%/min, respectively). Combined exposure to hypoxic-cold resulted in an even greater TTE reduction (-589 s), likely due to an increase in the rate of peripheral fatigue development (increased by 7.6%/min), but this was without significant interaction between stressors (P > 0.198). In contrast, combined exposure to hypoxic heat reduced TTE by 609 s, showing a significant antagonistic interaction (P = 0.003) similarly supported by an increased rate of peripheral fatigue development (which increased by 8.3%/min). A small decline (<0.4%/min) in voluntary muscle activation was observed only in thermoneutral normoxia. In conclusion, interaction type is influenced by the impact magnitude of the effect of the individual stressors' effect on exercise capacity, whereby the greater the effect of stressors, the greater the probability that one stressor will be abolished by the other. This indicates that humans respond to severe and simultaneous physiological strains on the basis of a worst-strain-takes-precedence principle.