Spinal and corticospinal excitability in response to reductions in skin and core temperature via whole-body cooling

Cold stress impairs fine and gross motor movements. Although peripheral effects of muscle cooling on performance are well understood, less is known about central mechanisms. This study characterized corticospinal and spinal excitability during surface cooling, reducing skin (Tsk) and core (Tes) temperature. Ten subjects (3 female) wore a liquid-perfused suit and were cooled (9°C perfusate, 90 min) and rewarmed (41°C perfusate, 30 min). Transcranial magnetic stimulation [eliciting motor evoked potentials (MEPs)], as well as transmastoid [eliciting cervicomedullary evoked potentials (CMEPs)] and brachial plexus [eliciting maximal compound motor action potentials (Mmax)] electrical stimulation, were applied at baseline, every 20 min during cooling, and following rewarming. Sixty minutes of cooling, reduced Tsk by 9.6°C (P<0.001) but Tes remained unchanged (P=0.92). Tes then decreased ~0.6℃ in the next 30 minutes of cooling (P<0.001). Eight subjects shivered. During rewarming, shivering was abolished, and Tsk returned to baseline while Tes did not increase. During cooling and rewarming, Mmax, MEP, and MEP/Mmax were unchanged from baseline. However, CMEP and CMEP/Mmax increased during cooling by ~85% and 79% (P<0.001) respectively, and remained elevated post-rewarming. Results suggest that spinal excitability is facilitated by reduced Tsk during cooling, and reduced Tes during warming, while corticospinal excitability remains unchanged. ClinicalTrials.gov ID NCT04253730 Novelty: • This is the first study to characterize corticospinal, and spinal excitability during whole body cooling, and rewarming in humans. • Whole body cooling did not affect corticospinal excitability. • Spinal excitability was facilitated during reductions in both skin and core temperatures.

Effect of post-exercise muscle cooling on PGC-1α and VEGF mRNA expression in Thoroughbreds

Besides preventing exertional heat illness, muscle cooling can be a potential strategy to enhance exercise-training induced adaptations. This study aimed to examine the effects of post-exercise cooling on the mRNA expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and vascular endothelial growth factor (VEGF) in Thoroughbred skeletal muscle. Five Thoroughbred horses performed treadmill running until their pulmonary artery temperature reached 42 °C, followed by walking on the treadmill with no additional cooling (CONT) or muscle cooling with a shower using the tap water (26 °C, 0.4 l/s; COOL), for 30 min. Muscle biopsies were obtained before (PRE) and 3 h after exercise (3 Hr-REC) from the gluteus medius muscle. PGC-1α mRNA expression was elevated 3 h after exercise in both the CONT (PRE vs 3 Hr-REC: 1.0±0.1 vs 5.0±0.8, P<0.01) and COOL (PRE vs 3 Hr-REC: 1.1±0.3 vs 6.6±0.9, P<0.01) conditions; however, there was no difference between the two conditions at 3 h after exercise (P=0.17). VEGF mRNA expression was elevated 3 h after exercise in COOL (PRE vs 3 Hr-REC: 1.0±0.2 vs 2.2±0.2, P<0.05) but not in CONT (PRE vs 3 Hr-REC: 1.0±0.1 vs 1.8±0.3, P=0.08). VEGF mRNA expression at 3 h after exercise was significantly negatively correlated with rectal temperature at the end of the 30-min cooling period (r = -0.65, P<0.05). Our results suggest that the decline in body temperature after exercise may lead to greater expression of the key angiogenic gene in Thoroughbred horses.

Effects of Cryotherapy on Temperature Change in Caudal Thigh Muscles of Dogs

Objective The aim of the study reported here was to determine the effect of cryotherapy on the caudal thigh muscles of dogs. We hypothesized that temperature changes would be greatest in superficial tissues and decrease with tissue depth. Study Design Eight mixed-breed dogs (mean weight 21.2 kg, mean age 3.3 years) were studied. Temperature was measured at the skin surface and at depths of 1.0 and 3.0 cm below the skin using needle thermistor probes that were inserted beneath the site of cold pack application. Treatment consisted of a standard 1.0°C cold pack applied for 20 minutes. Temperature was recorded every minute for the 20 minute cold pack treatment, and for 80 minutes following treatment. Results Cutaneous temperatures significantly decreased (p < 0.01), with rapid rewarming of the skin following cold pack removal. Tissue cooling was less profound with increasing tissue depths, but was still significant (p < 0.05). There was no significant difference in muscle temperature between haired and clipped limbs. Conclusion A single application of a cold pack to the caudal thigh muscles of dogs for 20 minutes resulted in significant temperature reduction at all tissue depths (p < 0.05). This decrease persisted for ∼60 minutes. The presence of hair did not have a significant effect on muscle cooling (p > 0.05).

Effect of Local Cold Application During Exercise on Gene Expression Related to Mitochondrial Homeostasis

Exercise training increases mitochondrial content in active skeletal muscle. Previous work suggests that mitochondrial-related genes respond favorably to exercise in cold environments. However, the impact of localized tissue cooling is unknown. The purpose is to determine the impact of local muscle cooling during endurance exercise on human skeletal muscle mitochondrial-related gene expression. Twelve subjects (age 28±6 y) cycled at 65% Wpeak. One leg was cooled (C) for 30 minutes before and during exercise with a thermal wrap while the other leg was wrapped but not cooled, room temperature (RT). Muscle biopsies were taken from each VL before and 4 hours post-exercise for the analysis of gene expression. Muscle temperature was lower in C (29.2±0.7°C) than RT (34.1±0.3°C) after pre-cooling for 30 minutes before exercise (p<0.001) and remained lower after exercise in C (36.9±0.5) than RT (38.4±0.2, p<0.001). PGC-1α and NRF1 mRNA expression were lower in C (p=0.012 and p=0.045, respectively) than RT at 4-h post. There were no temperature related differences in other genes (p>0.05). These data suggest that local cooling has an inhibitory effect on exercise-induced PGC-1α and NRF1 expression in human skeletal muscle. Those considering using local cooling during exercise should consider other systemic cooling options. Novelty Bullets • Local cooling has an inhibitory effect on exercise-induced PGC-1α and NRF1 expression in human skeletal muscle. • Local cooling may lead to a less robust exercise stimulus compared to standard conditions.

Dorsiflexion But Not Plantarflexion Force Declines At Lower Foot Skin Temperatures

Background/Aims Muscle maximum voluntary force declines at skin temperature <20°C, attributed to cold muscle, however large muscle deep fibres remain at >20°C. Large muscle maximum voluntary force decline is comparable to that in small superficial muscle where muscle temperature remains close to skin temperature. Therefore, factors in addition to temperature may contribute to large muscle maximum voluntary force decline. This study compares the effects of foot and/or shank skin temperature on dorsiflexion and plantarflexion maximum voluntary force with the hypotheses that: dorsiflexion maximum voluntary force>plantarflexion maximum voluntary force decline at lower skin temperature; and, plantarflexion and dorsiflexion maximum voluntary force will decline at lower shank skin temperature independent of foot skin temperature. Methods A total of 24 adults (15 females, 9 males, 29 ± 11.8 years, 170.1 ± 8.0 cm, 66.9 ± 9.3 kg [mean ± standard deviation]) gave informed consent to participate on three visits – cooling/warming of: foot only; foot and shank; and shank only. Foot and/or shank temperature was adjusted using Cryocuff™ sleeves filled with ~45°C or ~4°C water. Temperature, taken before each maximum voluntary force set, was monitored through thermocouples placed on the limb. Plantarflexion and dorsiflexion maximum voluntary force were measured with a KinCom isokinetic dynamometer, with subjects seated on a plinth with knee fully extended and neutral ankle. A general mixed model was used to evaluate the effects of skin temperature on maximum voluntary force. Fixed effects were skin temperature and condition, with subject as a random effect. The skin temperature*condition interaction was also modelled. P-values were obtained by likelihood ratio tests. Results Foot skin temperature <18.5°C resulted in a 10% (χ2(1)=5.479, P=0.019) dorsiflexion maximum voluntary force decline, with a skin temperature*condition interaction (χ2(2)=11.031, P=0.004), this decline was 12% (χ2(1)=13.18, P=0.0003) in foot only and 8% (χ2(1)=4.675, P=0.031) in foot and shank. Leg skin temperature did not affect (χ2(1)=2.849, P=0.091) dorsiflexion maximum voluntary force. Plantarflexion maximum voluntary force did not change with foot skin temperature (χ2(1)=0.04, P=0.841) or leg skin temperature (χ2(1)=0.082, P=0.929). Conclusions Dorsiflexion maximum voluntary force declines at lower foot skin temperature independently of shank skin temperature, whereas plantarflexion maximum voluntary force is unaffected by skin temperature, possibly because in this protocol the shank was insufficiently cooled. Therefore factors other than direct muscle cooling must contribute to dorsiflexion maximum voluntary force decline. One theory is a rightward shift in the force-length relationship, due to stiffer tendon, could result in dorsiflexions operating in the descending limb of the force-length relationship. This warrants further investigation.

Repeatability and reliability of muscle relaxation properties induced by motor cortical stimulation

Impaired muscle relaxation is a feature of many neuromuscular disorders. However, few tests are available to quantify muscle relaxation. Transcranial magnetic stimulation (TMS) of the motor cortex can induce muscle relaxation by abruptly inhibiting corticospinal drive. The aim of our study was to investigate whether repeatability and reliability of TMS-induced relaxation are greater than voluntary relaxation. Furthermore, effects of sex, cooling, and fatigue on muscle relaxation properties were studied. Muscle relaxation of deep finger flexors was assessed in 25 healthy subjects (14 men and 11 women, age 39.1 ± 12.7 and 45.3 ± 8.7 yr, respectively) with handgrip dynamometry. All outcome measures showed greater repeatability and reliability in TMS-induced relaxation compared with voluntary relaxation. The within-subject coefficient of variability of normalized peak relaxation rate was lower in TMS-induced relaxation than in voluntary relaxation (3.0% vs. 19.7% in men and 6.1% vs. 14.3% in women). The repeatability coefficient was lower (1.3 vs. 6.1 s−1in men and 2.3 vs. 3.1 s−1in women) and the intraclass correlation coefficient was higher (0.95 vs. 0.53 in men and 0.78 vs. 0.69 in women) for TMS-induced relaxation compared with voluntary relaxation. TMS enabled demonstration of slowing effects of sex, muscle cooling, and muscle fatigue on relaxation properties that voluntary relaxation could not. In conclusion, repeatability and reliability of TMS-induced muscle relaxation were greater compared with voluntary muscle relaxation. TMS-induced muscle relaxation has the potential to be used in clinical practice for diagnostic purposes and therapy effect monitoring in patients with impaired muscle relaxation.NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS)-induced muscle relaxation demonstrates greater repeatability and reliability compared with voluntary relaxation, represented by the ability to demonstrate typical effects of sex, cooling, and fatigue on muscle relaxation properties that were not seen in voluntary relaxation. In clinical practice, TMS-induced muscle relaxation could be used for diagnostic purposes and therapy effect monitoring. Furthermore, fewer subjects will be needed for future studies when using TMS to demonstrate differences in muscle relaxation properties.

Examination of Intramuscular and Skin Temperature Decreases Produced by the PowerPlay Intermittent Compression Cryotherapy

Context: Previous research has found ice bags are more effective at lowering intramuscular temperature than gel packs. Recent studies have evaluated intramuscular temperature cooling decreases with ice bag versus Game Ready and with the PowerPlay system wetted ice bag inserts; however, intramuscular temperature decreases elicited by PowerPlay with the standard frozen gel pack inserts have not been examined. Objective: Evaluate the rate and magnitude of cooling using PowerPlay with frozen gel pack (PP-gel) option, PowerPlay with wetted ice bag (PP-ice) option, and control (no treatment) on skin and intramuscular temperature (2 cm subadipose). Design: Repeated-measures counterbalanced study. Setting: University research laboratory. Patients or Other Participants: Twelve healthy college-aged participants (4 men and 8 women; age = 23.08 (1.93) y, height = 171.66 (9.47) cm, mass = 73.67 (13.46) kg, and subcutaneous thickness = 0.90 (0.35) cm). Intervention(s): PowerPlay (70 mm Hg) with either wetted ice bag or frozen gel pack was applied to posterior aspect of nondominant calf for 30 minutes; control lay prone for 30 minutes. Participants underwent each treatment in counterbalanced order (minimum 4 d, maximum 10 d between). Main Outcome Measure(s): Muscle temperature was measured via 21-gauge catheter thermocouple (IT-21; Physitemp Instruments, Inc). Skin temperature was measured via surface thermocouple (SST-1; Physitemp Instruments, Inc). Results: Significant treatment-by-time interaction for muscle cooling (F10,80 = 11.262, P = .01, , observed β = 0.905) was observed. PP-ice cooled faster than both PP-gel and control from minutes 12 to 30 (all Ps < .05); PP-gel cooled faster than control from minutes 18 to 30 (all Ps < .05). Mean decreases from baseline: PP-ice = 4.8°C (2.8°C), PP-gel = 2.3°C (0.8°C), and control = 1.1°C (0.4°C). Significant treatment-by-time interaction for skin cooling (F10,80 = 23.920, P = .001, , observed β = 0.998) was observed. PP-ice cooled faster than both PP-gel and control from minutes 6 to 30 (all Ps < .05); PP-gel cooled faster than control from minutes 12 to 30 (all Ps < .05). Mean decreases from baseline: PP-ice = 14.6°C (4.8°C), PP-gel = 4.0°C (0.9°C), and control = 1.0°C (1.0°C). Conclusions: PP-ice produces clinically and statistically greater muscle and skin cooling compared with PP-gel and control.