Faster early rate of force development in warmer muscle: an in vivo exploration of fascicle dynamics and muscle-tendon mechanical properties.

Although heat exposure has been shown to increase the skeletal rate of force development (RFD), the underlying processes remain unknown. This study investigated the effect of heat on gastrocnemius medialis (GM) muscle-tendon properties and interactions. Sixteen subjects performed electrically evoked and voluntary contractions combined with ultrafast ultrasound under thermoneutral [control (CON): 25.8 ± 1.8°C, core temperature 37.0 ± 0.3°C, muscle temperature 34.0 ± 1.1°C] and passive heat exposure [hot (HOT): 47.4 ± 1.8°C, core temperature 38.4 ± 0.3°C, muscle temperature 37.0 ± 0.8°C] conditions. Maximal voluntary force changes did not reach statistical significance (-5.0 ± 11.3%, P = 0.052) whereas voluntary activation significantly decreased (-4.6 ± 8.7%, P = 0.038) in HOT. Heat exposure significantly increased voluntary RFD before 100 ms from contraction onset (+48.2 ± 62.7%; P = 0.013), without further changes after 100 ms. GM fascicle dynamics during electrically evoked and voluntary contractions remained unchanged between conditions. Joint velocity at a given force was higher in HOT (+7.1 ± 6.6%; P = 0.004) but the fascicle force-velocity relationship remained unchanged. Passive muscle stiffness and active tendon stiffness were lower in HOT than CON (P ≤ 0.030). This study showed that heat-induced increases in early voluntary RFD may not be attributed to changes in contractile properties. Late voluntary RFD was unaltered, possibly due to decreased soft tissues' stiffness in heat. Further investigations are required to explore the influence of neural drive and motor unit recruitment in the enhancement of explosive strength elicited by heat exposure.

Association between thermal responses, medical events, performance, heat acclimation and health status in male and female elite athletes during the 2019 Doha World Athletics Championships.

PURPOSE: To determine associations between thermal responses, medical events, performance, heat acclimation and health status during a World Athletics Championships in hot-humid conditions. METHODS: From 305 marathon and race-walk starters, 83 completed a preparticipation questionnaire on health and acclimation. Core (Tcore; ingestible pill) and skin (Tskin; thermal camera) temperatures were measured in-competition in 56 and 107 athletes, respectively. 70 in-race medical events were analysed retrospectively. Performance (% personal best) and did not finish (DNF) were extracted from official results. RESULTS: Peak Tcore during competition reached 39.6°C±0.6°C (maximum 41.1°C). Tskin decreased from 32.2°C±1.3°C to 31.0°C±1.4°C during the races (p<0.001). Tcore was not related to DNF (25% of starters) or medical events (p≥0.150), whereas Tskin, Tskin rate of decrease and Tcore-to-Tskin gradient were (p≤0.029). A third of the athletes reported symptoms in the 10 days preceding the event, mainly insomnia, diarrhoea and stomach pain, with diarrhoea (9% of athletes) increasing the risk of in-race medical events (71% vs 17%, p<0.001). Athletes (63%) who performed 5-30 days heat acclimation before the competition: ranked better (18±13 vs 28±13, p=0.009), displayed a lower peak Tcore (39.4°C±0.4°C vs 39.8°C±0.7°C, p=0.044) and larger in-race decrease in Tskin (-1.4°C±1.0°C vs -0.9°C±1.2°C, p=0.060), than non-acclimated athletes. Although not significant, they also showed lower DNF (19% vs 30%, p=0.273) and medical events (19% vs 32%, p=0.179). CONCLUSION: Tskin, Tskin rate of decrease and Tcore-to-Tskin gradient were important indicators of heat tolerance. While heat-acclimated athletes ranked better, recent diarrhoea represented a significant risk factor for DNF and in-race medical events.

Angle specific analysis of hamstrings and quadriceps isokinetic torque identify residual deficits in soccer players following ACL reconstruction: a longitudinal investigation.

 Analysing the isokinetic curve is important following ACL reconstruction as there may be deficits in torque production at specific points throughout the range of motion. We examined isokinetic (60°.s-1) torque-angle characteristics in 27 male soccer players (24.5 ± 3.9 years) at 3 time-points (17 ± 5; 25 ± 6; and 34 ± 7 weeks post-surgery). Extracted data included knee flexor and extensor peak torque, conventional hamstrings: quadriceps (H/Q) ratio, and angle-specific torque using Statistical Parametric Mapping (SPM). There were significant involved limb increases in extension peak torque at each assessment (p<0.001); however, asymmetry favouring the un-involved limb was observed (p<0.01). Flexion peak torque was greater on the un-involved limb at assessments 1 and 2 only (p<0.05). The angle of peak torque was not significantly different at individual time-points or within-limbs across the 3 assessments. SPM revealed involved limb angle-specific reductions in extension torque across the full range of motion at time-points 1 and 2 (p<0.001) but only in angles [51-80°] at assessment 3 (p<0.05). Between limb H/Q angle differences [33-45°] were shown at assessments 2 and 3. The ratio ranged from 1.60-0.74 depending on the angle tested. Angle-specific moment curves are useful for monitoring patterns of strength development during rehabilitation.

Six weeks of localized heat therapy does not affect muscle mass, strength and contractile properties in healthy active humans.

PURPOSE: Animal and human studies have shown that repeated heating may induce skeletal muscle adaptations, increasing muscle strength. The aim of this study is to investigate the effect of 6 weeks of localized heating on skeletal muscle strength, volume and contractile properties in healthy humans. METHODS: Fifteen active participants (8 males/7 females, 35 ± 6 years, 70 ± 14 kg, 173 ± 7 cm, average training of 87 min per week) were subjected to 6 weeks of single-leg heat therapy. Heat pads were applied for 8 h/day, 5 days/week, on one randomly selected calf of each participant, while the contralateral leg acted as control. The heat pads increased muscle temperature by 4.6 ± 1.2 °C (p < 0.001). Every 2 weeks, participants were tested for morphological (MRI), architectural (ultrasound), contractile (electrically evoked twitch), and force (isometric and isokinetic) adaptations. RESULTS: Repeated localized heating did not affect the cross-sectional area (p = 0.873) or pennation angle (p = 0.345) of the gastrocnemius muscles; did not change the evoked peak twitch amplitude (p = 0.574) or rate of torque development (p = 0.770) of the plantar flexors; and did not change maximal voluntary isometric (p = 0.214) or isokinetic (p = 0.973) plantar flexor torque. CONCLUSION: Whereas previous studies have observed improved skeletal muscle function following whole-body and localized heating in active and immobilized humans, respectively, the current data suggested that localized heating may not be a potent stimulus for muscle adaptations in active humans.

Epidemiology and risk factors for heat illness: 11 years of Heat Stress Monitoring Programme data from the FIVB Beach Volleyball World Tour.

OBJECTIVES: To analyse 11 years of FIVB heat stress-monitoring data to determine the relative influence of the different environmental parameters in increasing the likelihood of a heat-related medical time-out (MTOheat). METHODS: A total of 8530 matches were recorded. The referee measured air temperature, black globe temperature, relative humidity and wet-bulb globe temperature (WBGT) before the matches, and registered the MTOheat. The absolute humidity was computed at posteriori. RESULTS: There were 20 MTOheat cases, but only 3 resulted in forfeiting the match. MTOheat incidence was not statistically impacted by sex (p=0.59). MTOheat cases were more prevalent during the games played in Asia during the 4th quarter of the year (p<0.001). Two cases of MTOheat experienced diarrhoea or gastroenteritis during the 5 preceding days; both of them forfeited the match. A principal component analysis showed a specific environmental profile for the matches with MTOheat. They occurred at higher WBGT, temperatures and absolute humidity (p<0.001), but with a lower relative humidity (p=0.027). CONCLUSIONS: The current data showed that an increase in ambient or black globe temperature, but not relative humidity, increased the risk of a MTOheat; but that the absolute risk remained low in elite beach volleyball players. However, suffering or recovering from a recent illness may represent a risk factor for a MTOheat to lead to player forfeit.

Recording injuries only during winter competitive season underestimates injury incidence in elite alpine skiers.

Current epidemiological studies in elite alpine skiers are mostly limited to retrospective surveys on in-season injury. The aim of this study was to determine the risk and pattern of injury in elite alpine skiers during the winter competitive season (WCS) and the summer off-season (SOS). European Cup skiers were prospectively followed during 5 complete years. A total of 133 skier-seasons (79 males and 54 females) completed the study. All acute and overuse injuries that required medical attention were registered, representing a total of 166 injuries. Absolute injury incidence was 124.8 (95% CI [106-145]) injuries per 100 athletes per complete season and was higher during the WCS compared with the SOS, albeit in a smaller magnitude than expected (relative risk ratio (RR) 1.44 [1.06-1.96]). The absolute incidence of severe injuries (ie, time-loss >28 days) was twofold higher during WCS compared with SOS (RR 2.19 [1.21-3.95]). Most common injuries during the 2013-2015 seasons were knee followed by back (absolute incidences 56 [38-80] and 20 [10-36], respectively; RR 2.82 [1.42-5.61]), but this difference disappeared after 2015 (absolute incidences 25.6 [15.7-39.6] and 23.1 [13.7-36.5], respectively; RR 1.11 [0.59-2.10]). In summary, we found that 98 (59%) injuries occurred during WCS and 68 (41%) during SOS over 5 years. These results demonstrate the necessity to record injuries during the entire year to avoid a significant underestimation of the injury incidence. In addition, the current data showed an evolution of injury pattern over time and since previous reports, with back injuries being the main concern along knee joint injuries.

Discipline and Sex Differences in Angle-specific Isokinetic Analysis in Elite Skiers.

Standard outcomes of traditional isokinetic testing do not detect differences between various muscle mechanical properties. This study i) explored a novel analysis throughout the range of motion based on statistical parametric mapping and ii) examined the impact of sex and discipline on hamstrings/quadriceps torque in elite alpine skiers. Twenty-eight national team skiers (14 females, 14 males; 14 technical, 14 speed) undertook an isokinetic evaluation of the knee flexors/extensors (range 30-90°, 0° representing full extension). There was no effect of sex (p=0.864, d=0.03) and discipline (p=0.360, d=0.17) on maximal hamstrings-to-quadriceps ratio and no effect of discipline on maximal torque (p>0.156, d≤0.25). Hamstrings torque and hamstrings-to-quadriceps ratio were lower in females than males toward knee extension only (p<0.05). Quadriceps torque was greater after 72° of knee flexion in technicians than downhill skiers (p<0.05). The current data showed that statistical parametric mapping analysis identified angle-specific differences that could not be evidenced when analyzing only maximal torques and reconstructed ratios. This may enhance screening methods to identify pathologic knee function or monitor rehabilitation programs, and inform sex- and discipline-specific training in alpine skiing.

Thermoregulatory responses during road races in hot-humid conditions at the 2019 Athletics World Championships.

The purpose of this study was to characterize thermoregulatory and performance responses of elite road-race athletes, while competing in hot, humid, night-time conditions during the 2019 IAAF World Athletic Championships. Male and female athletes, competing in the 20 km racewalk (n = 20 males, 24 females), 50 km racewalk (n = 19 males, 8 females), and marathon (n = 15 males, 22 females) participated. Exposed mean skin (Tsk) and continuous core body (Tc) temperature were recorded with infrared thermography and ingestible telemetry pill, respectively. The range of ambient conditions (recorded roadside) was 29.3°C-32.7°C air temperature, 46%-81% relative humidity, 0.1-1.7 m·s-1 air velocity, and 23.5°C-30.6°C wet bulb globe temperature. Tc increased by 1.5 ± 0.1°C but mean Tsk decreased by 1.5 ± 0.4°C over the duration of the races. Tsk and Tc changed most rapidly at the start of the races and then plateaued, with Tc showing a rapid increase again at the end, in a pattern mirroring pacing. Performance times were between 3% and 20% (mean = 113 ± 6%) longer during the championships compared with the personal best (PB) of athletes. Overall mean performance relative to PB was correlated with the wet-bulb globe temperature (WBGT) of each race (R2 = 0.89), but not with thermophysiological variables (R2 ≤ 0.3). As previously reported in exercise heat stress, in this field study Tc rose with exercise duration, whereas Tsk showed a decline. The latter contradicts the commonly recorded rise and plateau in laboratory studies at similar ambient temperatures but without realistic air movement.NEW & NOTEWORTHY This paper provides a kinetic observation of both core and skin temperatures in 108 elite athletes, during various outdoor competition events, adding to the very limited data so far available in the literature taken during elite competitions. The field skin temperature findings contrast previous laboratory findings, likely due to differences in relative air velocity and its impact on the evaporation of sweat. The rapid rise in skin temperature following cessation of exercise highlights the importance of infrared thermography measurements being taken during motion, not during breaks, when being used as a measurement of skin temperature during exercise.

Repeated practice runs during on-snow training do not generate any measurable neuromuscular alterations in elite alpine skiers.

Background: Alpine skiers typically train using repeated practice runs requiring high bursts of muscle activity but there is little field-based evidence characterizing neuromuscular function across successive runs. Purpose: To examine the impact of repeated ski runs on electromyographic activity (EMG) of the knee extensors and flexors in elite alpine skiers. Methods: Nineteen national team alpine skiers were tested during regular ski training [Slalom (SL), Giant Slalom (GS), Super Giant Slalom and Downhill (Speed)] for a total of 39 training sessions. The surface EMG of the vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM), biceps femoris (BF) and semimembranosus/semitendinosus (SMST) muscles was continuously recorded along with right knee and hip angles. The EMG root mean square signal was normalized to a maximal voluntary contraction (%MVC). The first and fourth runs of the training session were compared. Results: There was no meaningful main effect of run on EMG relative activation time or mean power frequency beyond the skier's intrinsic variability. However, EMG activity of the vastii increased from the first to the fourth run in SL [VM, ~+3%MVC for IL and outside leg (OL), p = 0.035)], speed (VL, IL:+6%/OL:+11%, p = 0.015), and GS (VM, IL:0/OL:+7%, p < 0.001); the later with an interaction with leg (p < 0.001) due to a localized increase on the OL. The run time and turn time did not change from the first to the fourth run. There were no meaningful changes in angular velocities, amplitude of movement, or maximal and minimal angles. Conclusion: Neuromuscular activity remains highly stable in elite skiers with low variability across four runs.

Muscle-tendon unit length changes in knee extensors and flexors during alpine skiing.

This study characterised the thigh muscle-tendon unit length variations (MTUv) in elite alpine skiing. MTUv were modelled for seven muscles from knee and hip angles in 17 national team skiers during 1602 cycles (546 Slalom (SL), 908 Giant-Slalom (GS), 136 Super-Giant (SG) and 12 Downhill (DH) cycles). The biarticular rectus femoris showed a dynamic pattern on both legs, especially in SL. On the other side, vastii displayed a stable length on the inside leg in all disciplines (37-46% of the cycle), contrasting with their dynamic behaviour on the outside leg being quasi-static for only 3% (SL) to 27% (DH) of the cycle. SL showed the largest amplitude of MTUv followed by GS, SG and DH. For vastus lateralis, MTUv was ~60%Lr.s-1 in SL but only ~12%Lr.s-1 in DH. In SL, a fine desynchronisation between both joints led to nearly constant MTUv (slow lengthening lasting ~45-51% of the cycle) for the biarticular hamstrings separated by faster variations during turn switch from outside to inside leg. In summary, biarticular MTUv were not characterised by single-joint behaviours, suggesting that extrapolating contraction regimen from knee joint only is not accurate in alpine skiing.

Thermoregulation and shivering responses in elite alpine skiers.

This study aimed to characterise the thermoregulatory responses of elite alpine skiers. Core (gastro-intestinal) and skin temperatures were recorded continuously from 4 national team skiers representing 18 training-sessions for a total of 83 ski runs (average ambient temperature 1.9°C). Shivering responses were estimated from quadriceps and hamstrings EMG recordings during 77 chairlifts. Temperature responses were also obtained from 6 accompanying staffs representing 16 training-sessions (passive in 10 cases, following the skiers in 6 cases). Core temperature of the skiers (P < .001) but not the staff (P = .103) increased during training and was higher in skiers than staff after one-third of the training session (P < .006). Conversely, average skin temperature of the skiers decreased during training (P < .001) and was lower in skiers than staff for the second half of the training. Muscle activity during chairlifts remained below 1% and there was no meaningful shivering. Muscle activity decreased whilst mean and median power frequency increased during the chairlift (P < .020). Muscle activity was also lower during the last lift than the first one whilst mean and median power frequency were higher (P < .020). In conclusion, skin temperature decreased during Slalom and Giant Slalom in elite alpine skiers, likely due to the limited insulative capacity of the racing suit and the high velocity increasing convective heat loss. Conversely, core temperature increased during the training session suggesting that the metabolic heat production was larger than the convective heat loss, and was also sufficient to maintain homeostasis without shivering.

Hyperthermia reduces electromechanical delay via accelerated electrochemical processes.

The present study aimed to determine the effect of hyperthermia on both electrochemical and mechanical components of the electromechanical delay (EMD), using very-high-frame-rate ultrasound. Electrically evoked peak twitch force, EMD, electrochemical (Dm; i.e., delay between stimulation and muscle fascicle motion), and mechanical (Tm; i.e., delay between fascicle motion and force production onset) components of EMD were assessed in 16 participants. Assessments were conducted in a control ambient environment (CON; 26°C, 34% relative humidity) and in a hot ambient environment (HOT; 46-50°C, 18% relative humidity, after ∼127 min of heat exposure). Following heat exposure, gastrocnemius medialis temperature was 37.0 ± 0.6°C in HOT vs. 34.0 ± 0.8°C in CON (P < 0.001). EMD was shorter (9.4 ± 0.8 ms) in HOT than in CON (10.8 ± 0.6 ms, P < 0.001). Electrochemical processes were shorter in HOT than in CON (4.0 ± 0.8 ms vs. 5.5 ± 0.9 ms, respectively, P < 0.001), whereas mechanical processes were unchanged (P = 0.622). These results demonstrate that hyperthermia reduces electromechanical delay via accelerated electrochemical processes, whereas force transmission along the active and passive parts of the series elastic component is not affected following heat exposure. The present study demonstrates that heat exposure accelerates muscle contraction thanks to faster electrochemical processes. Further investigations during voluntary contractions would contribute to better understand how these findings translate into motor performance.NEW & NOTEWORTHY Hyperthermia (targeted core temperature: 38.5°C) reduces the time between gastrocnemius medialis stimulation and the onset of plantar flexor force production in vivo. This reduction in electromechanical delay is concomitant to an earlier motion of muscle fascicle compared with thermoneutral environment. However, hyperthermia has no impact on the duration of force transmission along aponeurosis and tendon, thereby reflecting different effects of heat exposure on contractile and elastic properties of the muscle-tendon unit.

On-Field Ski Kinematic According to Leg and Discipline in Elite Alpine Skiers.

This study used wireless technology to investigate joint kinematic characteristics of the four alpine skiing disciplines. Knee and hip angles were measured in 20 national team alpine skiers during 253 ski runs under FIS regulation, including: 85 Slalom (SL), 123 Giant Slalom (GS), 29 Super Giant Slalom (SG), and 16 Downhill (DH). Data were analyzed by outside (OL, n = 2,087) and inside leg (IL, n = 2,015). The proportion of concentric and eccentric phases (extension and flexion respectively for the knee extensors) as well as the proportion of the quasi-isometric phase defined between ±20°.s-1 depended on the discipline in interaction with the IL/OL (p < 0.001). The results showed a lower knee quasi-isometric duration on OL in SL (11%) than other disciplines (DH: 38%; SG: 42%; GS: 34%, p < 0.001, d > 1.8), suggesting a highly dynamic style. Quasi-isometric mode was significantly longer on OL than IL in GS (34 vs. 20%, p < 0.001, d = 1.16) and SG (42 vs. 28%, p < 0.001, d = 1.11) but was significantly longer on IL than OL in SL (19 vs. 11%, p < 0.001, d = 0.64). Thus, GS and SG showed similarities, with a significantly faster knee eccentric mean angular velocity on IL compared to OL (GS -58 vs. -54°.s-1, SG -52 vs. -45°.s-1, p < 0.001, d ≥ 0.22) whereas SL showed an opposite pattern (-72 vs. -89°.s-1, p < 0.001, d = 1.10). The quasi-isometric phase was overlooked in previous studies but is crucial to consider. The current data may be used to train the outside and inside leg specificities incorporating discipline-specific contraction modes and exercises.

Hypoxia and Fatigue Impair Rapid Torque Development of Knee Extensors in Elite Alpine Skiers.

This study examined the effects of acute hypoxia on maximal and explosive torque and fatigability in knee extensors of skiers. Twenty-two elite male alpine skiers performed 35 maximal, repeated isokinetic knee extensions at 180°s-1 (total exercise duration 61.25 s) in normoxia (NOR, FiO2 0.21) and normobaric hypoxia (HYP, FiO2 0.13) in a randomized, single-blind design. Peak torque and rate of torque development (RTD) from 0 to 100 ms and associated Vastus Lateralis peak EMG activity and rate of EMG rise (RER) were determined for each contraction. Relative changes in deoxyhemoglobin concentration of the VL muscle were monitored by near-infrared spectroscopy. Peak torque and peak EMG activity did not differ between conditions and decreased similarly with fatigue (p < 0.001), with peak torque decreasing continuously but EMG activity decreasing significantly after 30 contractions only. Compared to NOR, RTD, and RER values were lower in HYP during the first 12 and 9 contractions, respectively (both p < 0.05). Deoxyhemoglobin concentration during the last five contractions was higher in HYP than NOR (p = 0.050) but the delta between maximal and minimal deoxyhemoglobin for each contraction was similar in HYP and NOR suggesting a similar muscle O2 utilization. Post-exercise heart rate (138 ± 24 bpm) and blood lactate concentration (5.8 ± 3.1 mmol.l-1) did not differ between conditions. Arterial oxygen saturation was significantly lower (84 ± 4 vs. 98 ± 1%, p < 0.001) and ratings of perceived exertion higher (6 ± 1 vs. 5 ± 1, p < 0.001) in HYP than NOR. In summary, hypoxia limits RTD via a decrease in neural drive in elite alpine skiers undertaking maximal repeated isokinetic knee extensions, but the effect of hypoxic exposure is negated as fatigue develops. Isokinetic testing protocols for elite alpine skiers should incorporate RTD and RER measurements as they display a higher sensitivity than peak torque and EMG activity.

Effect of Cold on Proprioception and Cognitive Function in Elite Alpine Skiers.

PURPOSE: To determine the effect of cold ambient conditions on proprioception and cognitive function in elite alpine skiers. METHODS: 22 high-level alpine skiers and 14 control participants performed a proprioceptive-acuity (active movement-extent discrimination) and a cognitive (planning task) test in cold (8°C) and temperate (24°C) ambient conditions. RESULTS: All participants displayed an increase in thermal discomfort and the amount of negative affects in the cold environment (all P < .05). Average proprioceptive acuity was significantly better in the elite skiers (0.46° ± 0.12°) than in the control group (0.55° ± 0.12°) (P < .05) and was not affected by cold ambient conditions, except for a shift in the pattern of error (over- vs underestimation, P < .05). Cognitive performance was similar between elite skiers and control participants in temperate environments but decreased in the cold in the control group only (P < .05) becoming lower than in elite skiers (P < .05). CONCLUSION: Elite alpine skiers showed a significantly better proprioceptive acuity than a control population and were able to maintain their performance during a cognitive task in a cold environment.