The impact of cold, hypoxia, and physical exertion on pistol accuracy and tactical performance

The purpose of this study was to examine the impact of independent cold and combined cold and hypoxic exposures on operational-specific task performance including pistol marksmanship, pistol magazine reload ability, and subjective and objective thermal indices before and after a whole-body physical exertional task. Twelve participants were exposed to Thermoneutral Normoxic (24 °C; FiO2 21%), Cold Normoxic (10 °C; FiO2 21%), and Cold Hypoxic (10 °C; FiO2 14%) conditions for 30min before performing pistol marksmanship at distances of 6.40 and 13.72m and a pistol magazine reload task before and after 3 sets of sandbag deadlifts at 50% body mass. Thermal perception and hand temperatures were collected before and after the physical exertion task. There were no significant differences in Pistol Accuracy performance at distances of 6.40 and 13.72m due to physical exertion, cold, or hypoxia. Following physical exertion, Pistol Accuracy was similar between Thermoneutral and Cold Normoxic conditions but lead to 17% and a 10% reduction in performance during the Cold Hypoxic condition, compared to Thermoneutral and Cold Normoxic conditions. There was no change in Pistol Accuracy for the Thermoneutral Normoxic condition. The pistol magazine reload task was not impacted by physical exertion, but there was a reduction in performance in Cold Normoxic 21% (4.04s) and Cold Hypoxic 16% (3.08s) conditions. Physical exertion did not impact hand temperature but did increase thermal perception scores for all conditions. These findings indicate that cold exposure reduced both tactical dexterity and pistol marksmanship, however, physical exertion may offset these deficits via an increase in thermal perception. Additionally, hypoxemia was the primary mediator of marksmanship performance in cold hypoxic environments following an acute bout of physical exertion. Thus, in cold mountainous environments, marksmen should be aware of their elevation and utilize brief episodes of physical activity to enhance their thermal state when marksmanship is a priority for operational success.

Changes in Neuromuscular Response Patterns After 4 Weeks of Leg Press Training During Isokinetic Leg Extensions

ABSTRACT

Salmon, OF, Housh, TJ, Hill, EC, Keller, JL, Anders, JPV, Johnson, GO, Schmidt, RJ, and Smith, CM. Changes in neuromuscular response patterns after 4 weeks of leg press training during isokinetic leg extensions. J Strength Cond Res 37(7): e405-e412, 2023-The purpose of this study was to identify velocity-specific changes in electromyographic root mean square (EMG RMS), EMG frequency (EMG MPF), mechanomyographic RMS (MMG RMS), and MMG MPF during maximal unilateral isokinetic muscle actions performed at 60° and 240°·s -1 velocities within the right and left vastus lateralis (VL) after 4 weeks of dynamic constant external resistance (DCER) bilateral leg press training. Twelve resistance-trained men (age: mean ± SD = 21.4 ± 3.6 years) visited the laboratory 3d·wk -1 to perform resistance training consisting of 3 sets of 10 DCER leg presses. Four, three-way analysis of variance were performed to evaluate changes in neuromuscular responses (EMG RMS, EMG MPF, MMG RMS, and MMG MPF) from the right and left VL during 1 single-leg maximal isokinetic leg extension performed at 60° and 240°·s -1 before and after 4 weeks of DCER leg press training ( p < 0.05). The results indicated a 36% increase in EMG RMS for the right leg, as well as a 23% increase in MMG RMS and 10% decrease in MMG MPF after training, collapsed across velocity and leg. In addition, EMG RMS was 65% greater in the right leg than the left leg following training, whereas EMG MPF was 11% greater for the left leg than the right leg throughout training. Thus, 4 weeks of DCER leg press training provides sufficient stimuli to alter the neuromuscular activation process of the VL but not velocity-specific neuromuscular adaptations in trained males.

Moderate and Severe Acute Normobaric Hypoxia and the 3-Repetition Deadlift, Hand-Release Push-Up, and Leg Tuck Events From the Army Combat Fitness Test

INTRODUCTION

The newly implemented Army Combat Fitness Test (ACFT) of the U.S. Army seeks to revolutionize the Army's fitness culture and reduce the rate of preventable injuries among soldiers. The initial rollout of the ACFT has been met with several challenges, including a gender-neutral scoring system. The ACFT has undergone several revisions to adapt to the present state of U.S. Army physical fitness; however, the test faces several more obstacles as more data become available. The ACFT was designed to measure combat readiness, a useful tool for units facing deployment or a change in duty station to a high-altitude environment. Reduced oxygen availability (hypoxia) at high altitude influences many physiological functions associated with physical fitness, such that there is an increased demand for oxygen in exercising muscle. Therefore, the purpose was to investigate the effects of normoxic and two levels of hypoxia exposure (moderate and severe; fraction of inspired oxygen [FiO2]: 16.0% and 14.3%) during the 3-repetition deadlift (MDL), hand-release push-up (HRP), and leg tuck (LTK) events of the ACFT.

MATERIALS AND METHODS

Fourteen recreationally active men (n = 10) and women (n = 4) soldier analogs (27.36 ± 1.12 years, height 1.71 ± 2.79 m, weight 80.60 ± 4.24 kg) completed the MDL, HRP, and LTK at normoxia and acute normobaric moderate (MH; FiO2 16%) and severe (SH; FiO2 14.3%) hypoxic exposure. Scores and performance were recorded for each event, and heart rate (HR) and total body oxygen saturation (SpO2) were monitored throughout. Repeated-measures analysis of variance (ANOVA) was used to assess differences in modified ACFT scores, performance, HR, and SpO2 among hypoxic conditions, with follow-up one-way ANOVA and paired t-test when appropriate.

RESULTS

Total body oxygen saturation was decreased at MH and SH conditions compared to normoxia but did not vary between ACFT events. Heart rate was not influenced by altitude but did increase in response to exercise. Scores of the modified total and individual ACFT events were not different between normoxia, MH, and SH. There was also no difference in performance based on the amount of weight lifted during the MDL and number of repetitions of the HRP and LTK events in response to hypoxic exposure.

CONCLUSIONS

Performance and scores of the modified ACFT were not influenced by acute normobaric MH and SH exposure compared to normoxia. Further investigations should examine the full testing battery of the ACFT to provide a comprehensive analysis and potential evidence for such differences.

Neuromuscular and Muscle Tissue Hemodynamic Responses When Exposed to Normobaric Hypoxia during Lower-Body Fatiguing Muscle Actions

OBJECTIVES

This study examined effects of acute hypoxia on the neuromuscular responses (electromyographic (EMG) amplitude and EMG frequency) and localized muscle tissue oxygenated hemoglobin (oxygenated hemoglobin (Oxy_Hb), deoxygenated hemoglobin (Deoxy_Hb), total hemoglobin (Total_Hb), and muscle tissue oxygenation saturation (StO₂) during the process of fatigue.

METHODS

Fifteen male participants (21.4±2.8yr) performed leg extension repetitions to failure at 70% 1-repetition maximum until volitional exhaustion under Normoxic (FiO₂:21%) and Hypoxic (FiO₂:12.9%) conditions. Electromyographic amplitude, EMG frequency, Oxy_Hb, Deoxy_Hb, Total_Hb, and StO₂ were measured from the vastus lateralis at Initial, 20, 40, 60, 80, and 100% of the repetitions to failure.

RESULTS

There was no significant difference in the patterns of responses for EMG amplitude, Oxy_Hb, or Deoxy_Hb between Normoxia and Hypoxia. For EMG frequency, Hypoxia was greater than Normoxia and decreased with fatigue. Total_Hb and StO₂ were greater under Normoxia compared to Hypoxia. The patterns of responses for EMG amplitude, Deoxy_Hb, and Total_Hb increased throughout the repetitions to failure. Oxy_Hb and StO₂ exhibited decreases throughout the repetitions to failure for Normoxic and Hypoxic conditions.

CONCLUSION

The EMG and oxygenation measurements non-invasively suggest a sympathoexcitatory response (indicated by EMG frequency) and provided complimentary information regarding the process of fatigue in normoxic and hypoxic states.

Effect of moderate and Severe Hypoxic exposure coupled with fatigue on psychomotor vigilance testing, muscle tissue oxygenation, and muscular performance

Purpose

The purpose of this study is to examine the effects of fatigue on muscular performance, oxygenation saturation, and cognition following acute hypoxic exposure at Normoxia, Moderate Hypoxia (MH), and Severe Hypoxia (SH).

Methods

Twelve males performed 3 sets of leg extensions to failure under Normoxia (FiO₂: 21%), MH (Fi0₂: 15.4%), and SH (Fi0₂: 12.9%). Heart rate, peripheral oxygenation saturation, total saturation index, psychomotor vigilance testing reaction time, psychomotor vigilance error rate, maximum strength, and repetitions to failure were measured throughout each visit.

Results

The primary findings indicated that MH and SH resulted in significant decreases in psychomotor vigilance test performance (MH: 388.25–427.17 ms, 0.41–0.33 error rate; SH: 398.17–445.42 ms reaction time, 0.25–1.00 error rate), absolute muscle tissue oxygen saturation (Abs-StO₂) (MH:67.22% compared to SH:57.56%), but similar muscular strength, heart rate, and patterns of muscle tissue oxygen saturation responses (StO₂%) during fatigue when compared to Normoxia. There was an acute decrease in the ability to remain vigilant and/or respond correctly to visual stimuli as indicated by the worsened reaction time (PVT_RT) during MH (FiO₂: 15.4%) and increased PVT_RT and error rate (PVT_E) during SH (FiO₂: 12.9%) conditions.

Conclusions

Acute hypoxic exposure in the current study was not a sufficient stimuli to elicit hypoxic-related changes in HR, muscular strength (1-RM), or repetitions to failure. The SpO₂ responses were hypoxic-level dependent with increasing levels of hypoxia resulting in greater and more sustained reductions in SpO₂. The combined SpO₂ and StO₂ responses at MH and SH suggested a balance between the muscles metabolic demand remaining lower than the muscle oxygen diffusion capacity. During the SH condition, Abs-StO₂ suggested greater metabolic stress than Normoxia and MH conditions during the fatiguing leg extensions. The patterns of responses for StO₂% during the three sets of leg press to failure indicated that exercise is a more potent influencer to muscle oxygenation status than hypoxic conditions (FiO₂: 15.4 and 12.9%).

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Moderate Hypoxia (FiO₂: 15.4%) coupled with fatigue resulted in a decrease in reaction time.

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Severe Hypoxia (FiO₂: 12.9%) coupled with fatigue resulted in increased error rate and a decrease in reaction time.

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Fatigue was the primary influencer on heart rate, muscular strength, and muscular endurance and not hypoxic exposure.

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Severe Hypoxia (FiO₂: 12.9%) resulted in greatly reduced muscle tissue oxygenation saturation before and after fatigue.

Neuromuscular responses at acute moderate and severe hypoxic exposure during fatiguing exercise of the biceps brachii

Purpose

The present study examined acute normobaric hypoxic exposure on the number of repetitions to failure, electromyographic (EMG) repetition duration (Time), EMG root mean square (RMS) and EMG mean power frequency (MPF) during biceps brachii (BB) dynamic constant external resistance (DCER) exercise.

Methods

Thirteen subjects performed two sets of fatiguing DCER arm curl repetitions to failure at 70% of their one repetition maximum under normoxic (NH), moderate hypoxia FiO₂ = 15% (MH) and severe hypoxia FiO₂ = 13% (SH). Electromyography of the BB was analyzed for EMG Time, EMG RMS, and EMG MPF. Repetitions were selected as 25%, 50%, 75%, and 100% of total repetitions (%Fail) completed. Pulse oximetry (SpO₂) was measured pre-and post-fatigue.

Results

There was no significant three-way (Condition x Set x %Fail) or two-way (Condition x Set) interaction for any variable. The number of repetitions to failure significantly decreased from (mean ± SEM) 18.2 ± 1.4 to 9.5 ± 1.0 with each Set. In addition, EMG Time increased (25% < 50%<75% < 100%), EMG RMS decreased (50% > 75%>100%), and EMG MPF decreased (75% > 100%) as a result of fatiguing exercise. SpO₂ was lower during MH (Δ5.3%) and SH (Δ9.2%) compared to NH and as a result of fatiguing exercise increased only in MH (Δ2.1%) and SH (Δ5.7%).

Conclusion

The changes in BB EMG variables indicated exercise caused myoelectric manifestations of fatigue, however, acute moderate or severe hypoxia had no additional influence on the rate of fatigue development or neuromuscular parameters.

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Acute MH (FiO₂ 15%) and SH (FiO₂ 14%) did not alter the muscle contractile process.

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Arm curl repetitions to failure decreased MU recruitment and conduction velocity.

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EMG fatigue analysis, hypoxia and arm curls to failure, EMG RMS, EMG MPF and Time.

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SpO₂ was lower at MH and SH which increased following fatiguing exercise.