Sex-related differences in motor unit behavior are influenced by myosin heavy chain during high- but not moderate-intensity contractions

AIMS

Motor unit recruitment and firing rate patterns of the vastus lateralis (VL) have not been compared between sexes during moderate- and high-intensity contraction intensities. Additionally, the influence of fiber composition on potential sex-related differences remains unquantified.

METHODS

Eleven males and 11 females performed 40% and 70% maximal voluntary contractions (MVCs). Surface electromyographic (EMG) signals recorded from the VL were decomposed. Recruitment thresholds (RTs), MU action potential amplitudes (MUAPAMP ), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N-EMGRMS ) at steady torque were analyzed. Y-intercepts and slopes were calculated for MUAPAMP , IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies.

RESULTS

There were no sex-related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAPAMP , whereas females displayed greater slopes (p = 0.001-0.007) for the IFR and MFR versus RT relationships. N-EMGRMS at 70% MVC was greater for females (p < 0.001). Type I %MHC was greater for females (p = 0.006), and was correlated (p = 0.018-0.031) with the slopes for the MUAPAMP , IFR, and MFR versus RT relationships at 70% MVC (r = -0.599-0.585).

CONCLUSION

Both sexes exhibited an inverse relationship between MU firing rates and recruitment thresholds. However, the sex-related differences in MU recruitment and firing rate patterns and N-EMGRMS at 70% MVC were likely due to greater type I% MHC and smaller twitch forces of the higher threshold MUs for the females. Evidence is provided that muscle fiber composition may explain divergent MU behavior between sexes.

High-Intensity Cycling Training Necessitates Increased Neuromuscular Demand of the Vastus Lateralis During a Fatiguing Contraction

Purpose: To examine the effects of a 5-week continuous cycling training intervention on electromyographic amplitude (EMG_RMS)- and mechanomyographic amplitude (MMG_RMS)-torque relationships of the vastus lateralis (VL) during a prolonged contraction. Methods: Twenty-four sedentary, young adults performed maximal voluntary contractions (MVCs) and a prolonged isometric trapezoidal contraction at the same absolute 40% MVC for the knee extensors before (PRE) and after training (POST_ABS). Individual b- (slopes) and a-terms (y-intercepts) were calculated from the log-transformed electromyographic amplitude (EMG_RMS)- and mechanomyographic amplitude (MMG_RMS)-torque relationships during the increasing and decreasing segments of the trapezoid. EMG_RMS and MMG_RMS was normalized for the 45-s steady torque segment. Results: At PRE, b-terms for the EMG_RMS-torque relationships during the linearly decreasing segment were greater than the increasing segment (p < .001), and decreased from PRE to POST_ABS (p = .027). a-terms were greater during the linearly increasing than decreasing segment at PRE, while the a-terms for the linearly decreasing segment increased from PRE to POST_ABS (p = .027). For the MMG_RMS-torque relationships, b-terms during the linearly decreasing segment decreased from PRE to POST_ABS (p = .013), while a-terms increased from PRE to POST_ABS when collapsed across segments (p = .022). Steady torque EMG_RMS increased for POST_ABS (p < .001). Conclusion: Although cycling training increased aerobic endurance, incorporating resistance training may benefit athletes/individuals as the alterations in neuromuscular parameters post-training suggest a greater neural cost (EMG_RMS) and mechanical output (MMG_RMS) to complete the same pre-training fatiguing contraction.

Change in measures of moral function following acute bouts of Marine Corps Martial Arts Training

The Marine Corps Martial Arts Program (MCMAP) was developed to prepare Marines for complex battlefield situations that include hand-to-hand combat and ethical decision making. The objective of this study was to quantify changes in ethical decision-making following MCMAP training. Fifty-five, active duty, newly enlisted U.S. Marines (Males: n = 37; age = 19 ± 1 years; height = 176 ± 7 cm; mass = 74 ± 7 kg; Females: n = 18; age = 20 ± 2 years; height = 164 ± 6 cm; mass = 61 ± 6 kg) volunteered for this investigation and were assessed three times over 6 weeks, with 3 weeks between each visit, using serial blood samples for cortisol, norepinephrine, and epinephrine collected before training and during recovery [Immediate Post (IP), 15, 30, 45 and 60 min]. The Moral Functioning Continuum was used to quantify moral function before training, IP, 30, and 60 min post. Moral intention exhibited an acute response to training with significantly impaired decision making immediately post training. Moreover, both moral intention and moral judgement worsened over the visits suggesting a chronic impairment related to time in training suggesting a functional change in ethical decision-making following acute bouts of MCMAP.

Transcranial Direct Current Stimulation of the Temporal Lobe Does Not Affect High-Intensity Work Capacity

Ciccone, AB, Deckert, JA, Schlabs, CR, Tilden, MJ, Herda, TJ, Gallagher, PM, and Weir, JP. Transcranial direct current stimulation of the temporal lobe does not affect high-intensity work capacity. J Strength Cond Res 33(8): 2074-2086, 2019-Stimulation of the left insular cortex may affect heart rate variability (HRV) and exercise effort perception. These studies investigated the effects transcranial direct current stimulation (tDCS) and electrode orientation on HRV and repeated maximal knee extensions. In study 1, after sham stimulation, anodal left temporal lobe stimulation, or anodal right temporal lobe stimulation, 10 male and 10 female subjects (age = 21.0 ± 1.5 years) completed 50 maximum isokinetic extensions at 180°·s. There was a significant effect of stimulation condition on HRV for only 1 (SD2; p = 0.037; η = 0.159) of 5 HRV metrics. There was no significant effect on isokinetic fatigue percent or isokinetic work (all p ≥ 0.278; all η ≤.065). It has been proposed that placing the cathode electrode on the shoulder may differentially affect tDCS. Therefore, in study 2, the effects of electrode orientation on tDCS-induced changes in HRV was assessed in 10 healthy females and 8 healthy males (21.6 ± 2.5 years) who completed cephalic, extracephalic, and sham trials. In the cephalic montage, the anode was placed over the left temporal lobe and the cathode was placed over right prefrontal cortex. In the extracephalic montage, the cathode was placed on the shoulder on the same side of the body as the anode. Neither cephalic nor extracephalic montages affected HRV (all p ≥ 0.152; all η ≤.105). These data suggest that anodal tDCS of the insular cortex has little effect on HRV, and does not improve high-intensity exercise performance in the current population. Therefore, anodal tDCS applied over the left temporal lobe is not recommended for high-intensity performance enhancement.

T cell responses to exercise stress do not adequately model those from tactical stress events

Military training environments are rigorous, requiring service members to endure not only physical and psychological stress but also sleep deprivation, caloric restriction, and severe thermic challenges. The convention has been to use immunoendocrine responses induced by various exercise regimens to generalize results to the layered stress environments of wartime. The purpose of this study was to determine whether exercise models are accurate representations of the physiologic response to the stress experienced in an operational environment. To achieve this aim we identified changes in T cell proliferative capacity (costimulation through CD3+CD28 or stimulation with PHA) and relevant immunoendocrine interactions following an environment comparable to that experienced in Basic Combat Training. An increased proliferation response was observed post-exercise in T cells isolated from whole blood in the layered stress group with those increases still significant at 6 h post-exercise. In contrast, the moderate exercise group saw no significant changes in proliferative ability immediately after exercise exhibiting what could be expressed as a characteristic exercise response. Analyses of serum stress hormones, immunomodulatory cytokines, and immunoglobulins -G, -M, and -A failed to reveal any correlated variations that could clarify the T cell findings. These data suggest that exposure to a high stress environment leads to increased T cell proliferation that appears to be independent of changes in stress hormone concentrations and immunomodulatory cytokines. We submit that variations in exercise intensity and duration do not necessarily approximate military operational or tactical stress responses.

Reminder: RMSSD and SD1 are identical heart rate variability metrics

Assessment of heart rate variability (HRV) is a common approach to examine cardiac autonomic nervous system modulation that has been employed in a variety of settings. Frequently, both the root mean square of successive differences (RMSSD) and SD1, which is a Poincaré plot component, have been used to quantify short-term heart rate variability. It is not typically appreciated, however, that RMSSD and SD1 are identical metrics of HRV. As a reminder to clinicians and researchers who use and study HRV, we show both empirically and mathematically that RMSSD and SD1 are identical metrics. Because the homology between RMSSD and SD1 is not commonly known, the inclusion of both measures has been reported in many recent publications. The inappropriate use of such redundant data may affect the interpretation of HRV studies. Muscle Nerve 56: 674-678, 2017.

T cell activation and proliferation following acute exercise in human subjects is altered by storage conditions and mitogen selection

Recent work investigating exercise induced changes in immunocompetence suggests that some of the ambiguity in the literature is resultant from different cell isolation protocols and mitogen selection. To understand this effect, we compared post-exercise measures of T cell activation and proliferation using two different stimulation methods (costimulation through CD28 or stimulation with phytohaemagglutinin [PHA]). Further, we investigated whether exercise induced changes are maintained when T cell isolation from whole blood is delayed overnight in either a room temperature or chilled (4°C) environment. As expected, an increased proliferation response was observed post-exercise in T cells isolated from whole blood of previously trained individuals immediately after blood collection. Also, cells stimulated with PHA after resting overnight in whole blood were not adversely impacted by the storage conditions. In contrast, allowing cells to rest overnight in whole blood prior to stimulation through CD28, lessened the proliferation observed by cells following exercise rendering both the room temperature and chilled samples closer to the results seen in the control condition. Changes in early markers of activation (CD25), followed a similar pattern, with activation in PHA stimulated cells remaining fairly robust after overnight storage; whereas cell activation following stimulation through CD3+CD28 was disproportionately decreased by the influence of overnight storage. These findings indicate that decisions regarding cell stimulation methods need to be paired with the timeline for T cell isolation from whole blood. These considerations will be especially important for field based studies of immunocompetence where there is a delay in getting whole blood samples to a lab for processing as well as clinical applications where a failure to isolate T cells in a timely manner may result in loss of the response of interest.

Perception of Exercise-Induced Bronchoconstriction in College Athletes

BACKGROUND

Exercise-induced bronchoconstriction (EIB) can lead to long-term respiratory illness and even death. EIB prevalence rates are both high and variable in college athletes. Also, prevalence rates may be underestimated due to ineffective screening. The purpose of this study is to investigate the prevalence of EIB and the perceived impact of EIB in college athletes via a self-report questionnaire.

METHODS

A self-report EIB questionnaire was administered to college athletes on 8 different sports teams. Information collected was used to identify athletes who self-reported: (1) a history of EIB and/or asthma, (2) respiratory symptoms during exercise, (3) medication use, and (4) concern about EIB.

RESULTS

Results showed that 56 of 196 athletes (28.6%) self-reported a history of EIB or asthma. Over half (52%) reported a history of EIB/asthma or current EIB symptoms. Forty-six of the 140 athletes (32.9%) who did not report a history of EIB or asthma indicated symptoms of EIB during sports, training, or exercise. Fourteen of 56 athletes (25%) self-reporting a history of EIB or asthma did not report the use of a respiratory medication. Nineteen of 196 athletes (9.7%) reported being concerned that EIB was adversely affecting their sports performance.

CONCLUSIONS

College athletes self-report a high prevalence of EIB or asthma. Although college athletes may not report a history of EIB or asthma, they indicate symptoms of EIB. A majority of athletes reported a history or current symptoms related to EIB or asthma. Many athletes with a history of EIB or asthma are not taking any asthma medication. Last, athletes report concern about EIB adversely affecting their sports performance. More work is needed using a combination of a screening questionnaire and standardized EIB testing to develop a validated tool for accurately screening and diagnosing EIB in college athletes.