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.

Sex-related differences in mCSA and MUAP of the higher-threshold MUs were likely the result of larger muscle fibres expressing type II characteristics for males.

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, following sham stimulation, anodal left temporal lobe stimulation, or anodal right temporal lobe stimulation, 10 male and 10 female subjects (age=21.0±1.5 yr) completed 50 maximum isokinetic extensions at 180°s. There was a significant effect of stimulation condition on HRV for only one (SD2; p=.037; η=.159) of five HRV metrics. There was no significant effect on isokinetic fatigue percent, or isokinetic work (all p≥.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 ten healthy females and eight healthy males (21.6±2.5 yr) 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≥.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.

The purpose of this study was to determine the effects of deadlift chain variable resistance on surface electromyography (EMG) of the gluteus maximus, erector spinae, and vastus lateralis muscles, ground reaction forces (GRFs), and rate of force development (RFD). Thirteen resistance-trained men (24.0 ± 2.1 years, 179.3 ± 4.8 cm, 87.0 ± 10.6 kg) volunteered for the study. On day 1, subjects performed 1 repetition maximum (1RM) testing of the deadlift exercise. On day 2, subjects performed one set of 3 repetitions with a load of 85% 1RM with chains (CH) and without chains (NC). The order of the CH and NC conditions was randomly determined for each subject. For the CH condition, the chains accounted for approximately 20% (19.9 ± 0.6%) of the 85% 1RM load, matched at the top of the lift. Surface EMG was recorded to differentiate muscle activity between conditions (CH, NC), range of motion (ROM; bottom, top), and phase (concentric, eccentric). Peak GRFs and RFD were measured using a force plate. Electromyography results revealed that for the gluteus maximus there was significantly greater EMG activity during the NC condition vs. the CH condition. For the erector spinae, EMG activity was greater at the bottom than the top ROM (p ≤ 0.05). Force plate results revealed that deadlifting at 85% 1RM with an accommodating chain resistance of approximately 20% results in a reduction in GRFs (p ≤ 0.05) and no change in RFD (p > 0.05). Collectively, these results suggest that the use of chain resistance during deadlifting can alter muscle activation and force characteristics of the lift.

Nonevaporative and respiratory heat loss mechanisms failed to offset decreased sweat evaporation. Nonlinear increases in Tre appeared to catalyze responses in most other variables, which demonstrated similar but temporally delayed patterns. Under these circumstances, an RH threshold for increased thermal strain plausibly existed between 55-70RH; environmental characteristics indicated the threshold occurred not where heat stress became uncompensable, but instead where Tre surpassed the "balance point," triggering compensatory responses.