Activities of daily living are characterized by continuous changes in exercise intensity and duration resulting in many abrupt changes in metabolic demand. The ability to readily meet these demands is described by the rate of adaptation in oxygen uptake measured at either the mouth (VO2p) or at the site of active muscle. Many studies have used a single step transition to assess this rate of adaptation. However, a continually changing exercise, such as a pseudorandom binary sequence (PRBS) protocol, provides more transition phases to analyze. We examined the VO2p and microvascular oxygenation responses using near infrared spectroscopy (NIRS) during PRBS exercise. Five healthy males (30 ± 6 yrs, 83.4 ± 20.9 kg, VO2pk, 38.9 ± 5.9 mL/kg/min; mean ± SD) completed a ramp exercise to fatigue for the determination of the lactate threshold (LT) and peak VO2p (VO2pk). Each bout of PRBS exercise transitioned from a baseline of 20 W to a WR corresponding to either 90% of LT (MOD, 100 ± 7 W)) or to a WR at 40% of the difference between LT and VO2pk (HVY, 173 ± 13 W). The PRBS exercise consisted of 30 units of 15 s that alternated between BSL and the higher WR in a pseudorandom pattern; the pattern was repeated 3 times. VO2p was measured breath‐by‐breath using a metabolic cart and interpolated to 1 s intervals. For each subject and WR, the 3 repetitions for each trial were ensembled averaged yielding a single 450 s response. Changes in hemoglobin oxygenation (deoxy‐[Hb+Mb]) and total hemoglobin concentration ([THC]) were measured from the vastus lateralis (VL) using frequency‐domain NIRS. Data were collected at 1 Hz and ensembled averaged to yield a single 450 s response. VO2p and deoxy‐[Hb+Mb] responses were transformed into the frequency domain and analyzed using standard approaches and the mean‐normalized gain (MNG). The harmonic amplitudes (Amph) for VO2p/WR were lower (p<0.05) during HVY at 0.0067 Hz (Mod, 56.9 ± 13.0 %; HVY, 49.3 ± 6.8 %), 0.011 Hz (Mod, 37.6 ± 17.0 %, HVY: 25.8 ± 5.7 %) and 0.013 Hz (Mod, 33.2 ± 12.8 %; HVY, 15.8 ± 10.2 %). The Amph for deoxy‐[Hb+Mb]/WR were lower (p<0.05) for VO­2p/WR in HVY compared to MOD (Mod, 39.5 ± 6.0 %; HVY, 30.2 ± 3.9 %). MNG was also lower (p<0.05) for deoxy‐[Hb+Mb]/WR in HVY compared to MOD (Mod, 77.4 ± 26.6 %; HVY, 47.3 ± 10.0 %). The VO2p and deoxy‐[Hb+Mb] responses from a PRBS trial represent the responses to individual sinusoidal inputs at each frequency tested. The normalized Amph and MNG measured in the current study described the temporal dynamics of the variables of interest (i.e., VO2p and deoxy‐[Hb+Mb]). The results of the current study showed a significant decrease in both normalized Amph and MNG for VO2p and deoxy‐[Hb+Mb] suggesting that exercise performed above the LT results in a slowing of both the VO2p and microvascular deoxy‐[Hb+Mb] responses. These findings are consistent with previous studies demonstrating a slowing of VO2p above the LT. Compared to studies using a single step protocol, responses to PRBS protocols include many transition periods to identify...

Dietary nitrate supplementation in the form of beetroot juice has been the subject of numerous studies in recent literature with inconclusive results. Beetroot juice (BRJ) has been consistently tested using constant‐load or maximal‐exertion exercise, but the influence of nitrate on pulmonary oxygen uptake (VO2p) and microvascular oxygenation to continually‐changing exercise demands has received less attention. Five healthy males (30 ± 6 yrs, 83.4 ± 20.9 kg, VO2pk, 38.9 ± 5.9 mL/kg/min; mean ± SD) completed a ramp exercise to fatigue for the determination of the lactate threshold (LT) and peak VO2p (VO2pk). Each bout of PRBS exercise transitioned from a baseline of 20 W to a WR corresponding to 90% of LT (MOD, 100 ± 7 W). The PRBS exercise consisted of 30 units of 15 s that alternated between BSL and 90% LT in a pseudorandom pattern; the pattern was repeated 3 times. VO2p was measured breath‐by‐breath using a metabolic cart and interpolated to 1 s intervals. For each subject and WR, the 3 repetitions for each trial were ensemble averaged yielding a single 450 s response. Changes in hemoglobin oxygenation (deoxy‐[Hb+Mb]) and total hemoglobin concentration ([THC]) were measured from the vastus lateralis (VL) using frequency‐domain NIRS. Data were collected at 1 Hz and ensemble averaged to yield a single 450 s response. VO2p gain was calculated in the time domain by dividing the ΔVO2p by the ΔWR. VO2p and deoxy‐[Hb+Mb] responses were transformed into the frequency domain and analyzed using standard approaches and the mean‐normalized gain (MNG). Subjects completed the first trial under control conditions (CON), and then were put on a dosing regimen of 400mg of dietary nitrate (Beet It! Beetroot Juice) 2x/day for 3 days before returning to the lab for the second trial (BRJ). The gain (VO2/WR) was decreased in the BRJ condition versus CON (CON: 5.58 ± 0.65 mL/min/W, BRJ: 4.66 ± 1.03 mL/min/W, p = 0.049). The VO2 harmonics were not different between conditions. The deoxy‐[Hb+Mb] harmonics were lower at all frequencies in the BRJ condition versus CON (p < 0.05). MNG is a measure that isolates the temporal characteristics of the investigated variable, using a normalization procedure. MNG for VO2 was not significantly different between conditions. The deoxy‐[Hb+Mb] MNG was lower in BRJ (CON: 77.4 ± 26.6 %, BRJ: 42.3 ± 11.8 %, p = 0.011). The results of the current study are consistent with previous studies showing that BRJ supplementation decreases the oxygen cost of exercise for a given work rate. The similarity in the harmonics and the MNG for VO2 suggest that there was no impact of BRJ on the speed of VO2 adaptation, but the significant decrease in the MNG for the deoxy‐[Hb+Mb] suggests that the rate of adaptation of deoxy‐[Hb+Mb] was slower in the BRJ condition. The PRBS protocol has many on‐ and off‐transitions that may identify influences of dietary nitrate that are not discernable during the rapid adaptation of deoxy‐[Hb+Mb] to a single‐step transition. Additionally, the results of the current study indicate that fur...

The 3 min all‐out cycling test has been used in several studies to establish peak oxygen uptake (VO2pk) and critical power. We have previously shown that the prior recruitment of motor units in excess of that needed to meet the metabolic demands of a subsequent bout of exercise had little impact on both the absolute or relative O2 cost. However, the effect of continuous supra‐maximal exercise, such as that performed during a 3 min all‐out test, has not been systematically compared to constant load, submaximal exercise that is performed at the same absolute work rate (WR). Thus, the purpose of this study was to compare VO2p and motor unit activation during a 3 min all‐out maximal exercise test versus a constant load exercise test performed at the same absolute WR. Five healthy males (25 ± 3 yrs, 84.2 ± 9.1 kg, 178.2 ± 10.5 cm, VO2pk: 40.4 ± 7.3 mL/kg/min; mean ± SD) completed a ramp exercise test (25 W/min) to volitional fatigue for the identification of the lactate threshold (LT) and peak VO2p (VO2pk) as well as the linear factor for use in the subsequent 3 min all‐out test. Critical power (CP) was determined using a 3 min all‐out exercise test (3MAO) performed on a cycle ergometer set to the linear mode where resistance is dependent on cadence. The subject was instructed to pedal as fast as possible for 3 min. After no less than 48 hrs of rest, subjects completed a 6 min constant load exercise test (CLCP) at CP (i.e. the WR corresponding to the last 30 s of the 3MAO test). VO2p was measured breath‐by‐breath using a metabolic cart and interpolated to 1s intervals. For both the 3MAO and CLCP tests, end exercise VO2p (EEVO2p) was determined as the mean VO2p during the last 30 s of exercise. VO2p gain was calculated as ΔEEVO2p/ΔWR. sEMG was measured using electrodes placed over the vastus lateralis (VL) and vastus medialis (VM) muscles. The root mean square (RMS) was normalized as the percent (%) increase above RMS EMG data collected at a fixed WR of 75W for each subject. There was no difference (p>0.05) between the normalized VL and VM responses within 3MAO or CLCP trials; therefore, overall motor unit activation at end exercise for each exercise condition is reported as the mean RMS for the VL and VM. Relative to VO2p, there was no difference in EEVO2p for 3MAO (90.5 ± 10.0 %, p>0.05) or CLCP (81.9 ± 15.5%, p=0.06) nor was there a difference (p=0.07) in EEVO2p between 3MAO (3093 ± 575 mL/min) and CLCP (2777 ± 523 mL/min).ΔEEVO2p/ΔWR was higher (p<0.05) following 3MAO (13.4 ± 0.3 mL/min/W) compared to CLCP (12.0 ± 1.2 mL/min/W). Normalized RMS was higher (p<0.05) following 3MAO (243 ± 40 %) compared to CLCP (169 ± 42 %). EEVO2p, when expressed relative to normalized RMS, was lower (p<0.05) during 3MAO compared to CLCP. Both the EEVO2p relative to muscle activation as well as the higher gain during the 3MAO compared to the CLCP trials found in the present study are similar to those of previous studies demonstrating a dissociation of motor unit activation and O2 cost during heavy‐to‐severe intensity exercise. Further...