Oxygen Uptake and Near Infrared Spectroscopy Responses during Heavy Intensity Pseudorandom Binary Sequence Exercise

Abstract

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 potential control mechanisms that are more discernable during non-steady state exercise.