Motor Unit Recruitment and VO 2 Responses to All Out Maximal versus Constant Load Exercise Tests

Abstract

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 investigations into the effect of muscle fatigue on the integrated responses of force production and the resulting metabolic responses during heavy-to-severe intensity exercise are warranted.