The study and application of the critical power (CP) concept has spanned many decades. The CP test provides estimates of two distinct parameters, CP and W′, that describe aerobic and anaerobic metabolic capacities, respectively. Various mathematical models have been used to estimate the CP and W′ parameters across exercise modalities. Recently, the CP model has been applied to dynamic constant external resistance (DCER) exercises. The same hyperbolic relationship that has been established across various continuous, whole-body, dynamic movements has also been demonstrated for upper-, lower-, and whole-body DCER exercises. The asymptote of the load versus repetition relationship is defined as the critical load (CL) and the curvature constant is L′. The CL and L′ can be estimated from the same linear and non-linear mathematical models used to derive the CP. The aims of this review are to (1) provide an overview of the CP concept across continuous, dynamic exercise modalities; (2) describe the recent applications of the model to DCER exercise; (3) demonstrate how the mathematical modeling of DCER exercise can be applied to further our understanding of fatigue and individual performance capabilities; and (4) make initial recommendations regarding the methodology for estimating the parameters of the CL test.

As been applied to heart rate (HR) measurements during cycle ergometry to derive a fatigue threshold called the critical heart rate (CHR). This study had 2 purposes: (a) determine if the CHR model for cycle ergometry could be applied to treadmill running and (b) examine the times to exhaustion (Tlim) and the V[Combining Dot Above]O2 responses during constant HR runs at the CHR. Thirteen runners (mean ± SD; age = 23 ± 3 years) performed an incremental treadmill test to exhaustion. On separate days, 4 constant velocity runs to exhaustion were performed. The total number of heart beats (HBlim) for each velocity was calculated as the product of the average 5-second HR and Tlim. The CHR was the slope coefficient of the HBlim vs. Tlim relationship. The Tlim and V[Combining Dot Above]O2 responses were recorded during a constant HR run at the CHR. Polynomial regression analyses were used to examine the patterns of responses for V[Combining Dot Above]O2 and velocity. The HBlim vs. Tlim relationship (r2 = 0.995–1.000) was described by the linear equation: HBlim = a + CHR (Tlim). The CHR (176 ± 7 b·min−1, 91 ± 3% HRpeak) was maintained for 47.84 ± 11.04 minutes. There was no change in HR but quadratic decreases in velocity and V[Combining Dot Above]O2. These findings indicated that the CHR model for cycle ergometry was applicable to treadmill running and represented a sustainable (30–60 minutes) intensity but cannot be used to demarcate exercise intensity domains....

Objectives: This study examined the time course of changes and patterns of responses in electromyographic amplitude (EMG AMP) and EMG mean power frequency (MPF) for the superficial quadriceps muscles during exhaustive treadmill runs within the severe exercise intensity zones (SIZ(1) and SIZ(2)). Methods: The EMG signals for the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) as well as times to exhaustion (T-lim) were recorded in ten runners during two exhaustive treadmill runs (SIZ(1) and SIZ(2)). The composite and individual responses were compared among muscles and between intensities. Results: The composite patterns of responses in EMG AMP (linear, quadratic, and cubic increases; r(2)/R-2=0.684-0.848) and EMG MPF (linear, quadratic, and cubic decreases; r(2)/R-2=0.648-0.852) for the VL and RF were consistent with neuromuscular fatigue in both zones, but those for the VM were not (quadratic, cubic, and non-significant relationships with responses near baseline). The RF tended to demonstrate greater fatigue (EMG MPF decreased from 80-100% T-lim). There was large inter-individual variability (only 10-60% of responses consistent with composite) in response to fatiguing treadmill running. Conclusions: The current findings support the examination and characterization of neuromuscular fatigue on an intensity, muscle, and subject-by-subject basis.

This study examined the effects of curcumin and fenugreek soluble fiber supplementation on the ventilatory threshold (VT) and peak oxygen consumption ( $\dot{\mathrm{V}}$ O₂ peak). Methods: Forty-five untrained men and women were randomly assigned to one of three supplementation groups: placebo (PLA, n = 13), 500 mg·day⁻¹ CurQfen^® (CUR, n = 14), or 300 mg·day⁻¹ fenugreek soluble fiber (FEN, n = 18). Participants completed a maximal graded exercise test on a cycle ergometer to determine the VT and $\dot{\mathrm{V}}$ O₂ peak before (PRE) and after (POST) 28 days of daily supplementation. Separate, one-way analyses of covariance (ANCOVAs) were used to examine the between-group differences for adjusted POST VT and $\dot{\mathrm{V}}$ O₂ peak values, covaried for the respective PRE-test values. Results: The adjusted POST VT $\dot{\mathrm{V}}$ O₂ values for the CUR (mean ± SD = 1.593 ± 0.157 L·min⁻¹) and FEN (1.597 ± 0.157 L·min⁻¹) groups were greater than (p = 0.039 and p = 0.025, respectively) the PLA (1.465 ± 0.155 L·min⁻¹) group, but the FEN and CUR groups were not different (p = 0.943). There were no differences in the adjusted $\dot{\mathrm{V}}$ O₂ peak values (F = 0.613, p = 0.547) among groups. Conclusion: These findings indicated that fenugreek soluble fiber was responsible for the improvements in the submaximal performance index for both CUR and FEN groups.

Herrick, LP, Goh, J, Menke, W, Campbell, MS, Fleenor, BS, Abel, MG, and Bergstrom, HC. Effects of curcumin and fenugreek soluble fiber on the physical working capacity at the fatigue threshold, peak oxygen consumption, and time to exhaustion. J Strength Cond Res 34(12): 3346–3355, 2020—The purpose of this study was to examine the effects of curcumin in combination with fenugreek soluble fiber (CUR + FEN) or fenugreek soluble fiber alone (FEN) on the neuromuscular fatigue threshold (PWC_FT), peak oxygen consumption (V˙o₂peak), and time to exhaustion (T_lim) on a graded exercise test (GXT), in untrained subjects. The PWC_FT estimates the highest power output that can be maintained without evidence of neuromuscular fatigue. Forty-seven untrained, college-aged subjects were randomly assigned to one of 3 supplementation groups; placebo (PLA, n = 15), CUR + FEN (500 mg·d⁻¹, n = 18), or FEN (300 mg·d⁻¹, n = 14). The subjects completed a maximal GXT on a cycle ergometer to determine the PWC_FT, V˙o₂peak, and T_lim before (PRE) and after (POST) 28 days of daily supplementation. Surface electromyographic signals were recorded from a bipolar electrode arrangement on the vastus lateralis of the right leg during each test. Separate one-way analysis of covariances were used to determine if there were between-group differences for adjusted POST-PWC_FT, POST-V˙o₂peak, and POST-T_lim values, covaried for the respective PRE-test scores. The adjusted POST-PWC_FT for the CUR + FEN group (mean ± SD: 196 ± 58 W) was greater (p = 0.016) than the PLA group (168 ± 49 W) but the FEN group (185 ± 32 W) was not different from the CUR + FEN or PLA groups (p > 0.05). There were no differences for adjusted POST-V˙o₂peak (p = 0.612) or POST-T_lim (p = 0.508) among the groups. These findings suggested curcumin combined with fenugreek soluble fiber might delay neuromuscular fatigue.