Physiological Evidence that the Critical Torque Is a Phase Transition Not a Threshold

Introduction: Distinct physiological responses to exercise occur in the heavy and severe-intensity domains, which are separated by the critical power or critical torque (CT). However, how the transition between these intensity domains actually occurs is not known. We tested the hypothesis that CT is a sudden threshold, with no gradual transition from heavy- to severe-intensity behavior within the confidence limits associated with the CT. Methods: Twelve healthy participants performed four exhaustive severe-intensity trials for the determination of CT, and four 30-minute trials in close proximity to CT (one or two standard errors above or below each participant’s CT estimate; CT–2, CT–1, CT+1, CT+2). Muscle O2 uptake (mV[Combining Dot Above]O2), rectified EMG and torque variability and complexity were monitored throughout each trial, and maximal voluntary contractions with femoral nerve stimulation were performed before and after each trial to determine central and peripheral fatigue responses. Results: The rates of change in fatigue-related variables, mV[Combining Dot Above]O2, EMG amplitude and torque complexity were significantly faster in the severe trials compared to CT–2. For example, the fall in maximal voluntary contraction (MVC) torque was –1.5 ± 0.8 N.m.min-1 in CT–2 vs. –7.9 ± 2.5 N.m.min-1 in the lowest severe-intensity trial (S1; P < 0.05). Individual analyses showed a low frequency of severe responses even in the circa-CT trials ostensibly above the CT, but also the rare appearance of severe-intensity responses in all circa-CT trials. Conclusion: These data demonstrate that the transition between heavy- and severe-intensity exercise occurs gradually rather than suddenly

Ischemic Preconditioning Blunts Loss of Knee Extensor Torque Complexity with Fatigue

Introduction: Neuromuscular fatigue reduces the temporal structure, or complexity, of muscle torque output, purportedly through an effect on motor unit behaviour. Ischaemic pre-conditioning (IPC), an emerging ergogenic aid, has been demonstrated to have a potent effect on muscular output and endurance. We therefore tested the hypothesis that IPC would attenuate the fatigueinduced loss of muscle torque complexity. Methods: Ten healthy participants (6 male/4 female) performed intermittent isometric knee extension contractions (6 s contraction, 4 s rest) to task failure at 40% maximal voluntary contraction (MVC). Contractions were preceded by either IPC (three bouts of 5 minutes proximal thigh occlusion at 225 mmHg, interspersed with 5 minutes rest) or SHAM (as IPC, but occlusion at only 20 mmHg) treatments. Torque and EMG signals were sampled continuously. Complexity and fractal scaling were quantified using approximate entropy (ApEn) and the detrended fluctuation analysis (DFA) α scaling exponent. Muscle oxygen consumption (mV̇O$_2$) was determined using near-infrared spectroscopy. Results: IPC increased time to task failure by 43 ± 13% (mean ± SEM, P = 0.047). Complexity decreased in both trials (decreased ApEn, increased DFA α; both P < 0.001), though the rate of decrease was significantly lower following IPC (ApEn, –0.2 ± 0.1 vs. –0.4 ± 0.1, P = 0.013; DFA α, 0.2 ± 0.1 vs. 0.3 ± 0.1, P = 0.037). Similarly, the rates of increase in EMG amplitude (P = 0.022) and mV̇O$_2$ (P = 0.043) were significantly slower following IPC. Conclusion: These results suggest the ergogenic effect of IPC observed here is of neural origin and accounts for the slowing of the rates of change in torque complexity, EMG amplitude and muscle oxygen consumption as fatigue develops

A Dual X-Ray Absorptiometry Validated Geometric Model for the Calculation of Body Segment Inertial Parameters of Young Females

The purpose of this study was to validate a new geometric solids model, developed to address the lack of female specific models for body segment inertial parameter estimation. A second aim was to determine the effect of reducing the number of geometric solids used to model the limb segments on model accuracy. The ‘full’ model comprised 56 geometric solids, the ‘reduced’ 31, and the ‘basic’ 16. Predicted whole-body inertial parameters were compared with direct measurements (reaction board, scales), and predicted segmental parameters with those estimated from whole-body DXA scans for 28 females. The percentage root mean square error (%RMSE) for whole-body volume was <2.5% for all models, and 1.9% for the full model. The %RMSE for whole-body center of mass location was <3.2% for all models. The %RMSE whole-body mass was <3.3% for the full model. The RMSE for segment masses was <0.5 kg (<0.5%) for all segments; Bland-Altman analysis showed the full and reduced models could adequately model thigh, forearm, foot and hand segments, but the full model was required for the trunk segment. The proposed model was able to accurately predict body segment inertial parameters for females, more geometric solids are required to more accurately model the trunk

Relationship between muscle metabolic rate and muscle torque complexity during fatiguing intermittent isometric contractions in humans

To test the hypothesis that a system’s metabolic rate and the complexity of fluctuations in the output of that system are related, thirteen healthy participants performed intermittent isometric knee extensor contractions at intensities where a rise in metabolic rate would (40% maximal voluntary contraction, MVC) and would not (20% MVC) be expected. The contractions had a 60% duty factor (6 sec contraction, 4 sec rest) and were performed until task failure or for 30 min, whichever occurred sooner. Torque and surface EMG signals were sampled continuously. Complexity and fractal scaling of torque were quantified using approximate entropy (ApEn) and the detrended fluctuation analysis (DFA) α scaling exponent. Muscle metabolic rate was determined using near‐infrared spectroscopy. At 40% MVC, task failure occurred after (mean ± SD) 11.5 ± 5.2 min, whereas all participants completed 30 min of contractions at 20% MVC. Muscle metabolic rate increased significantly after 2 min at 40% MVC (2.70 ± 1.48 to 4.04 ± 1.23 %·s$^{‐1}$, P < 0.001), but not at 20% MVC. Similarly, complexity decreased significantly at 40% MVC (ApEn, 0.53 ± 0.19 to 0.15 ± 0.09; DFA α, 1.37 ± 0.08 to 1.60 ± 0.09; both P < 0.001), but not at 20% MVC. The rates of change of torque complexity and muscle metabolic rate at 40% MVC were significantly correlated (ApEn, ρ = −0.63, P = 0.022; DFA, ρ = 0.58, P = 0.037). This study demonstrated that an inverse relationship exists between muscle torque complexity and metabolic rate during high‐intensity contractions

Loss of knee extensor torque complexity during fatiguing isometric muscle contractions occurs exclusively above the critical torque

The complexity of knee extensor torque time series decreases during fatiguing isometric muscle contractions. We hypothesised that, due to peripheral fatigue, this loss of torque complexity would occur exclusively during contractions above the critical torque (CT). Nine healthy participants performed isometric knee extension exercise (6 s contraction, 4 s rest) on 6 occasions for 30 min or to task failure, whichever occurred sooner. Four trials were performed above CT (trials S1-S4, S1 being the lowest intensity), and two were performed below CT (at 50% and 90% of CT). Global, central and peripheral fatigue were quantified using maximal voluntary contractions (MVCs) with femoral nerve stimulation. The complexity of torque output was determined using approximate entropy (ApEn) and the Detrended Fluctuation Analysis ? scaling exponent (DFA ?). The MVC torque was reduced in trials below CT (by [Mean ± SEM] 19 ± 4% in 90%CT), but complexity did not decrease (ApEn for 90%CT: from 0.82 ± 0.03 to 0.75 ± 0.06, 95% paired-samples confidence intervals, 95% CI = –0.23, 0.10; DFA ? from 1.36 ± 0.01 to 1.32 ± 0.03, 95% CI –0.12, 0.04). Above CT, substantial reductions in MVC torque occurred (of 49 ± 8% in S1), and torque complexity was reduced (ApEn for S1: from 0.67 ± 0.06 to 0.14 ± 0.01, 95% CI = –0.72, –0.33; DFA ? from 1.38 ± 0.03 to 1.58 ± 0.01, 95% CI 0.12, 0.29). Thus, in these experiments, the fatigue-induced loss of torque complexity occurred exclusively during contractions performed above the CT

Caffeine Ingestion Attenuates Fatigue-induced Loss of Muscle Torque Complexity

Purpose: We tested the hypothesis that caffeine administration would attenuate the fatigue-induced loss of torque complexity. Methods: Eleven healthy participants performed intermittent isometric contractions of the knee extensors to task failure at a target torque of 50% maximal voluntary contraction (MVC), with a 60% duty factor (6 s contraction, 4 s rest), 60 min after ingesting 6 mg·kg?1 caffeine or a placebo. Torque and surface EMG signals were sampled continuously. Complexity and fractal scaling of torque were quantified using approximate entropy (ApEn) and the detrended fluctuation analysis (DFA) ? scaling exponent. Global, central and peripheral fatigue were quantified using MVCs with femoral nerve stimulation. Results: Caffeine ingestion increased endurance by 30 ± 16% (mean ± SD, P = 0.019). Complexity decreased in both trials (decreased ApEn, increased DFA ?; both P < 0.01), as global, central and peripheral fatigue developed (all P < 0.01). Complexity decreased significantly more slowly following caffeine ingestion (ApEn, -0.04 ± 0.02 vs. –0.06 ± 0.01, P = 0.004; DFA ?, 0.03 ± 0.02 vs. 0.04 ± 0.03, P = 0.024), as did the rates of global (-18.2 ± 14.1 vs. –23.0 ± 17.4 N.m.min?1, P = 0.004) and central (-3.5 ± 3.4 vs. –5.7 ± 3.9 %·min?1, P = 0.02) but not peripheral (-6.1 ± 4.1 vs. –7.9 ± 6.3 N.m.min?1, P = 0.06) fatigue. Conclusion: Caffeine ingestion slowed the fatigue-induced loss of torque complexity and increased the time to task failure during intermittent isometric contractions, most likely through central mechanisms

Fatigue-induced changes in knee-extensor torque complexity and muscle metabolic rate are dependent on joint angle

Abstract Purpose Joint angle is a significant determinant of neuromuscular and metabolic function. We tested the hypothesis that previously reported correlations between knee-extensor torque complexity and metabolic rate ($${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 ) would be conserved at reduced joint angles (i.e. shorter muscle lengths). Methods Eleven participants performed intermittent isometric knee-extensor contractions at 50% maximum voluntary torque for 30 min or until task failure (whichever occurred sooner) at joint angles of 30º, 60º and 90º of flexion (0º = extension). Torque and surface EMG were sampled continuously. Complexity and fractal scaling of torque were quantified using approximate entropy (ApEn) and detrended fluctuation analysis (DFA) α. $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 was determined using near-infrared spectroscopy. Results Time to task failure/end increased as joint angle decreased (P &lt; 0.001). Over time, complexity decreased at 90º and 60º (decreased ApEn, increased DFA α, both P &lt; 0.001), but not 30º. $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 increased at all joint angles (P &lt; 0.001), though the magnitude of this increase was lower at 30º compared to 60º and 90º (both P &lt; 0.01). There were significant correlations between torque complexity and $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 at 90º (ApEn, r =  − 0.60, P = 0.049) and 60º (ApEn, r =  − 0.64, P = 0.035; DFA α, ρ = 0.68, P = 0.015). Conclusion The lack of correlation between $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 and complexity at 30º was likely due to low relative task demands, given the similar kinetics of $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 and torque complexity. An inverse correlation between $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 and knee-extensor torque complexity occurs during high-intensity contractions at intermediate, but not short, muscle lengths. </jats:sec