Acute performance fatigability following continuous vs. intermittent cycling protocols is not proportional to total work done.

Classical training theory postulates that performance fatigability following a training session should be proportional to the total work done (TWD); however, this notion has been questioned. This study investigated indices of performance and perceived fatigability after primary sessions of high-intensity interval (HIIT) and constant-work rate (CWR) cycling, each followed by a cycling time-to-task-failure (TTF) bout. On separate days, 16 participants completed an incremental cycling test, and, in a randomized order, i) a TTF trial at 80% of peak power output (PPO), ii) a HIIT session and iii) a CWR session, both of which were immediately followed by a TTF trial at 80% PPO. Central and peripheral aspects of performance fatigability were measured using interpolated twitch technique, and perceptual measures were assessed prior to and following the HIIT and CWR trials, and again following the TTF trial. Despite TWD being less following HIIT (P=0.029), subsequent TTF trial was an average of 125 s shorter following HIIT vs. CWR (P<0.001), and this was accompanied by greater impairments in voluntary and electrically evoked forces (P<0.001), as well as exacerbated perceptual measures (P<0.001); however, there were no differences in any fatigue measure following the TTF trial (P≥0.149). There were strong correlations between the decline in TTF and indices of peripheral (r=0.70) and perceived fatigability (r≥0.80) measured at the end of HIIT and CWR. These results underscore the dissociation between TWD and performance fatigability and highlight the importance of peripheral components of fatigability in limiting endurance performance during high-intensity cycling exercise.

Evidence of non-Maxwellian ion velocity distributions in spherical shock-driven implosions.

The ion velocity distribution functions of thermonuclear plasmas generated by spherical laser direct drive implosions are studied using deuterium-tritium (DT) and deuterium-deuterium (DD) fusion neutron energy spectrum measurements. A hydrodynamic Maxwellian plasma model accurately describes measurements made from lower temperature (<10 keV), hydrodynamiclike plasmas, but is insufficient to describe measurements made from higher temperature more kineticlike plasmas. The high temperature measurements are more consistent with Vlasov-Fokker-Planck (VFP) simulation results which predict the presence of a bimodal plasma ion velocity distribution near peak neutron production. These measurements provide direct experimental evidence of non-Maxwellian ion velocity distributions in spherical shock driven implosions and provide useful data for benchmarking kinetic VFP simulations.

Rapid, non-invasive, in vivo measurement of tissue mechanical properties using gravitational loading and a nonlinear virtual fields method.

Measuring the mechanical properties of soft tissues in vivo is important in biomechanics and for diagnosis and staging of diseases, but challenging because it is difficult to control the boundary conditions. We present a novel, non-invasive method for measuring tissue properties using gravitational loading. MRI images of an organ in different positions are registered to measure tissue displacements due to gravitational forces in different positions. Considering equilibrium between stresses and gravity, we established a nonlinear virtual fields method to identify the tissue properties. The method was applied to the human brain as a proof of concept, using an Ogden model. Sensitivity analysis showed that the bulk modulus could be identified accurately while the shear modulus was identified with greater uncertainty; the strains were too small to identify the strain stiffening exponent. The measured properties agreed well with published in vitro data. The technique offers very promising perspectives, allowing the non-invasive measurement of otherwise inaccessible tissues and providing new information such as the bulk modulus under static loading, which has never previously been measured in vivo.

Validity of the Entralpi force plate in the assessment of finger flexor performance metrics in rock climbers.

This study assessed the validity of the Entralpi force plate in the assessment of finger flexor performance in rock climbers. In addition to a static force evaluation, peak force, peak impulse, and total impulse were measured during 30 all-out performance trials by 15 participants, in which force during the trials was recorded simultaneously by the Entralpi and a Pasco force plate. Agreement between devices was assessed by a variety of statistical analyses, including intraclass correlation coefficient (ICC), coefficient of variation (CV), and Bland-Altman analyses. The static force evaluation showed a mean relative error of 0.41% and excellent day-to-day reliability (ICC = 1; CV = 0.03%). Peak force, peak impulse, and total impulse from the performance trials demonstrated strong agreement (ICC ≥ 0.991, CV ≤ 1.9%, Bland-Altman mean bias ≤ 0.5%). These results illustrate that the Entralpi force plate provides accurate and reliable data for rock climbing related tasks at an affordable cost.

Test-Retest Reliability of a 4-Minute All-Out Critical Force Test in Rock Climbers.

The purpose of this study was to assess the test-retest reliability of a 4-minute all-out critical force test in well-trained rock climbers. Thirteen rock climbers (n=4 females) completed a familiarization session and two all-out critical force tests on different days. During each trial, participants completed 24 repetitions of 7s right-handed, maximal effort hangs from a 20mm edge interspersed with 3 s rest. The end-test force (EF; i.e., critical force), impulse above EF (IEF), and peak force achieved during the test were analyzed with paired t-tests to determine differences between trials. Intraclass correlation coefficient (ICC), coefficient of variation (CV), and Bland-Altman analysis were performed to quantify the relative and absolute reliability of the measure, respectively. The level of significance for this study was set at p<0.05. There were no significant differences between trials for any of the reported variables (P≥0.455). For EF, IEF, and peak force, ICC was 0.848, 0.820, and 0.938, respectively; and CV was 21.0%, 13.2%, and 5.6%, respectively. Bland-Altman analyses showed a mean relative bias of -2.3%, -2.8%, and -1.3%, with 95% limits of agreement (LoA) of -62.6% to 58.1%, -40.5% to 30.9%, and -17.2% to 14.6% for EF, IEF, and peak force, respectively, however linear regression revealed a significant proportional bias for EF (p = 0.026, R2 = 0.377). The reliability of this protocol was good to excellent for all parameters; however, there was larger intra-individual variability for EF and IEF. This study suggests that when using the 4-min all-out critical force test in rock climbers, coaches and athletes should be aware that there may be a trade-off between the test's practicality and the precision of its results.

Ischemic Preconditioning, But Not Priming Exercise, Improves Exercise Performance in Trained Rock Climbers.

ABSTRACT: MacDougall, KB, McClean, ZJ, MacIntosh, BR, Fletcher, JR, and Aboodarda, SJ. Ischemic preconditioning, but not priming exercise, improves exercise performance in trained rock climbers. J Strength Cond Res 37(11): 2149-2157, 2023-To assess the effects of ischemic preconditioning (IPC) and priming exercise on exercise tolerance and performance fatigability in a rock climbing-specific task, 12 rock climbers completed familiarization and baseline tests, and constant-load hangboarding tests (including 7 seconds on and 3 seconds off at an intensity estimated to be sustained for approximately 5 minutes) under 3 conditions: (a) standardized warm-up (CON), (b) IPC, or (c) a priming warm-up (PRIME). Neuromuscular responses were assessed using the interpolated twitch technique, including maximum isometric voluntary contraction (MVC) of the finger flexors and median nerve stimulation, at baseline and after the performance trial. Muscle oxygenation was measured continuously using near-infrared spectroscopy (NIRS) across exercise. Time to task failure (T lim ) for IPC (316.4 ± 83.1 seconds) was significantly greater than CON (263.6 ± 69.2 seconds) ( p = 0.028), whereas there was no difference between CON and PRIME (258.9 ± 101.8 seconds). At task failure, there were no differences in MVC, single twitch force, or voluntary activation across conditions; however, recovery of MVC and single twitch force after the performance trial was delayed for IPC and PRIME compared with CON ( p < 0.05). Despite differences in T lim , there were no differences in any of the NIRS variables assessed. Overall, despite exercise tolerance being improved by an average of 20.0% after IPC, there were no differences in neuromuscular responses at task failure, which is in line with the notion of a critical threshold of peripheral fatigue. These results indicate that IPC may be a promising precompetition strategy for rock climbers, although further research is warranted to elucidate its mechanism of action.

Experimental Measurements of Ion Diffusion Coefficients and Heating in a Multi-Ion-Species Plasma Shock.

Collisional plasma shocks generated from supersonic flows are an important feature in many astrophysical and laboratory high-energy-density plasmas. Compared to single-ion-species plasma shocks, plasma shock fronts with multiple ion species contain additional structure, including interspecies ion separation driven by gradients in species concentration, temperature, pressure, and electric potential. We present time-resolved density and temperature measurements of two ion species in collisional plasma shocks produced by head-on merging of supersonic plasma jets, allowing determination of the ion diffusion coefficients. Our results provide the first experimental validation of the fundamental inter-ion-species transport theory. The temperature separation, a higher-order effect reported here, is valuable for advancements in modeling HED and ICF experiments.

MRI safety, imaging artefacts, and grid distortion evaluated for FFP3 respiratory masks worn throughout the COVID-19 pandemic.

AIM: To determine which filtering face piece (FFP3) respirators worn throughout the COVID-19 pandemic are safe for magnetic resonance imaging (MRI). MATERIALS AND METHODS: Three clinical MRI sequences were performed to assess imaging artefacts, grid distortion, and local heating for eight commercially available FFP3 respirators. All examinations were performed at Cardiff University Brain Research Imaging Centre using a 3 T Siemens Magnetom Prisma with a 64-channel head and neck coil. Each FFP3 mask was positioned on a custom-developed three-dimensional (3D) head phantom for testing. RESULTS: Five of the eight FFP3 masks contained ferromagnetic components and were regarded as "MRI unsafe". One mask was considered "MRI conditional" and only two masks were deemed "MRI safe" for both MRI staff and patients. Temperature strips positioned at the nasal bridge of the phantom did not exhibit local heating. A maximum grid distortion of 5 mm was seen in the anterior portion of the head of the ferromagnetic FFP3 masks. CONCLUSION: This study has demonstrated the importance of assessing respiratory FFP3 masks for use in and around MRI machines. Future research involving FFP3 masks can be conducted safely by following the procedures laid out in this study.

Efficiency of cycling exercise: Quantification, mechanisms, and misunderstandings.

The energetics of cycling represents a well-studied area of exercise science, yet there are still many questions that remain. Efficiency, broadly defined as the ratio of energy output to energy input, is one key metric that, despite its importance from both a scientific as well as performance perspective, is commonly misunderstood. There are many factors that may affect cycling efficiency, both intrinsic (e.g., muscle fiber type composition) and extrinsic (e.g., cycling cadence, prior exercise, and training), creating a complex interplay of many components. Due to its relative simplicity, the measurement of oxygen uptake continues to be the most common means of measuring the energy cost of exercise (and thus efficiency); however, it is limited to only a small proportion of the range of outputs humans are capable of, further limiting our understanding of the energetics of high-intensity exercise and any mechanistic bases therein. This review presents evidence that delta efficiency does not represent muscular efficiency and challenges the notion that the slow component of oxygen uptake represents decreasing efficiency. It is noted that gross efficiency increases as intensity of exercise increases in spite of the fact that fast-twitch fibers are recruited to achieve this high power output. Understanding the energetics of high-intensity exercise will require critical evaluation of the available data.

Fully kinetic simulations of strong steady-state collisional planar plasma shocks.

We report on simulations of strong, steady-state collisional planar plasma shocks with fully kinetic ions and electrons, independently confirmed by two fully kinetic codes (an Eulerian continuum and a Lagrangian particle-in-cell). While kinetic electrons do not fundamentally change the shock structure as compared with fluid electrons, we find an appreciable rearrangement of the preheat layer, associated with nonlocal electron heat transport effects. The electron heat-flux profile qualitatively agrees between kinetic- and fluid-electron models, suggesting a certain level of "stiffness," though substantial nonlocality is observed in the kinetic heat flux. We also find good agreement with nonlocal electron heat-flux closures proposed in the literature. Finally, in contrast to the classical hydrodynamic picture, we find a significant collapse in the "precursor" electric-field shock at the preheat layer leading edge, which correlates with the electron-temperature gradient relaxation.

Does treatment sequence affect outcomes in patients with metaplastic breast cancer?

INTRODUCTION: We compared characteristics and outcomes by treatment sequence among patients with metaplastic breast cancer (MBC), an aggressive subtype. METHODS: Women ≥18 years old with newly diagnosed Stage I-III MBC from 2003 to 2018 who received any treatment in our health system were identified. Unadjusted overall survival (OS) was estimated with the Kaplan-Meier method; the log-rank test was used to compare survival differences between recipients of neoadjuvant (NACT) and adjuvant chemotherapy (ACT). RESULTS: Of the 91 MBC patients identified, 60 received chemotherapy. NACT recipients (n = 20, median age 46.5 y) were younger than ACT recipients (n = 40, median age 60.5 y, p < 0.001) but similar with regards to race and radiation receipt. There was no significant OS difference between NACT and ACT recipients (log-rank p = 0.15), which remained true when patients were stratified by age (≥50 y vs < 50 y). CONCLUSIONS: Among MBC patients, NACT recipients were younger than ACT recipients, but there was no survival difference by treatment sequence.

Multiscale simulation of plasma flows using active learning.

Plasma flows encountered in high-energy-density experiments display features that differ from those of equilibrium systems. Nonequilibrium approaches such as kinetic theory (KT) capture many, if not all, of these phenomena. However, KT requires closure information, which can be computed from microscale simulations and communicated to KT. We present a concurrent heterogeneous multiscale approach that couples molecular dynamics (MD) with KT in the limit of near-equilibrium flows. To reduce the cost of gathering information from MD, we use active learning to train neural networks on MD data obtained by randomly sampling a small subset of the parameter space. We apply this method to a plasma interfacial mixing problem relevant to warm dense matter, showing considerable computational gains when compared with the full kinetic-MD approach. We find that our approach enables the probing of Coulomb coupling physics across a broad range of temperatures and densities that are inaccessible with current theoretical models.

Additional in-series compliance does not affect the length dependence of activation in rat medial gastrocnemius.

NEW FINDINGS: What is the central question of this study? The length dependence of activation (LDA) is typically explained by a length-dependent increase in calcium sensitivity, but recently calcium-independent mechanisms have been suggested: does active muscle shortening provided by a compliant in-series component impact the muscle length at which force output is maximized, thus contributing to LDA? What is the main finding and its importance? Using an in situ rat medial gastrocnemius set-up and varying the magnitude of muscle shortening via an artificial compliant series-elastic component, we were unable to observe any change in optimal length between conditions, contrary to some previous findings. More research is therefore required to explain these discrepancies. ABSTRACT: The force-length relationship dictates the amount of force a muscle can produce as a function of its length, during maximal isometric contractions. When activation is submaximal, it has been shown that the length at which force production is highest (the optimal length) is longer. This is typically explained by a length-dependent increase in Ca2+ sensitivity, known as the 'length dependence of activation'. Recent reports have implicated shortening against in-series compliance to be a potential factor in the observed optimal length (L0 ) of muscle, via the phenomenon of shortening-induced force depression (a phenomenon which describes the relative reduction in muscle force when a muscle is actively shortening to a given length compared to contracting isometrically at that same length). In the current study, rat medial gastrocnemius was stimulated with single and triple pulses (200 Hz) over a range of lengths, both with and without additional in-series compliance provided by a small piece of silicon tubing in series with the muscle, which allowed greater fascicle shortening upon activation. Fascicle length was measured using sonomicrometry crystals, and peak force (Fpeak ) and L0 were estimated by curve-fitting of the force-length data. The additional in-series compliance significantly reduced Fpeak by approximately 14% and 25% for the single and triple pulses, respectively (P = 0.003, P < 0.001), yet L0 remained unchanged (P = 0.405), suggesting that in our model, shortening against in-series compliance does not affect L0 . We offer potential explanations for the discrepancies seen and discuss whether the velocity of shortening may have a role in the length dependence of force.

A comparison of floor surfaces for injury prevention in care settings: impact forces and horizontal pulling force required to move wheeled equipment.

Shock-absorbing flooring is one potential solution to prevent fall-related injuries. No standards exist to characterize shock-absorbing healthcare flooring. This study explores two mechanical tests for impact force reduction and horizontal force required to move wheeled objects. An appropriately designed rubber underlay can reduce peak impact by 25% compared with 1% with standard vinyl. INTRODUCTION: Severe falls often occur in hospitals and care homes. Shock-absorbing flooring is one potential solution to prevent fall-related injuries; however, no standards exist for characterizing flooring as an injury prevention measure. Shock-absorbing flooring use in high-risk settings may influence both patients (injury-saving potential) and staff (manoeuvring equipment). We aimed to explore two tests to characterize floors, to determine shock absorbency and horizontal pulling force required to move wheeled objects. METHODS: Mechanical testing was performed according to the Canadian Standards Association Z325 Hip Protectors document. This test was developed for hip protectors but is applicable to compliant surfaces that form part of the floor. Tests were performed on commercially available floor materials (suitable for care settings) to assess the force required to initiate movement of a wheeled object across the floor. We explored the relationships between horizontal force required to pull wheeled objects, impact force, floor thickness, and core material. RESULTS: Considerable differences were identified between floor samples in their ability to reduce the peak impact force (range 0.7-25%). A peak force reduction of up to 25% can be achieved with a specially designed rubber underlay. Horizontal pulling force increased with floor thickness but was lower for rubber floors. There was no direct relationship between impact attenuation and horizontal pulling force. Whilst thickness and core material explain some variations (66.5% for wheel movement; 82.3% for impact), other unmeasured factors clearly influence floor performance. CONCLUSIONS: These results can inform the development of flooring and the establishment of standards needed to underpin practice, research, and development in this field.

Force-frequency relationship during fatiguing contractions of rat medial gastrocnemius muscle.

The force-frequency relationship presents the amount of force a muscle can produce as a function of the frequency of activation. During repetitive muscular contractions, fatigue and potentiation may both impact the resultant contractile response. However, both the apparent fatigue observed, and the potential for activity-dependent potentiation can be affected by the frequency of activation. Thus, we wanted to explore the effects that repetitive stimulation had on the force-frequency relationship. The force-frequency relationship of the rat medial gastrocnemius muscle was investigated during consecutive bouts of increasing fatigue with 20 to 100 Hz stimulation. Force was measured prior to the fatiguing protocol, during each of three levels of fatigue, and after 30 min of recovery. Force at each frequency was quantified relative to the pre-fatigued 100 Hz contractions, as well as the percentage reduction of force from the pre-fatigued level at a given frequency. We observed less reduction in force at low frequencies compared to high frequencies, suggesting an interplay of fatigue and potentiation, in which potentiation can "protect" against fatigue in a frequency-dependent manner. The exact mechanism of fatigue is unknown, however the substantial reduction of force at high frequency suggests a role for reduced force per cross-bridge.