The effect of Nordic hamstring exercise intervention volume on eccentric strength and muscle architecture adaptations : a systematic review and meta-analyses

Although performance of the Nordic hamstring exercise (NHE) has been shown to elicit adaptations that may reduce hamstring strain injury (HSI) risk and occurrence, compliance in NHE interventions in professional soccer teams is low despite a high occurrence of HSI in soccer. A possible reason for low compliance is the high dosages prescribed within the recommended interventions. The aim of this review was to investigate the effect of NHE-training volume on eccentric hamstring strength and biceps femoris fascicle length adaptations. A literature search was conducted using the SPORTDiscus, Ovid, and PubMed databases. A total of 293 studies were identified prior to application of the following inclusion criteria: (1) a minimum of 4 weeks of NHE training was completed; (2) mean ± standard deviation (SD) pre- and post-intervention were provided for the measured variables to allow for secondary analysis; and (3) biceps femoris muscle architecture was measured, which resulted in 13 studies identified for further analysis. The TESTEX criteria were used to assess the quality of studies with risk of bias assessment assessed using a fail-safe N (Rosenthal method). Consistency of studies was analysed using I as a test of heterogeneity and secondary analysis of studies included Hedges' g effect sizes for strength and muscle architecture variables to provide comparison within studies, between-study differences were estimated using a random-effects model. A range of scores (3-11 out of 15) from the TESTEX criteria were reported, showing variation in study quality. A 'low risk of bias' was observed in the randomized controlled trials included, with no study bias shown for both strength or architecture (N = 250 and 663, respectively; p < 0.001). Study consistency was moderate to high for strength (I  = 62.49%) and muscle architecture (I  = 88.03%). Within-study differences showed that following interventions of ≥ 6 weeks, very large positive effect sizes were seen in eccentric strength following both high volume (g = 2.12) and low volume (g = 2.28) NHE interventions. Similar results were reported for changes in fascicle length (g ≥ 2.58) and a large-to-very large positive reduction in pennation angle (g ≥ 1.31). Between-study differences were estimated to be at a magnitude of 0.374 (p = 0.009) for strength and 0.793 (p < 0.001) for architecture. Reducing NHE volume prescription does not negatively affect adaptations in eccentric strength and muscle architecture when compared with high dose interventions. These findings suggest that lower volumes of NHE may be more appropriate for athletes, with an aim to increase intervention compliance, potentially reducing the risk of HSI

Hydrodynamic singularities and clustering in a freely cooling inelastic gas

We employ hydrodynamic equations to follow the clustering instability of a freely cooling dilute gas of inelastically colliding spheres into a well-developed nonlinear regime. We simplify the problem by dealing with a one-dimensional coarse-grained flow. We observe that at a late stage of the instability the shear stress becomes negligibly small, and the gas flows solely by inertia. As a result the flow formally develops a finite time singularity, as the velocity gradient and the gas density diverge at some location. We argue that flow by inertia represents a generic intermediate asymptotic of unstable free cooling of dilute inelastic gases.Comment: 4 pages, 4 figure

Hyperfine Populations Prior to Muon Capture

It is shown that the 1S level hyperfine populations prior to muon capture will be statistical when either target or beam are unpolarised independent of the atomic level at which the hyperfine interaction becomes appreciable. This assertion holds in the absence of magnetic transitions during the cascade and is true because of minimal polarisation after atomic capture and selective feeding during the cascade.Comment: (revtex, 6 preprint pages, no figures

The Effect of Altering Body Posture and Barbell Position on the Between-Session Reliability of Force-Time Curve Characteristics in the Isometric Mid-Thigh Pull

Seventeen strength and power athletes (n = 11 males, 6 females; height: 177.5 ± 7.0 cm, 165.8 ± 11.4 cm; body mass: 90.0 ± 14.1 kg, 66.4 ± 13.9 kg; age: 30.6 ± 10.4 years, 30.8 ± 8.7 years), who regularly performed weightlifting movements during their resistance training programs, were recruited to examine the effect of altering body posture and barbell position on the between-session reliability of force-time characteristics generated in the isometric mid-thigh pull (IMTP). After participants were familiarised with the testing protocol, they undertook two testing sessions which were separated by seven days. In each session, the participants performed three maximal IMTP trials in each of the four testing positions examined, with the order of testing randomized. In each position, no significant differences were found between sessions for all force-time characteristics (p = \u3e0.05). Peak force (PF), time-specific force (F50, F90, F150, F200, F250) and IMP time-bands (0–50, 0–90, 0–150, 0–200, 0–250 ms) were reliable across each of the four testing positions (ICC ≥ 0.7, CV ≤ 15%). Time to peak force, peak RFD, RFD time-bands (0–50, 0–90, 0–150, 0–200, 0–250 ms) and peak IMP were unreliable regardless of the testing position used (ICC =15%). Overall, the use of body postures and barbell positions during the IMTP that do not correspond to the second pull of the clean have no adverse effect on the reliability of the force-time characteristics generated

Heart rate variability and direct current measurement characteristics in professional mixed martial arts athletes

This study’s purpose was to examine heart rate variability (HRV) and direct current potential (DC) measures’ sensitivity and correlations between changes in the acute recovery and stress scale (ARSS) and the previous day’s training load. Training load, HRV, DC and ARSS data were collected from fourteen professional mixed martial arts athletes (32.6 ± 5.3 years, 174.8 ± 8.8 cm, 79.2 ± 17.5 kg) the following morning after hard, easy and rest days. Sensitivity was expressed as a signal-to-noise ratio (SNR, inter-day typical error (TE) or coefficient of variation (%CV) divided by intra-day TE or %CV). Correlations between HRV, DC and ARSS with training load were also examined. The SNRs for the various HRV and DC measures were acceptable to good (1.02–2.85). There was a 23.1% CV average increase between measures taken between different locations versus the same location. Training load changes were not correlated with HRV/DC but were correlated with ARSS stress variables. Practitioners should be aware of HRV/DC variability; however the daily training signal was greater than the test-retest error in this investigation. Upon awakening, HRV/DC measures appear superior for standardization and planning. HRV and DC measures were less sensitive to the previous day’s training load than ARSS measures

The agreement between a portable contact-mat and force-plates during bilateral vertical jumps

Force plates are commonly used when assessing vertical jumping performance but are not always affordable or practical tools for all testing situations. Twenty-four participants volunteered to take part in a study investigating the agreement between bilateral force plates and a new commercially available contact mat that records jump height, flight-time (FT), and FT of individual limbs during both countermovement (CMJ) and squat (SJ) jumps. Each participant performed six jumps of each type while standing on a contact mat placed upon a pair of in-ground force plates. When compared to the force plate via ordinary least products regression, the contact mat agreed with force plate CMJ and SJ jump height, individual limb FT during CMJs, and left-leg FT during SJs. The bilateral contact mat provided valid assessment of individual limb FT during CMJs, but not SJs. Practitioners can therefore use a bilateral contact mat interchangeably with bilateral force plates to measure SJ and CMJ performance

The second and third Sonine coefficients of a freely cooling granular gas revisited

In its simplest statistical-mechanical description, a granular fluid can be modeled as composed of smooth inelastic hard spheres (with a constant coefficient of normal restitution $\alpha$) whose velocity distribution function obeys the Enskog-Boltzmann equation. The basic state of a granular fluid is the homogeneous cooling state, characterized by a homogeneous, isotropic, and stationary distribution of scaled velocities, $F(\mathbf{c})$. The behavior of $F(\mathbf{c})$ in the domain of thermal velocities ($c\sim 1$) can be characterized by the two first non-trivial coefficients ($a_2$ and $a_3$) of an expansion in Sonine polynomials. The main goals of this paper are to review some of the previous efforts made to estimate (and measure in computer simulations) the $\alpha$-dependence of $a_2$ and $a_3$, to report new computer simulations results of $a_2$ and $a_3$ for two-dimensional systems, and to investigate the possibility of proposing theoretical estimates of $a_2$ and $a_3$ with an optimal compromise between simplicity and accuracy.Comment: 12 pages, 5 figures; v2: minor change

The Agreement Between a Portable Contact-Mat and Force-Plates During Bilateral Vertical Jumps

International Journal of Exercise Science 15(1): 632-644, 2022. Force plates are commonly used when assessing vertical jumping performance but are not always affordable or practical tools for all testing situations. Twenty-four participants volunteered to take part in a study investigating the agreement between bilateral force plates and a new commercially available contact mat that records jump height, flight-time (FT), and FT of individual limbs during both countermovement (CMJ) and squat (SJ) jumps. Each participant performed six jumps of each type while standing on a contact mat placed upon a pair of in-ground force plates. When compared to the force plate via ordinary least products regression, the contact mat agreed with force plate CMJ and SJ jump height, individual limb FT during CMJs, and left-leg FT during SJs. The bilateral contact mat provided valid assessment of individual limb FT during CMJs, but not SJs. Practitioners can therefore use a bilateral contact mat interchangeably with bilateral force plates to measure SJ and CMJ performance

Dynamics and stress in gravity driven granular flow

We study, using simulations, the steady-state flow of dry sand driven by gravity in two-dimensions. An investigation of the microscopic grain dynamics reveals that grains remain separated but with a power-law distribution of distances and times between collisions. While there are large random grain velocities, many of these fluctuations are correlated across the system and local rearrangements are very slow. Stresses in the system are almost entirely transfered by collisions and the structure of the stress tensor comes almost entirely from a bias in the directions in which collisions occur.Comment: 4 pages, 3 eps figures, RevTe