Neuromuscular electrical stimulation during maximal voluntary contraction: a Delphi survey with expert consensus.

PURPOSE: The use of electrical stimulation to assess voluntary activation of muscle/s is a popular method employed in numerous exercise science and health research settings. This Delphi study aimed to collate expert opinion and provide recommendations for best practice when using electrical stimulation during maximal voluntary contractions. METHODS: A two-round Delphi study was undertaken with 30 experts who completed a 62-item questionnaire (Round 1) comprising of open- and closed-ended questions. Consensus was assumed if ≥ 70% of experts selected the same response; such questions were removed from the subsequent Round 2 questionnaire. Responses were also removed if they failed to meet a 15% threshold. Open-ended questions were analysed and converted into closed-ended questions for Round 2. It was assumed there was no clear consensus if a question failed to achieve a ≥ 70% response in Round 2. RESULTS: A total of 16 out of 62 (25.8%) items reached consensus. Experts agreed that electrical stimulation provides a valid assessment of voluntary activation in specific circumstances, such as during maximal contraction, and this stimulation can be applied at either the muscle or the nerve. Experts recommended using doublet stimuli, self-adhesive electrodes, a familiarisation session, real-time visual or verbal feedback during the contraction, a minimum current increase of + 20% to ensure supramaximal stimulation, and manually triggering stimuli. CONCLUSION: The results of this Delphi consensus study can help researchers make informed decisions when considering technical parameters when designing studies involving electrical stimulation for the assessment of voluntary activation.

Concurrent adaptations in maximal aerobic capacity, heat tolerance, microvascular blood flow and oxygen extraction following heat acclimation and ischemic preconditioning.

We investigated the effects of: 1) Ischemic pre-conditioning (IPC) plus a concurrent five-day heat acclimation + IPC (IPC + HA), 2) five-day HA with sham IPC (HA), or 3) control (CON) on thermoneutral measurements of endurance performance, resting measures of skeletal muscle oxygenation and blood flow. Twenty-nine participants were randomly allocated to three groups, which included: 1) five-days of repeated leg occlusion (4 x 5-min) IPC at limb occlusive pressure, plus fixed-intensity (55% V˙ O2max) cycling HA at ~36 °C/40% humidity; 2) HA plus sham IPC (20 mmHg) or 3) or CON (thermoneutral 55% V˙ O2max plus sham IPC). In IPC + HA and HA, there were increases in maximal oxygen consumption (O2max) (7.8% and 5.4%, respectively; P < 0.05), ventilatory threshold (VT) (5.6% and 2.4%, respectively, P < 0.05), delta efficiency (DE) (2.0% and 1.4%, respectively; P < 0.05) and maximum oxygen pulse (O2pulse-Max) (7.0% and 6.9%, respectively; P < 0.05) during an exhaustive incremental test. There were no changes for CON (P > 0.05). Changes (P < 0.05) in resting core temperature (TC), muscle oxygen consumption (m V˙ O2), and limb blood flow (LBF) were also found pre-to-post intervention among the HA and IPC + HA groups, but not in CON (P > 0.05). Five-days of either HA or IPC + HA can enhance markers of endurance performance in cooler environments, alongside improved muscle oxygen extraction, blood flow, exercising muscle efficiency and O2 pulse at higher intensities, thus suggesting the occurrence of peripheral adaptation. Both HA and IPC + HA enhance the adaptation of endurance capacity, which might partly relate to peripheral changes.

The time course of adaptations in thermoneutral maximal oxygen consumption following heat acclimation.

PURPOSE: This study investigated the effects of a 10-day heat acclimation (HA) programme on the time course of changes in thermoneutral maximal oxygen uptake ([Formula: see text]O2max) during and up to 10 days post-HA. METHODS: Twenty-two male cyclists were assigned to a HA or control (Con) training group following baseline ramp tests of thermoneutral [Formula: see text]O2max. Ten days of fixed-intensity (50% baseline [Formula: see text]O2max) indoor cycling was performed in either ~ 38.0 °C (HA) or ~ 20 °C (Con). [Formula: see text]O2max was re-tested on HA days 5, 10 and post-HA days 1, 2, 3, 4, 5 and 10. RESULTS: [Formula: see text]O2max initially declined across time in both groups during training (P < 0.05), before increasing in the post-HA period in both groups (P < 0.05). However, [Formula: see text]O2max was higher than control by post-HA day 4 in the HA group (P = 0.046). CONCLUSIONS: The non-linear time course of [Formula: see text]O2max adaptation suggests that post-testing should be performed 96-h post-training to identify the maximal change for most individuals. In preparation for training or testing, athletes can augment their aerobic power in thermoneutral environments by performing 10 days HA, but the full effects will manifest at varying stages of the post-HA period.

Corticospinal responses of resistance-trained and un-trained males during dynamic muscle contractions.

Little is known regarding the modulation and the plasticity of the neural pathway interconnecting elements of the central nervous system and skeletal muscle in resistant-trained individuals. The aim of the study was to compare corticospinal and spinal responses measured during dynamic muscle contractions of the tibialis anterior in resistance trained (RT) and un-trained (UT) males. Nine UT and 10 RT male volunteers reported to the laboratory 24h following a familiarisation session. Motor evoked potentials (MEPs) and the cortical silent period were evoked using transcranial magnetic stimulation at a range of contraction intensities and was delivered as the ankle passed 90° during shortening and lengthening contractions. The Hoffmann reflex (H-reflex) and V-waves were evoked with peripheral nerve stimulation. Despite the RT group being significantly stronger during shortening (28%; P=0.023: CI=1.27-15.1Nm), lengthening (25%; P=0.041: CI=0.27-17.0Nm) and isometric muscle actions (20%; P=0.041; CI=0.77-14.9Nm), no differences between the groups existed for corticospinal or spinal variables. Lack of detectable differences between RT and UT individuals may be linked to minimal exposure to task specific, isolated high intensity resistance training of the TA muscle.

Recovery time of motor evoked potentials following lengthening and shortening muscle action in the tibialis anterior.

Motor evoked potentials (MEP) at rest remain facilitated following an isometric muscle contraction. Because the pre-synaptic and post-synaptic control of shortening (SHO) and lengthening (LEN) contractions differs, the possibility exists that the recovery of the MEP is also task specific. The time course of MEP recovery was assessed in the tibialis anterior following SHO and LEN (0.26 rad/s) at 25% and 80% of maximal voluntary contraction. Following LEN and SHO contractions, the MEP recovered to baseline levels within 10s. Despite task-specific differences between SHO and LEN contractions, the MEP facilitation from the augmented neurotransmitter release appears to be short lasting and not influenced by contraction type.

Photochemistry in dried polymer films incorporating the deionized blue membrane form of bacteriorhodopsin.

The preparation and photochemical properties of dried deionized blue membrane (dIbR600; lambdamax approximately 600 nm, epsilon approximately 54, 760 cm-1 M-1, f approximately 1.1) in polyvinyl alcohol films are studied. Reversible photoconversion from dIbR600 to the pink membrane (dIbR485; lambdamax approximately 485 nm) is shown to occur in these films under conditions of strong 647-nm laser irradiation. The pink membrane analog, dIbR485, has a molar extinction coefficient of approximately 39,000 cm-1 M-1 (f approximately 1.2). The ratio of pink --> blue and blue --> pink quantum efficiencies is 33 +/- 5. We observe an additional blue-shifted species (dIbR455, lambdamax approximately 455 nm) with a very low oscillator strength (f approximately 0.6, epsilon approximately 26,000 cm-1 M-1). This species is the product of fast thermal decay of dIbR485. Molecular modeling indicates that charge/charge and charge/dipole interactions introduced by the protonation of ASP85 are responsible for lowering the excited-state all-trans --> 9-cis barrier to approximately 6 kcal mol-1 while increasing the corresponding all-trans --> 13-cis barrier to approximately 4 kcal mol-1. Photochemical formation of both 9-cis and 13-cis photoproducts are now competitive, as is observed experimentally. We suggest that dIbR455 may be a 9-cis, 10-s-distorted species that partially divides the chromophore into two localized conjugated segments with a concomitant blue shift and decreased oscillator strength of the lambdamax absorption band.

Effective photochromic nonlinearity of dried blue-membrane bacteriorhodopsin films.

We report the effective nonlinearity for photochromic conversion in a blue-membrane bacteriorhodopsin film hosted in a dry polyvinyl alcohol matrix. The shift in absorption maximum on photoconversion in this film is larger than that of the same material in hydrated form, thus offering a larger modulation of the refractive index. The photoexcited index modulation is stable for several months, which provides for holographic data recording and long-term photochromic data storage. The effective index modulation is experimentally measured and is in good agreement with the theoretical predictions based on the Kramers-Kronig transformation.

On the molecular origin of photoreceptor noise.

Retinal photoreceptors are noisy. They generate discrete electrical events in the dark indistinguishable from those evoked by light and thereby limit visual sensitivity at low levels of illumination. The random spontaneous events are strongly temperature-dependent and have been attributed to thermal isomerizations of the vitamin A chromophore of rhodopsin, the light-sensitive molecule in photoreceptors. But thermal generation of dark events in both vertebrate and invertebrate photoreceptors requires activation energies in the range of 23 to 27 kcal mol-1, which are significantly less than the energy barrier of 45 kcal mol-1, for photoisomerization of the chromophore of native rhodopsin. We propose that photoreceptor noise results from the thermal isomerization of a relatively unstable form of rhodopsin, one in which the Schiff-base linkage between the chromophore and protein is unprotonated. This molecular mechanism is supported by both theoretical calculations of the properties of rhodopsin and experimental measurements of the properties of photoreceptor noise.

Conformational energetics and excited state level ordering in 11-cis retinal.

Semiempirical molecular orbital theory and semiclassical solvent effect theory are used to analyze the conformational and electronic properties of the 12-s-cis and 12-s-trans conformers of 11-cis retinal. The goal is to examine the influence of solvent environment on the equilibrium geometries of these conformers as well as to provide a perspective on the electronic transitions that contribute to the four band systems that are observed in the 200-500 nm region of the optical spectrum. We conclude that the 12-s-cis isomer is more stable in vacuum, but that the 12-s-trans conformer is preferentially stabilized in both polar and nonpolar solvent environment due to dispersive as well as electrostatic interactions. This observation is in substantial agreement with previous literature results. In contrast, our analysis of the excited state manifold indicates that the spectral features observed in the absorption spectrum are associated with a complex set of overlapping transitions. A total of 18 pi*<--pi transitions contribute to the four bands, and in some cases, conformation changes the relative contribution of the individual transitions that define the overall band shape. This study provides the first definitive assignments for all four band systems.