Pre-Exercise Rehydration Attenuates Central Fatigability during 2-Min Maximum Voluntary Contraction in Hyperthermia

Background and objectives: Hyperthermia with dehydration alters several brain structure volumes, mainly by changing plasma osmolality, thus strongly affecting neural functions (cognitive and motor). Here, we aimed to examine whether the prevention of significant dehydration caused by passively induced whole-body hyperthermia attenuates peripheral and/or central fatigability during a sustained 2-min isometric maximal voluntary contraction (MVC). Materials and Methods: Ten healthy and physically active adult men (21 ± 1 years of age) performed an isometric MVC of the knee extensors for 2 min (2-min MVC) under control (CON) conditions, after passive lower-body heating that induced severe whole-body hyperthermia (HT, Tre > 39 °C) with dehydration (HT-D) and after HT with rehydration (HT-RH). Results: In the HT-D trial, the subjects lost 0.94 ± 0.15 kg (1.33% ± 0.13%) of their body weight; in the HT-RH trial, their body weight increased by 0.1 ± 0.42 kg (0.1% ± 0.58%). After lower-body heating, the HT-RH trial (vs. HT-D trial) was accompanied by a significantly lower physiological stress index (6.77 ± 0.98 vs. 7.40 ± 1.46, respectively), heart rate (47.8 ± 9.8 vs. 60.8 ± 13.2 b min−1, respectively), and systolic blood pressure (−12.52 ± 5.1 vs. +2.3 ± 6.4, respectively). During 2-min MVC, hyperthermia (HT-D; HT-RH) resulted in greater central fatigability compared with the CON trial. The voluntary activation of exercising muscles was less depressed in the HT-RH trial compared with the HT-D trial. Over the exercise period, electrically (involuntary) induced torque decreased less in the HT-D trial than in the CON and HT-RH trials. Conclusions: Our results suggest that pre-exercise rehydration might have the immediate positive effect of reducing physiological thermal strain, thus attenuating central fatigability even when exercise is performed during severe (Tre > 39 °C) HT, induced by passive warming of the lower body.

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The Effect of Heating and Cooling on Time Course of Voluntary and Electrically Induced Muscle Force Variation

Medicina ◽

10.3390/medicina47010006 ◽

2011 ◽

Vol 47 (1) ◽

pp. 6 ◽

Cited By ~ 3

Author(s):

◽

Keyword(s):

Muscle Fatigue ◽

Time Course ◽

Muscle Force ◽

Peak Torque ◽

Voluntary Contraction ◽

Knee Extensors ◽

Lower Body ◽

Heating And Cooling ◽

Force Variation ◽

Voluntary Contractions

The aim of this study was to investigate the effect of heating and cooling on time course of voluntary and electrically induced muscle force variation. Material and Methods. Ten volunteers performed 50 maximal voluntary and electrically induced contractions of the knee extensors at an angle of 120 degrees under the control conditions and after passive lower body heating and cooling in the control, heating, and cooling experiments. Peak torque, torque variation, and half-relaxation time were assessed during the exercise. Results. Passive lower body heating increased muscle and core temperatures, while cooling lowered muscle temperature, but did not affect core temperature. We observed significantly lower muscle fatigue during voluntary contraction compared with electrically induced contractions. Body heating (opposite to cooling) increased involuntarily induced muscle force, but caused greater electrically induced muscle fatigue. In the middle of the exercise, the coefficient of correlation for electrically induced muscle torque decreased significantly as compared with the beginning of the exercise, while during maximal voluntary contractions, this relation for torque remained significant until the end of the exercise. Conclusion. It was shown that time course of voluntary contraction was more stable than in electrically induced contractions.

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Skeletal muscle mass and distribution in 468 men and women aged 18–88 yr

Journal of Applied Physiology ◽

10.1152/jappl.2000.89.1.81 ◽

2000 ◽

Vol 89 (1) ◽

pp. 81-88 ◽

Cited By ~ 1215

Author(s):

Ian Janssen ◽

Steven B. Heymsfield ◽

ZiMian Wang ◽

Robert Ross

Keyword(s):

Skeletal Muscle ◽

Gender Differences ◽

Body Weight ◽

Upper Body ◽

Whole Body ◽

Lower Body ◽

Large Measure ◽

Imaging Protocol ◽

Men And Women ◽

Magnetic Resonance Imaging Protocol

We employed a whole body magnetic resonance imaging protocol to examine the influence of age, gender, body weight, and height on skeletal muscle (SM) mass and distribution in a large and heterogeneous sample of 468 men and women. Men had significantly ( P < 0.001) more SM in comparison to women in both absolute terms (33.0 vs. 21.0 kg) and relative to body mass (38.4 vs. 30.6%). The gender differences were greater in the upper (40%) than lower (33%) body ( P < 0.01). We observed a reduction in relative SM mass starting in the third decade; however, a noticeable decrease in absolute SM mass was not observed until the end of the fifth decade. This decrease was primarily attributed to a decrease in lower body SM. Weight and height explained ∼50% of the variance in SM mass in men and women. Although a linear relationship existed between SM and height, the relationship between SM and body weight was curvilinear because the contribution of SM to weight gain decreased with increasing body weight. These findings indicate that men have more SM than women and that these gender differences are greater in the upper body. Independent of gender, aging is associated with a decrease in SM mass that is explained, in large measure, by a decrease in lower body SM occurring after the fifth decade.

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Effects of whole-body vibrations on neuromuscular fatigue: a study with sets of different durations

PeerJ ◽

10.7717/peerj.10388 ◽

2020 ◽

Vol 8 ◽

pp. e10388

Author(s):

Miloš Kalc ◽

Ramona Ritzmann ◽

Vojko Strojnik

Keyword(s):

High Frequency ◽

Maximum Voluntary Contraction ◽

Peak Torque ◽

Voluntary Contraction ◽

Neuromuscular Fatigue ◽

Voluntary Activation ◽

Whole Body ◽

Peripheral Fatigue ◽

Single Twitch ◽

Whole Body Vibrations

Background Whole body vibrations have been used as an exercise modality or as a tool to study neuromuscular integration. There is increasing evidence that longer WBV exposures (up to 10 minutes) induce an acute impairment in neuromuscular function. However, the magnitude and origin of WBV induced fatigue is poorly understood. Purpose The study aimed to investigate the magnitude and origin of neuromuscular fatigue induced by half-squat long-exposure whole-body vibration intervention (WBV) with sets of different duration and compare it to non-vibration (SHAM) conditions. Methods Ten young, recreationally trained adults participated in six fatiguing trials, each consisting of maintaining a squatting position for several sets of the duration of 30, 60 or 180 seconds. The static squatting was superimposed with vibrations (WBV30, WBV60, WBV180) or without vibrations (SHAM30, SHAM60, SHAM180) for a total exercise exposure of 9-minutes in each trial. Maximum voluntary contraction (MVC), level of voluntary activation (%VA), low- (T20) and high-frequency (T100) doublets, low-to-high-frequency fatigue ratio (T20/100) and single twitch peak torque (TWPT) were assessed before, immediately after, then 15 and 30 minutes after each fatiguing protocol. Result Inferential statistics using RM ANOVA and post hoc tests revealed statistically significant declines from baseline values in MVC, T20, T100, T20/100 and TWPT in all trials, but not in %VA. No significant differences were found between WBV and SHAM conditions. Conclusion Our findings suggest that the origin of fatigue induced by WBV is not significantly different compared to control conditions without vibrations. The lack of significant differences in %VA and the significant decline in other assessed parameters suggest that fatiguing protocols used in this study induced peripheral fatigue of a similar magnitude in all trials.

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Thermoregulatory and blood responses during exercise at graded hypohydration levels

Journal of Applied Physiology ◽

10.1152/jappl.1985.59.5.1394 ◽

1985 ◽

Vol 59 (5) ◽

pp. 1394-1401 ◽

Cited By ~ 222

Author(s):

M. N. Sawka ◽

A. J. Young ◽

R. P. Francesconi ◽

S. R. Muza ◽

K. B. Pandolf

Keyword(s):

Heart Rate ◽

Body Weight ◽

Plasma Volume ◽

Thermal Strain ◽

Plasma Osmolality ◽

Stress Tests ◽

Sweating Rate ◽

Large Increment ◽

Male Subjects ◽

Humidity Environment

We studied the effects of graded hypohydration levels on thermoregulatory and blood responses during exercise in the heat. Eight heat-acclimated male subjects attempted four heat-stress tests (HSTs). One HST was attempted during euhydration, and three HSTs were attempted while the subjects were hypohydrated by 3, 5, and 7% of their body weight. Hypohydration was achieved by an exercise-heat regimen on the day prior to each HST. After 30 min of rest in a 20 degrees C antechamber the HST consisted of a 140-min exposure (4 repeats of 10 min rest and 25 min treadmill walking) in a hot-dry (49 degrees C, 20% relative humidity) environment. The following observations were made: 1) a low-to-moderate hypohydration level primarily reduced plasma volume with little effect on plasma osmolality, whereas a more severe hypohydration level resulted in no further plasma volume reduction but a large increment in plasma osmolality; 2) core temperature and heart rate responses increased with severity of hypohydration; 3) sweating rate responses for a given rectal temperature were systematically decreased with severity of hypohydration; and 4) the reduction in sweating rate was more strongly associated with plasma hyperosmolality than hypovolemia. In conclusion, an individual's thermal strain increases linearly with the severity of hypohydration during exercise in the heat, and plasma hyperosmolality influences the reduction in sweating more profoundly than hypovolemia.

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The activity of a single muscle sympathetic vasoconstrictor nerve unit is affected by physiological stress in humans

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.00184.2005 ◽

2006 ◽

Vol 290 (2) ◽

pp. H853-H860 ◽

Cited By ~ 29

Author(s):

Hisayoshi Murai ◽

Shigeo Takata ◽

Michiro Maruyama ◽

Manabu Nakano ◽

Daisuke Kobayashi ◽

...

Keyword(s):

Single Unit ◽

Physiological Stress ◽

Valsalva Maneuver ◽

Muscle Sympathetic Nerve Activity ◽

Maximum Voluntary Contraction ◽

Voluntary Contraction ◽

Accurate Information ◽

Neural Firing ◽

Handgrip Exercise ◽

New Strategy

Recording of neural firing from single-unit muscle sympathetic nerve activity (MSNA) is a new strategy offering information about the frequency of pure sympathetic firing. However, it is uncertain whether and when single-unit MSNA would be more useful than multiunit MSNA for analysis of various physiological stresses in humans. In 15 healthy subjects, we measured single-unit and multiunit MSNA before and during handgrip exercise at 30% of maximum voluntary contraction for 3 min and during the Valsalva maneuver at 40 mmHg expiratory pressure for 15 s. Shapes of individual single-unit MSNA were proved to be consistent and suitable for further evaluation. Single-unit and multiunit MSNA exhibited similar responses during handgrip exercise. However, acceleration of neural firing determined from single-unit MSNA became steeper than multiunit MSNA during the Valsalva maneuver. During the Valsalva maneuver, unlike handgrip exercise, the distribution of multiunit burst between 0, 1, 2, 3, and 4 spikes was significantly shifted toward multiple spikes within a given burst ( P < 0.05). These results indicated that evaluation of single-unit MSNA could provide more detailed and accurate information concerning the role and responses of neuronal discharges induced by various physiological stresses in humans, especially amid intense sympathetic activity.

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Effects of static and dynamic training on the stiffness and blood volume of tendon in vivo

Journal of Applied Physiology ◽

10.1152/japplphysiol.91381.2008 ◽

2009 ◽

Vol 106 (2) ◽

pp. 412-417 ◽

Cited By ~ 56

Author(s):

Keitaro Kubo ◽

Toshihiro Ikebukuro ◽

Katsutoshi Yaeshima ◽

Hideaki Yata ◽

Naoya Tsunoda ◽

...

Keyword(s):

Blood Volume ◽

Maximum Voluntary Contraction ◽

Voluntary Contraction ◽

Muscle Volume ◽

Knee Extensors ◽

Neural Activation ◽

Patella Tendon ◽

Activation Level ◽

Before And After

The purpose of this study was to investigate the effects of static and dynamic training on the stiffness and blood volume of the human tendon in vivo. Ten subjects completed 12 wk (4 days/wk) of a unilateral training program for knee extensors. They performed static training on one side [ST; 70% of maximum voluntary contraction (MVC)] and dynamic training on the other side (DT; 80% of one repetition maximum). Before and after training, MVC, neural activation level (by interpolated twitch), muscle volume (by magnetic resonance imaging), stiffness of tendon-aponeurosis complex and patella tendon (by ultrasonography), and blood volume of patella tendon (by red laser lights) were measured. Both protocols significantly increased MVC (49% for ST, 32% for DT; both P < 0.001), neural activation level (9.5% for ST, 7.6% for DT; both P < 0.01), and muscle volume (4.5% for ST, 5.6% for DT; both P < 0.01). The stiffness of tendon-aponeurosis complex increased significantly after ST (55%; P = 0.003) and DT (30%; P = 0.033), while the stiffness of patella tendon increased significantly after ST (83%; P < 0.001), but not for DT ( P = 0.110). The blood volume of patella tendon increased significantly after DT (47%; P = 0.016), but not for ST ( P = 0.205). These results implied that the changes in the blood volume of tendon would be related to differences in the effects of resistance training on the tendon properties.

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Joint Flexibility and Isometric Strength Parameters Are Not Relevant Determinants for Countermovement Jump Performance

International Journal of Environmental Research and Public Health ◽

10.3390/ijerph18052510 ◽

2021 ◽

Vol 18 (5) ◽

pp. 2510

Author(s):

Andreas Konrad ◽

Marina Maren Reiner ◽

Daniel Bernsteiner ◽

Christoph Glashüttner ◽

Sigrid Thaller ◽

...

Keyword(s):

Range Of Motion ◽

Maximum Voluntary Contraction ◽

Knee Extensor ◽

Voluntary Contraction ◽

Knee Extensors ◽

Plantar Flexors ◽

Countermovement Jump ◽

Jump Performance ◽

Leg Muscles ◽

Knee Flexors

Vertical jumps are of great importance as a performance predictor for many types of sports that require speed and agility. However, to date, it is not clear if flexibility and/or the strength of the different leg muscles are determinants for countermovement jump (CMJ) performance. Therefore, the purpose of this study was to relate isometric maximum voluntary contraction (MVC) torque and the flexibility of various muscle groups of the lower body with CMJ performance. Thirty-six healthy male volunteers participated in this study. The participants performed MVCs of the knee extensors, knee flexors, and plantar flexors on a dynamometer. Moreover, range of motion of the hip flexors and plantar flexors was assessed with 3D motion capture, and the range of motion of the knee flexors (hamstrings) was assessed with a Sit n’ Reach® box. CMJs were assessed with a force platform. The correlation analysis revealed a significant moderate correlation of CMJ height with the flexibility of the hip flexors (rP = −0.39) and plantar flexors (rP = 0.47), but not the knee flexors. Moreover, we found that absolute MVC values are not related to CMJ height. However, we did find that knee extensor MVC relative to body mass is significantly related to CMJ height (rP = 0.33) with a moderate magnitude. Although we found significant correlations, the magnitudes of correlations vary between trivial and large according to a 90% confidence interval. Thus, this indicates that range of motion or strength of the assessed leg muscles can explain CMJ performance only to a limited extent.

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Divergent roles of plasma osmolality and the baroreflex on sweating and skin blood flow

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00411.2011 ◽

2012 ◽

Vol 302 (5) ◽

pp. R634-R642 ◽

Cited By ~ 24

Author(s):

Aaron G. Lynn ◽

Daniel Gagnon ◽

Konrad Binder ◽

Robert C. Boushel ◽

Glen P. Kenny

Keyword(s):

Blood Flow ◽

Heat Stress ◽

Core Temperature ◽

Skin Blood Flow ◽

Negative Pressure ◽

Lower Body Negative Pressure ◽

Plasma Osmolality ◽

Whole Body ◽

Lower Body ◽

Change In Mean

Plasma hyperosmolality and baroreceptor unloading have been shown to independently influence the heat loss responses of sweating and cutaneous vasodilation. However, their combined effects remain unresolved. On four separate occasions, eight males were passively heated with a liquid-conditioned suit to 1.0°C above baseline core temperature during a resting isosmotic state (infusion of 0.9% NaCl saline) with (LBNP) and without (CON) application of lower-body negative pressure (−40 cmH2O) and during a hyperosmotic state (infusion of 3.0% NaCl saline) with (LBNP + HYP) and without (HYP) application of lower-body negative pressure. Forearm sweat rate (ventilated capsule) and skin blood flow (laser-Doppler), as well as core (esophageal) and mean skin temperatures, were measured continuously. Plasma osmolality increased by ∼10 mosmol/kgH2O during HYP and HYP + LBNP conditions, whereas it remained unchanged during CON and LBNP ( P ≤ 0.05). The change in mean body temperature (0.8 × core temperature + 0.2 × mean skin temperature) at the onset threshold for increases in cutaneous vascular conductance (CVC) was significantly greater during LBNP (0.56 ± 0.24°C) and HYP (0.69 ± 0.36°C) conditions compared with CON (0.28 ± 0.23°C, P ≤ 0.05). Additionally, the onset threshold for CVC during LBNP + HYP (0.88 ± 0.33°C) was significantly greater than CON and LBNP conditions ( P ≤ 0.05). In contrast, onset thresholds for sweating were not different during LBNP (0.50 ± 0.18°C) compared with CON (0.46 ± 0.26°C, P = 0.950) but were elevated ( P ≤ 0.05) similarly during HYP (0.91 ± 0.37°C) and LBNP + HYP (0.94 ± 0.40°C). Our findings show an additive effect of hyperosmolality and baroreceptor unloading on the onset threshold for increases in CVC during whole body heat stress. In contrast, the onset threshold for sweating during heat stress was only elevated by hyperosmolality with no effect of the baroreflex.

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Acute Effect of Sodium Bicarbonate Supplementation on Symptoms of Gastrointestinal Discomfort, Acid‐Base Balance, and Performance of Jiu‐Jitsu Athletes

Journal of Human Kinetics ◽

10.2478/hukin-2020-0039 ◽

2020 ◽

Vol 75 (1) ◽

pp. 85-93

Author(s):

Luciano Ragone ◽

João Guilherme Vieira ◽

Mateus Camaroti Laterza ◽

Luis Leitão ◽

Jefferson da Silva Novaes ◽

...

Keyword(s):

Body Weight ◽

Sodium Bicarbonate ◽

Isometric Contraction ◽

Maximum Voluntary Contraction ◽

Acute Effect ◽

Voluntary Contraction ◽

Acid Base Balance ◽

Acid Base ◽

Gastrointestinal Discomfort ◽

Base Balance

Abstract The purpose of the present study was to verify the acute effect of sodium bicarbonate supplementation on symptoms of gastrointestinal discomfort, acid-base balance and intermittent isometric handgrip test performance in Jiu-Jitsu athletes. Ten male (22.2 ± 3.9 years; 174 ± 0.07 cm; 74.5 ± 8.9 kg) jiu-jitsu athletes participated in this counterbalanced double-blind crossover study. Two protocols, a) supplementation with 0.3 g.kg-1 of body weight of sodium bicarbonate, and b) supplementation with 0.045 g.kg-1 of body weight of placebo substance, were employed. Gastrointestinal tolerability was assessed by the questionnaire. Blood samples were collected at three time points (baseline, pre-ISO, and post-ISO) to determine the responses of potential hydrogenionic (pH), bicarbonate (HCO3- ), base excess (EB) and lactate concentrations. The maximum voluntary contraction test and the intermittent isometric contraction test were also performed. As a result, none of the athletes reported significant gastrointestinal discomfort (p > 0.05). HCO3- , pH, and EB at the pre-ISO and post-ISO moments were significantly higher for the sodium bicarbonate protocol. Lactate concentrations were significantly higher for both post-ISO protocols (p = 0.000). There was no significant difference in the performance of the maximum voluntary contraction test and the intermittent isometric contraction test (p > 0.05). Thus, we conclude that sodium bicarbonate supplementation does not generate adverse responses resulting in gastrointestinal discomfort, and does not benefit performance yet promotes a state of metabolic alkalosis.

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Development and Experimental Verification of an Ergonomic Backpack

BioMed Research International ◽

10.1155/2020/1437126 ◽

2020 ◽

Vol 2020 ◽

pp. 1-13

Author(s):

Mohamed Z. Ramadan ◽

Sultan N. Al-Tayyar

Keyword(s):

Body Weight ◽

Experimental Verification ◽

Continuous Monitoring ◽

Maximum Voluntary Contraction ◽

Voluntary Contraction ◽

The Body ◽

Oblique Muscle ◽

Independent Variables ◽

Reducing Activity ◽

Spinal Muscles

Carrying a heavy school backpack has extensively been reported as a prime cause of children’s body strain. It is suggested that the load should not exceed 10 percent of the child’s body weight; however, ensuring this requires continuous monitoring. The study explores how ergonomically designed school backpack based on the user anthropometric data (n=280) and ergonomic parameters help reduce force concentration on shoulders and back. It provides a validation process of the developed prototype by experimental verification. The developed design was assessed in a comparison experiment with a commercially available local school backpack. An experimental study was used which recruited thirty healthy college students (aged 19 to 23 years). Two independent variables evaluated were school backpack type (developed backpack versus commercial one) and load levels as a percentage of body weight. Three load levels were employed 10%, 15%, and 20%. These variables were measured on the responses: bag comfort scale and the percent of maximum voluntary contraction (%MVC) of six muscles (right and left of erector spine, right and left of external abdominal oblique muscle, and right and left of trapezius). The developed backpack provided astonishing performance at levels of 15% and 20% of body weight in terms of subjective measure and electromyography (EMG) responses. It also showed that increasing the carried weight more than 10% result in reducing activity on the erector spinal muscles, while it increases on abdominal oblique muscles. The developed backpack design confirmed the efficiency of its bases by distributing the carried weight among the trunk through side pockets, attached to the body through two upper and lower straps. It helped the body to distribute the carried weight and avoid concentrating pressure on specific areas.

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