Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for “central” fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal cha...

Spinal and Supraspinal Factors in Human Muscle Fatigue

SUMMARY Exertional heat illness can affect athletes during high-intensity or longduration exercise and result in withdrawal from activity or collapse during or soon after activity. These maladies include exercise associated muscle cramping, heat exhaustion, or exertional heatstroke. While certain individuals are more prone to collapse from exhaustion in the heat (i.e., not acclimatized, using certain medications, dehydrated, or recently ill), exertional heatstroke (EHS) can affect seemingly healthy athletes even when the environment is relatively cool. EHS is defined as a rectal temperature greater than 40-C accompanied by symptoms or signs of organ system failure, most frequently central nervous system dysfunction. Early recognition and rapid cooling can reduce both the morbidity and mortality associated with EHS. The clinical changes associated with EHS can be subtle and easy to miss if coaches, medical personnel, and athletes do not maintain a high level of awareness and monitor at-risk athletes closely. Fatigue and exhaustion during exercise occur more rapidly as heat stress increases and are the most common causes of withdrawal from activity in hot conditions. When athletes collapse from exhaustion in hot conditions, the term heat exhaustion is often applied. In some cases, rectal temperature is the only discernable difference between severe heat exhaustion and EHS in on-site evaluations. Heat exhaustion will generally resolve with symptomatic care and oral fluid support. Exercise associated muscle cramping can occur with exhaustive work in any temperature range, but appears to be more prevalent in hot and humid conditions. Muscle cramping usually responds to rest and replacement of fluid and salt (sodium). Prevention strategies are essential to reducing the incidence of EHS, heat exhaustion, and exercise associated muscle cramping.

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Exertional heat illness during training and competition

Several factors may alter apparent resting metabolic rate (RMR) during measurement with indirect calorimetry. Likewise, numerous indirect calorimetry measurement protocols have been developed over the years, and the methodology employed could influence test results. As part of a larger project to determine the role of indirect calorimetry in clinical practice, a systematic review of the literature was undertaken to determine the ideal subject condition and test methodology for obtaining reliable measurement of RMR with indirect calorimetry. Food, ethanol, caffeine, and nicotine affect RMR for a variable number of hours after consumption; therefore, intake of these items must be controlled before measurement. Activities of daily living increase metabolic rate, but a short rest (≤20 minutes) before testing is sufficient for the effect to dissipate. Moderate or vigorous physical activity has a longer carryover effect and therefore must be controlled in the hours before a measurement of RMR is attempted. Limited data were found regarding ideal ambient conditions for RMR testing. Measurement duration of 10 minutes with the first 5 minutes deleted and the remaining 5 minutes having a coefficient of variation

https://jandonline.org/article/S0002-8223(06)00146-5/pdf

Best Practice Methods to Apply to Measurement of Resting Metabolic Rate in Adults: A Systematic Review

ContextEphedra and ephedrine sometimes are used for weight loss or enhanced
athletic performance, but the efficacy and safety of these compounds are uncertain.ObjectiveTo assess the efficacy and safety of ephedra and ephedrine used for
weight loss and enhanced athletic performance.Data SourcesWe searched 9 databases using the terms ephedra, ephedrine, adverse effect, side effect, efficacy, effective, and toxic. We included unpublished
trials and non–English-language documents. Adverse events reported to
the US Food and Drug Administration MedWatch program were assessed.Study SelectionEligible studies were controlled trials of ephedra or ephedrine used
for weight loss or athletic performance and case reports of adverse events
associated with such use. Eligible studies for weight loss were human studies
with at least 8 weeks of follow-up; and for athletic performance, those having
no minimum follow-up. Eligible case reports documented that ephedra or ephedrine
was consumed within 24 hours prior to an adverse event or that ephedrine or
an associated product was found in blood or urine, and that other potential
causes had been excluded. Of the 530 articles screened, 52 controlled trials
and 65 case reports were included in the adverse events analysis. Of more
than 18 000 other case reports screened, 284 underwent detailed review.Data ExtractionTwo reviewers independently identified trials of efficacy and safety
of ephedra and ephedrine on weight loss or athletic performance; disagreements
were resolved by consensus. Case reports were reviewed with explicit and implicit
methods.Data SynthesisNo weight loss trials assessed duration of treatment greater than 6
months. Pooled results for trials comparing placebo with ephedrine (n = 5),
ephedrine and caffeine (n = 12), ephedra (n = 1), and ephedra and herbs containing
caffeine (n = 4) yielded estimates of weight loss (more than placebo) of 0.6
(95% confidence interval, 0.2-1.0), 1.0 (0.7-1.3), 0.8 (0.4-1.2), and 1.0
(0.6-1.3) kg/mo, respectively. Sensitivity analyses did not substantially
alter the latter 3 results. No trials of ephedra and athletic performance
were found; 7 trials of ephedrine were too heterogeneous to synthesize. Safety
data from 50 trials yielded estimates of 2.2- to 3.6-fold increases in odds
of psychiatric, autonomic, or gastrointestinal symptoms, and heart palpitations.
Data are insufficient to draw conclusions about adverse events occurring at
a rate less than 1.0 per thousand. The majority of case reports are insufficiently
documented to allow meaningful assessment.ConclusionsEphedrine and ephedra promote modest short-term weight loss (≈0.9
kg/mo more than placebo) in clinical trials. There are no data regarding long-term
weight loss, and evidence to support use of ephedra for athletic performance
is insufficient. Use of ephedra or ephedrine and caffeine is associated with
increased risk of psychiatric, autonomic, or gastrointestinal symptoms, and
heart palpitations.

/pdf/efficacy-and-safety-of-ephedra-and-ephedrine-for-weight-loss-v7tfxm8jzm.pdf

Efficacy and safety of ephedra and ephedrine for weight loss and athletic performance: a meta-analysis.

https://www.archives-pmr.org/article/S0003-9993(98)90142-3/pdf

Back and hip extensor fatigability in chronic low back pain patients and controls

In the last few years, epidemiological studies have shown correlations between the low endurance capacity of the erector spinae muscle and low back pain (LBP) In this study, the function of the erector spinae muscle and localized muscle fatigue in LBP were investigated in both LBP patients and control subjects without LBP, using surface multi-channel electromyograms (EMG) Four positions (L1, L2, L4, L5) were chosen as representative locations for the different functions of the erector spinae muscle Median frequency (fm) parameters of an EMG power density spectrum were monitored to quantify localized muscle fatigue This research indicated a statistically significant difference (P < 005) in fm, parameters between the LBP patients and the control subjects The integrated electromyogram (iEMG) signals of LBP patients were considerably higher than for the control subjects during endurance isometric contraction Our research suggested that human fatiguing erector spinae muscle, in submaximal voluntary contractions tests, induced a different firing order of motor unit spikes between two groups

Spectral analysis of electromyogram in lumbar muscles: fatigue induced endurance contraction.

Background Ingesting a combination of caffeine and ephedrine (C+E) has been shown to raise metabolic heat production and body temperature This side effect of C+E ingestion may be positive during a cold stress scenario, however, during heat stress it could prove to be detrimental Thus, the purpose of this study was to clarify the effect of C+E ingestion on body temperature regulation during moderate exercise in a hot dry environment Methods Ten, healthy, non heat acclimated, males exercised at 50% VO2peak in a 40 degrees C and 30% RH environment until rectal temperature reached 393 degrees C; heart rate (HR) remained at 95% of peak value or greater for 3 min, dizziness or nausea precluded further exercise, or 3 h had elapsed They did this four times at weekly intervals: familiarization (Fam), control (Cont), placebo, and C+E (5 mg  kg(-1) caffeine + 1 mg  kg(-1) ephedrine) trials The Fam and Cont treatments were done first and sequentially while the placebo and C+E treatments were balanced and double-blind Tolerance times, mean skin temperature (Tsk), rectal temperature (Tre), Vo2, Vco2, VE, sweat rate (SR), HR, and sensation of thermal comfort were measured Results Tolerance times (mean+/-SD in minutes) were similar for the placebo (1200+/-284) and C+E (1213+/-339) trials and both times were significantly longer than Cont (1066+/-240) trial C+E did not affect Tsk, initial TrC, delta Tre, SR or the sensation of thermal comfort VO2 and VF, were significantly increased by C+E HR was elevated by C+E compared with the other trials, but only during the initial 20 min of exercise Conclusion Although the metabolic rate was slightly increased with C+E treatment, it was sufficiently offset by increased heat loss mechanisms so that internal body temperature was not increased during moderate exercise in a hot, dry environment

Thermal regulation in the heat during exercise after caffeine and ephedrine ingestion

1. The present study was performed in order to determine the reliability of the portable calorimeter. 2. Resting energy expenditure (REE) was measured by two different apparatuses: one was the typical gas analyser, the other was the portable calorimeter. 3. Although there are differences among individuals, unless the subject has been exposed to severe physical activity prior to the measurement, a suitable resting time prior to the measurement of resting metabolic rate is 10 min. 4. For the measurement of resting metabolic rate, fluctuations in respiratory quotient (RQ) are extremely small; there is greater fluctuation due to variations in respiration. Therefore, for the screening of energy consumption, the use of a fixed value for RQ is sufficient when measuring only oxygen uptake. 5. Respiratory fluctuations vary from person to person and it is not possible to make stable measurements in 1 or 2 min. Therefore, a suitable measurement time for resting metabolic rate is from 3 to approximately 6 min. 6. The results indicate that this portable calorimeter is a useful apparatus for measuring REE in the field.

Examination of the reliability of the portable calorimeter

Stroke-prone spontaneously hypertensive rats (SHRSP) induce spontaneous osteoporosis. To elucidate the specific characteristics of bone metabolism, the SHRSP was compared with age matched Wistar-Kyoto (WKY) rats. We investigated the effects of prolonged swimming exercise training on bone mineral density (BMD) and metabolism in the SHRSP. Seven-week-old male SHRSP and WKY were divided into three groups; the sedentary control WKY group (n=6, WKY), the sedentary control SHRSP group (n=6, SP) and the swimming exercise training SHRSP group (n=6, SWIM) (in pool with 60 min./day, 5 days/week for 12 weeks). The femoral BMD, bone mineral content (BMC), strength, Ca and P contents (%) of SHRSP were approximately 17, 27, 25, 20 and 9%, respectively, lower than that of WKY (p<0.001). Serum alkaline phosphatase (AlP) had not changed between both of SP and WKY, but tartrate-resistant acid phosphatase (TrAcP) of SP approximately 3-fold higher than that of WKY (p<0.05). Both serum calcium (Ca) and intact parathyroid hormone (i-PTH) were similar between SP and WKY. However, serum phosphate (P) of SP was approximately 180wer than that of WKY (N.S.). These results suggested that SHRSP induces osteopenia by the bone turnover of the promoted osteoclast activity with disturbed phosphate homeostasis. On the other hand, the femoral BMD and strength were approximately 7% and 20%, respectively, decreased in the SWIM (p<0.001), and femoral bone Ca and P contents (%) were also approximately 11% and 14%, respectively, lower than that of SP (p<0.001). There were no significant difference between SWIM and SP on serum Ca, but serum P of SWIM was significantly lower than that of SP (p<0.05). These results suggested that the prolonged swimming exercise training in the SHRSP induces more cruelly hypophosphatemia, and leading to osteopenia eventually. We conclude that SHRSP induces osteopenia with disturbance of phosphate homeostasis, and the prolonged swimming exercise in the SHRSP might deteriorate hypophosphatemia and osteopenia.

Masami Miyazaki

Papers

Spectral analysis of electromyogram in lumbar muscles: fatigue induced endurance contraction.

Thermal regulation in the heat during exercise after caffeine and ephedrine ingestion

Examination of the reliability of the portable calorimeter

Prolonged swimming exercise training induce hypophosphatemic osteopenia in stroke-prone spontaneously hypertensive rats (SHRSP).

The effect of fatigue caused by electrical induction or voluntary contraction on Ia inhibition in human soleus muscle