Fatigue-related changes in torque output and electromyographic parameters of trunk muscles during isometric axial rotation exertion - An investigation in patients with back pain and in healthy subjects

Study Design. A cross-sectional case-control study. Objectives. To examine the effect of fatigue on torque output as well as electromyographic frequency and amplitude values of trunk muscles during isometric axial rotation exertion in back pain patients and to compare the results with a matched control group. Summary of Background Data. Back pain patients exhibited different activation strategies in trunk muscles during the axial rotation exertions. Fatigue changes of abdominal and back muscles during axial rotation exertion have not been examined in patients with back pain. Methods. Twelve back pain patients and 12 matched controls performed isometric fatiguing axial rotation to both sides at 80% maximum voluntary contraction in a standing position. During the fatiguing exertion, electromyographic changes of rectus abdominis, external oblique, internal oblique, latissimus dorsi, iliocostalis lumborum, and multifidus were recorded bilaterally. The primary torque in the transverse plane and the coupling torques in sagittal and coronal planes were also measured. Results. No difference in the endurance capacity was found between back pain and control groups. At the initial period of the exertion, back pain patients demonstrated a statistical trend (P = 0.058) of greater sagittal coupling torque as well as lower activity of rectus abdominis and multifidus and higher activity in external oblique. During the fatigue process similar changes of coupling torque were demonstrated in both sagittal and coronal planes, but a smaller fatigue rate for right external oblique, increase in median frequency for latissimus dorsi, and lesser increase in activity for back muscles were found in the back pain group compared with the control group. Conclusions. Alterations in electromyographic activation and fatigue rates of abdominal and back muscles demonstrated during the fatigue process provide insights into the muscle dysfunctions in back pain and may help clinicians to devise more rational treatment strategies.

A comparison of the cycling performance of cyclists and triathletes

The aim of this study was to compare the cycling performance of cyclists and triathletes. Each week for 3 weeks, and on different days, 25 highly trained male cyclists and 18 highly trained male triathletes performed: (1) an incremental exercise test on a cycle ergometer for the determination of peak oxygen consumption ((V) over dot O-2peak), peak power output and the first and second ventilatory thresholds, followed 15 min later by a sprint to volitional fatigue at 150% of peak power output; (2) a cycle to exhaustion test at the (V) over dot O-2peak power output; and (3) a 40-km cycle time-trial. There were no differences in (V) over dot O-2peak, peak power output, time to volitional fatigue at 150% of peak power output or time to exhaustion at (V) over dot O-2peak power output between the two groups. However, the cyclists had a significantly faster time to complete the 40-km time-trial (56:18 +/- 2:31 min:s; mean +/- s) than the triathletes (58:57 +/- 3:06 min:s; P < 0.01), which could be partially explained (r = 0.34-0.51; P < 0.05) by a significantly higher first (3.32 +/- 0.36 vs 3.08 +/- 0.36 l . min(-1)) and second ventilatory threshold (4.05 +/- 0.36 vs 3.81 +/- 0.29 l . min(-1); both P < 0.05) in the cyclists compared with the triathletes. In conclusion, cyclists may be able to perform better than triathletes in cycling time-trial events because they have higher first and second ventilatory thresholds.

Fatigue Prediction on Fibrin Poly-ε-caprolactone Macroporous Scaffolds

Tissue engineering applications rely on scaffolds that during its service life, either for in-vivo or in vitro applications, are under mechanical solicitations. The variation of the mechanical condition of the scaffold is strongly relevant for cell culture and has been scarcely addressed. Fatigue life cycle of poly-ε-caprolactone, PCL, scaffolds with and without fibrin as filler of the pore structure were characterized both dry and immersed in liquid water. It is observed that the there is a strong increase from 100 to 500 in the number of loading cycles before collapse in the samples tested in immersed conditions due to the more uniform stress distributions within the samples, the fibrin loading playing a minor role in the mechanical performance of the scaffolds

Effects of the length and timing of nighttime naps on task performance and physiological function

OBJECTIVE: To examine the effects of the length and timing of nighttime naps on performance and physiological functions, an experimental study was carried out under simulated night shift schedules. METHODS: Six students were recruited for this study that was composed of 5 experiments. Each experiment involved 3 consecutive days with one night shift (22:00-8:00) followed by daytime sleep and night sleep. The experiments had 5 conditions in which the length and timing of naps were manipulated: 0:00-1:00 (E60), 0:00-2:00 (E120), 4:00-5:00 (L60), 4:00-6:00 (L120), and no nap (No-nap). During the night shifts, participants underwent performance tests. A questionnaire on subjective fatigue and a critical flicker fusion frequency test were administered after the performance tests. Heart rate variability and rectal temperature were recorded continuously during the experiments. Polysomnography was also recorded during the nap. RESULTS: Sleep latency was shorter and sleep efficiency was higher in the nap in L60 and L120 than that in E60 and E120. Slow wave sleep in the naps in E120 and L120 was longer than that in E60 and L60. The mean reaction time in L60 became longer after the nap, and faster in E60 and E120. Earlier naps serve to counteract the decrement in performance and physiological functions during night shifts. Performance was somewhat improved by taking a 2-hour nap later in the shift, but deteriorated after a one-hour nap. CONCLUSIONS: Naps in the latter half of the night shift were superior to earlier naps in terms of sleep quality. However performance declined after a 1-hour nap taken later in the night shift due to sleep inertia. This study suggests that appropriate timing of a short nap must be carefully considered, such as a 60-min nap during the night shift.

Innovative methodologies for the enhancement of the flexural strengthening performance of NSM CFRP technique for RC beams

Tese de Doutoramento em Engenharia Civil

Mechanical, fatigue and wear properties of sintered Carbon Nanotube-based functionally graded materials

Tese de Doutoramento Engenharia Mecânica

Isokinetic rotator muscles fatigue in glenohumeral joint instability before and after Latarjet surgery: a pilot prospective study.

We aimed to analyze the changes in isokinetic internal (IR) and external (ER) rotator muscles fatigue (a) in patients with non-operated recurrent anterior instability, and (b) before and after shoulder surgical stabilization with the Bristow-Latarjet procedure. Thirty-seven patients with non-operated unilateral recurrent anterior post-traumatic instability (NG) were compared with 12 healthy subjects [control group (CG)]. Twenty patients with operated recurrent anterior instability group (OG) underwent isokinetic evaluation before and 3, 6, and 21 months after Bristow-Latarjet surgery. IR and ER muscles strength was evaluated with Con-Trex® dynamometer, with subjects seated and at a 45° shoulder abduction angle in scapular plane. IR and ER muscle fatigue was determined after 10 concentric repetitions at 180° · s(-1) through the fatigue index, the percent decrease in performance (DP), and the slope of peak torque decrease. There were no differences in rotator muscles fatigue between NG and CG. In OG, 3 months post-surgery, IR DP of operated shoulder was significantly (P < 0.001) higher than presurgery and 6 and 21 months post-surgery. Rotator muscles fatigability was not associated with recurrent anterior instability. After surgical stabilization, there was a significantly higher IR fatigability in the operated shoulder 3 months post-surgery, followed by recovery evidenced 6 months post-surgery and long-term maintenance over 21 months.

Increasing lactate turnover rate during exercise by means of altitude training and fructose ingestion : effects on substrate metabolism and endurance performance

Summary : With regard to exercise metabolism, lactate was long considered as a dead-end waste product responsible for muscle fatigue and a limiting factor for motor performance. However, a large body of evidence clearly indicates that lactate is an energy efficient metabolite able to link the glycolytic pathway with aerobic metabolism and has endocrine-like actions, rather than to be a dead-end waste product. Lactate metabolism is also known to be quickly upregulated by regular endurance training and is thought to be related to exercise performance. However, to what extent its modulation can increase exercise performance in already endurance-trained subjects is unknown. The general hypothesis of this work was therefore that increasing either lactate metabolic clearance rate or lactate availability could, in turn, increase endurance performance. The first study (Study I) aimed at increasing the lactate clearance rate by means of assumed interaction effects of endurance training and hypoxia on lactate metabolism and endurance performance. Although this study did not demonstrate any interaction of training and hypoxia on both lactate metabolism and endurance performance, a significant deleterious effect of endurance training in hypoxia was shown on glucose homeostasis. The methods used to determine lactate kinetics during exercise exhibited some limitations, and the second study did delineate some of the issues raised (Study 2). The third study (Study 3) investigated the metabolic and performance effects of increasing plasma lactate production and availability during prolonged exercise in the fed state. A nutritional intervention was used for this purpose: part of glucose feedings ingested during the control condition was substituted by fructose. The results of this study showed a significant increase of lactate turnover rate, quantified the metabolic fate of fructose; and demonstrated a significant decrease of lipid oxidation and glycogen breakdown. In contrast, endurance performance appeared to be unmodified by this dietary intervention, being at odds with recent reports. Altogether the results of this thesis suggest that in endurance athletes the relationship between endurance performance and lactate turnover rate remains unclear. Nonetheless, the result of the present study raises questions and opens perspectives on the rationale of using hypoxia as a therapeutic aid for the treatment of insulin resistance. Moreover, the results of the second study open perspectives on the role of lactate as an intermediate metabolite and its modulatory effects on substrate metabolism during exercise. Additionally it is suggested that the simple nutritional intervention used in the third study can be of interest in the investigation on the aforementioned roles of lactate. Résumé : Lorsque le lactate est évoqué en rapport avec l'exercice, il est souvent considéré comme un déchet métabolique responsable de l'acidose métabolique, de la fatigue musculaire ou encore comme un facteur limitant de la performance. Or la littérature montre clairement que le lactate se révèle être plutôt un métabolite utilisé efficacement par de nombreux tissus par les voies oxydatives et, ainsi, il peut être considéré comme un lien entre le métabolisme glycolytique et le métabolisme oxydatif. De plus on lui prête des propriétés endocrines. Il est connu que l'entraînement d'endurance accroît rapidement le métabolisme du lactate, et il est suggéré que la performance d'endurance est liée à son métabolisme. Toutefois la relation entre le taux de renouvellement du lactate et la performance d'endurance est peu claire, et, de même, de quelle manière la modulation de son métabolisme peut influencer cette dernière. Le but de cette thèse était en conséquence d'investiguer de quelle manière et à quel degré l'augmentation du métabolisme du lactate, par l'augmentation de sa clearance et de son turnover, pouvait à son tour améliorer la performance d'endurance de sujets entraînés. L'objectif de la première étude a été d'augmenter la clearance du lactate par le biais d'un entraînement en conditions hypoxiques chez des cyclistes d'endurance. Basé sur la littérature scientifique existante, on a fait l'hypothèse que l'entraînement d'endurance et l'hypoxie exerceraient un effet synergétique sur le métabolisme du lactate et sur la performance, ce qui permettrait de montrer des relations entre performance et métabolisme du lactate. Les résultats de cette étude n'ont montré aucun effet synergique sur la performance ou le métabolisme du lactate. Toutefois, un effet délétère sur le métabolisme du glucose a été démontré. Quelques limitations de la méthode employée pour la mesure du métabolisme du lactate ont été soulevées, et partiellement résolues dans la seconde étude de ce travail, qui avait pour but d'évaluer la sensibilité du modèle pharmacodynamique utilisé pour le calcul du turnover du lactate. La troisième étude a investigué l'effet d'une augmentation de la lactatémie sur le métabolisme des substrats et sur la performance par une intervention nutritionnelle substituant une partie de glucose ingéré pendant l'exercice par du fructose. Les résultats montrent que les composants dynamiques du métabolisme du lactate sont significativement augmentés en présence de fructose, et que les oxydations de graisse et de glycogène sont significativement diminuées. Toutefois aucun effet sur la performance n'a été démontré. Les résultats de ces études montrent que la relation entre le métabolisme du lactate et la performance reste peu claire. Les résultats délétères de la première étude laissent envisager des pistes de travail, étant donné que l'entraînement en hypoxie est considéré comme outil thérapeutique dans le traitement de pathologies liées à la résistance à l'insuline. De plus les résultats de la troisième étude ouvrent des perspectives de travail quant au rôle du lactate comme intermédiaire métabolique durant l'exercice ainsi que sur ses effets directs sur le métabolisme. Ils suggèrent de plus que la manipulation nutritionnelle simple qui a été utilisée se révèle être un outil prometteur dans l'étude des rôles et effets métaboliques que peut revêtir le lactate durant l'exercice.

Neuromuscular fatigue in racquet sports

This article describes the physiologic and neural mechanisms that cause neuromuscular fatigue in racquet sports: table tennis, tennis, squash, and badminton. In these intermittent and dual activities, performance may be limited as a match progresses because of a reduced central activation, linked to changes in neurotransmitter concentration or in response to afferent sensory feedback. Alternatively, modulation of spinal loop properties may occur because of changes in metabolic or mechanical properties within the muscle. Finally, increased fatigue manifested by mistimed strokes, lower speed, and altered on-court movements may be caused by ionic disturbances and impairments in excitation-contraction coupling properties. These alterations in neuromuscular function contribute to decrease in racquet sports performance observed under fatigue.

No variation of physical performance and perceived exertion after adrenal gland stimulation by synthetic ACTH (Synacthen) in cyclists.

There is anecdotal evidence that athletes use the banned substance Synacthen because of its perceived benefit with its associated rise in cortisol. To test the performance-enhancing effects of Synacthen, eight trained cyclists completed two, 2-day exercise sessions separated by 7-10 days. On the first day of each 2-day exercise session, subjects received either Synacthen (0.25 mg, TX) or placebo (PLA) injection. Performance was assessed by a 20-km time trial (TT) after a 90-min fatigue period on day 1 and without the fatiguing protocol on day 2. Plasma androgens and ACTH concentrations were measured during the exercise bouts as well as the rate of perceived exertion (RPE). Spot urines were analyzed for androgens and glucocorticoids quantification. Basal plasma hormones did not differ significantly between PLA and TX groups before and 24 h after the IM injection (P > 0.05). After TX injection, ACTH peaked at 30 min and hormone profiles were significantly different compared to the PLA trial (P < 0.001). RPE increased significantly in both groups as the exercise sessions progressed (P < 0.001) but was not influenced by treatment. The time to completion of the TT was not affected on both days by Synacthen treatment. In the present study, a single IM injection of synthetic ACTH did not improve either acute or subsequent cycling performance and did not influence perceived exertion. The investigated urinary hormones did not vary after treatment, reinforcing the difficulty for ACTH abuse detection.

Do childhood cancer survivors with physical performance limitations reach healthy activity levels?

BACKGROUND: The extent to which physical performance limitations affect the ability of childhood cancer survivors to reach healthy activity levels is unknown. Therefore this study aims to describe the effect of different types of limitations on activity levels in survivors. PROCEDURE: Within the Swiss Childhood Cancer Survivor Study we sent a questionnaire to all survivors (≥16 years) registered in the Swiss Childhood Cancer Registry, who survived >5 years and were diagnosed 1976-2005 aged <16 years. We measured healthy activity levels using international guidelines and assessed different kinds of performance limitations (visual impairment, weight and endurance problems, cardiorespiratory, musculoskeletal, and neurological problems, pain and fatigue syndromes). RESULTS: The sample included 1,560 survivors (75% response rate), of whom 209 (13.5%) reported they have performance limitations. Forty-two percent of survivors with limitations reached healthy activity levels, compared to 57% of survivors without limitations. Least active were survivors with vision impairments (25% active), weight and endurance problems (27.3%), cardiorespiratory problems (36.4%), and musculoskeletal problems (43.1%). After adjusting for socio-demographic variables and type of cancer, we found that survivors with limitations were 1.4 (95%CI 1.0-2.0; P = 0.047) times more likely to be inactive. CONCLUSIONS: Although many survivors with physical performance limitations maintain healthy activity levels, there is room for improvement. Adapted and targeted physical activity counseling for survivors with performance limitations might help them to raise level of activity and pursue a healthy lifestyle.

Fly Ash Soil Stabilization for Non-Uniform Subgrade Soils, Volume II: Influence of Subgrade Non-Uniformity on PCC Pavement Performance, TR-461, 2005

To provide insight into subgrade non-uniformity and its effects on pavement performance, this study investigated the influence of non-uniform subgrade support on pavement responses (stress and deflection) that affect pavement performance. Several reconstructed PCC pavement projects in Iowa were studied to document and evaluate the influence of subgrade/subbase non-uniformity on pavement performance. In situ field tests were performed at 12 sites to determine the subgrade/subbase engineering properties and develop a database of engineering parameter values for statistical and numerical analysis. Results of stiffness, moisture and density, strength, and soil classification were used to determine the spatial variability of a given property. Natural subgrade soils, fly ash-stabilized subgrade, reclaimed hydrated fly ash subbase, and granular subbase were studied. The influence of the spatial variability of subgrade/subbase on pavement performance was then evaluated by modeling the elastic properties of the pavement and subgrade using the ISLAB2000 finite element analysis program. A major conclusion from this study is that non-uniform subgrade/subbase stiffness increases localized deflections and causes principal stress concentrations in the pavement, which can lead to fatigue cracking and other types of pavement distresses. Field data show that hydrated fly ash, self-cementing fly ash-stabilized subgrade, and granular subbases exhibit lower variability than natural subgrade soils. Pavement life should be increased through the use of more uniform subgrade support. Subgrade/subbase construction in the future should consider uniformity as a key to long-term pavement performance.

The Effect of Two Speed Endurance Training Regimes on Performance of Soccer Players.

In order to better understand the specificity of training adaptations, we compared the effects of two different anaerobic training regimes on various types of soccer-related exercise performances. During the last 3 weeks of the competitive season, thirteen young male professional soccer players (age 18.5±1 yr, height 179.5±6.5 cm, body mass 74.3±6.5 kg) reduced the training volume by ~20% and replaced their habitual fitness conditioning work with either speed endurance production (SEP; n = 6) or speed endurance maintenance (SEM; n = 7) training, three times per wk. SEP training consisted of 6-8 reps of 20-s all-out running bouts followed by 2 min of passive recovery, whereas SEM training was characterized by 6-8 x 20-s all-out efforts interspersed with 40 s of passive recovery. SEP training reduced (p<0.01) the total time in a repeated sprint ability test (RSAt) by 2.5%. SEM training improved the 200-m sprint performance (from 26.59±0.70 to 26.02±0.62 s, p<0.01) and had a likely beneficial impact on the percentage decrement score of the RSA test (from 4.07±1.28 to 3.55±1.01%) but induced a very likely impairment in RSAt (from 83.81±2.37 to 84.65±2.27 s). The distance covered in the Yo-Yo Intermittent Recovery test level 2 was 10.1% (p<0.001) and 3.8% (p<0.05) higher after SEP and SEM training, respectively, with possibly greater improvements following SEP compared to SEM. No differences were observed in the 20- and 40-m sprint performances. In conclusion, these two training strategies target different determinants of soccer-related physical performance. SEP improved repeated sprint and high-intensity intermittent exercise performance, whereas SEM increased muscles' ability to maximize fatigue tolerance and maintain speed development during both repeated all-out and continuous short-duration maximal exercises. These results provide new insight into the precise nature of a stimulus necessary to improve specific types of athletic performance in trained young soccer players.

Effect of short-duration lipid supplementation on fat oxidation during exercise and cycling performance.

The effect of intramyocellular lipids (IMCLs) on endurance performance with high skeletal muscle glycogen availability remains unclear. Previous work has shown that a lipid-supplemented high-carbohydrate (CHO) diet increases IMCLs while permitting normal glycogen loading. The aim of this study was to assess the effect of fat supplementation on fat oxidation (Fox) and endurance performance. Twenty-two trained male cyclists performed 2 simulated time trials (TT) in a randomized crossover design. Subjects cycled at ∼53% maximal voluntary external power for 2 h and then followed 1 of 2 diets for 2.5 days: a high-CHO low-fat (HC) diet, consisting of CHO 7.4 g·kg(-1)·day(-1) and fat 0.5 g·kg(-1)·day(-1); or a high-CHO fat-supplemented (HCF) diet, which was a replication of the HC diet with ∼240 g surplus fat (30% saturation) distributed over the last 4 meals of the diet period. On trial morning, fasting blood was sampled and Fox was measured during an incremental exercise; a ∼1-h TT followed. Breath volatile compounds (VOCs) were measured at 3 time points. Mental fatigue, measured as reaction time, was evaluated during the TT. Plasma free fatty acid concentration was 50% lower after the HCF diet (p < 0.0001), and breath acetone was reduced (p < 0.05) "at rest". Fox peaked (∼0.35 g·kg(-1)) at ∼42% peak oxygen consumption, and was not influenced by diet. Performance was not significantly different between the HCF and HC diets (3369 ± 46 s vs 3398 ± 48 s; p = 0.39), nor were reaction times to the attention task and VOCs (p = NS for both). In conclusion, the short-term intake of a lipid supplement in combination with a glycogen-loading diet designed to boost intramyocellular lipids while avoiding fat adaptation did not alter substrate oxidation during exercise or 1-hour cycling performance.

Association between the increase in brain temperature and physical performance at different exercise intensities and protocols in a temperate environment

There is evidence that brain temperature (Tbrain) provides a more sensitive index than other core body temperatures in determining physical performance. However, no study has addressed whether the association between performance and increases in Tbrain in a temperate environment is dependent upon exercise intensity, and this was the primary aim of the present study. Adult male Wistar rats were subjected to constant exercise at three different speeds (18, 21, and 24 m/min) until the onset of volitional fatigue. Tbrain was continuously measured by a thermistor inserted through a brain guide cannula. Exercise induced a speed-dependent increase in Tbrain, with the fastest speed associated with a higher rate of Tbrain increase. Rats subjected to constant exercise had similar Tbrain values at the time of fatigue, although a pronounced individual variability was observed (38.7-41.7°C). There were negative correlations between the rate of Tbrain increase and performance for all speeds that were studied. These results indicate that performance during constant exercise is negatively associated with the increase in Tbrain, particularly with its rate of increase. We then investigated how an incremental-speed protocol affected the association between the increase in Tbrain and performance. At volitional fatigue, Tbrain was lower during incremental exercise compared with the Tbrain resulting from constant exercise (39.3±0.3 vs 40.3±0.1°C; P<0.05), and no association between the rate of Tbrain increase and performance was observed. These findings suggest that the influence of Tbrain on performance under temperate conditions is dependent on exercise protocol.