Longer static flexion duration elicits a neuromuscular disorder in the lumbar spine.

The objective of this study was to assess the impact of two sequential long, static, anterior lumbar flexions on the development of a neuromuscular disorder and to compare it with previously obtained data from a series of short static flexion periods of the same cumulative time (Sbriccoli P, Solomonow M, Zhou BH, Baratta RV, Lu Y, Zhu MP, and Burger EL, Muscle Nerve 29: 300-308, 2004). Static flexions with loads of 20, 40, and 60 N were applied to the lumbar spine over two 30-min periods with a 10-min rest in between. The reflex EMG activity from the multifidus muscles and supraspinous ligament displacement (creep) was recorded during the flexion periods. Creep and EMG were also monitored over 7 h of rest following the work-rest-work cycle. It was found that the creep that developed in the first 30-min flexion period did not recover completely during the following 10 min of rest, giving rise to a large cumulative creep at the end of the work-rest-work session. Spasms were frequently seen within the EMG during the static flexion. Initial and delayed hyperexcitabilities were observed in all of the preparations at any of the three loads explored during the 7-h rest period. ANOVA revealed a significant effect of time (P < 0.0001) on the postloading data. Larger loads elicited larger magnitudes of the initial and delayed hyperexcitabilities, yet were not statistically different. It was concluded that the 3:1 work-to-rest duration ratio resulted in a neuromuscular disorder, regardless of the load magnitude. The conclusions are reinforced in view of the results from a previous study using 60 min of flexion overall but at 1:1 work-to-rest ratio in which only the highest load elicited a delayed hyperexcitability (Sbriccoli et al., Muscle Nerve 29: 300-308, 2004). An optimal dose-to-duration ratio needs to be established to limit, attenuate, or prevent the adverse effects of static load on the lumbar spine while considering the loading duration as a major risk factor.     

Work to rest durations ratios exceeding unity are a risk factor for low back disorder; a feline model.

Low back disorders are prominent among the work force engaged in static anterior flexion during the workday. As a continuing part of a long-term research aimed to identify the biomechanical and physiological processes and corresponding risk factors leading to such cumulative trauma disorder (CTD), we ventured to assess the effect of rest and the work-to-rest duration ratios that may prevent CTD. Three groups of the feline model were subjected to three load/rest paradigms: two 30 min loading periods spaced by 10 min rest in Group I, two 30 min loading period spaced by 30 min rest in Group II and one 60 min loading period for Group III. The cumulative loading duration in the three groups was 60 min. Each of the groups were allowed 7h of rest while monitoring EMG and lumbar viscoelastic tissue creep each hour. The results demonstrate that for two 30 min load periods with a 30 min in between rest, an acute neuromuscular disorder was not present whereas for two 30 min loading with a 10 min rest it was. Similarly, for a 60 min loading with long-term rest, the disorder was present. Post hoc Fisher analysis demonstrated significant differences in the delayed hyperexcitability between the first and second group (P<0.0001) and the third and second (P<0.0001) group. Statistical difference in the displacement data of the three groups was not present. ANOVA showed a significant effect of time post-loading (P<0.0001 and different rest durations (P<0.0001) on the EMG data during the 7h recovery. The new data allow us to conclude that a work-to-rest duration ratio of 1:1 can prevent the development of CTD as long as the work periods are not too long (<60 min). Longer static flexion durations do not respond favorably to rest even if it is of equal or longer duration. It is suggested that appropriate durations of rest may be a viable tool to avert CTD in a certain range whereas long static flexion durations should be avoided at all cost.     

Influence of the knee flexion on muscle activation and transmissibility during whole body vibration

The influence of the knee flexion on muscle activation and transmissibility during whole body vibration is controversially discussed in the literature. In this study, 34 individuals had electromyography activity (EMG) of the vastus lateralis and the acceleration assessed while squatting with 60° and 90° of knee flexion either with or without whole-body vibration (WBV). The conditions were maintained for 10 s with 1 min of rest between each condition. The main findings were (1) the larger the angle of knee flexion (90° vs. 60°), the greater the EMG (p < 0.001), with no difference on acceleration transmissibility; (2) for both angles of knee flexion, the addition of WBV produced no significant difference in EMG and higher acceleration compared to without WBV (p < 0.001). These results suggest that the larger the knee flexion angle (60° vs. 90°), the greater the muscle activation without acceleration modification. However, the addition of WBV increases the transmissibility of acceleration in the lower limbs without modification in EMG of vastus lateralis.     

Muscular dysfunction elicited by creep of lumbar viscoelastic tissue.

The biomechanics, histology and electromyography of the lumbar viscoelastic tissues and multifidus muscles of the in vivo feline were investigated during 20 min of static as well as cyclic flexion under load control and during 7 h of rest following the flexion. It was shown that the creep developed in the viscoelastic tissues during the 20 min of static or cyclic flexion did not fully recover over the 7 h of following rest. It was further seen that a neuromuscular disorder with five distinct components developed during and after the static and cyclic flexion. The neuromuscular disorder consisted of a decreasing magnitude of reflexive EMG from the multifidus upon flexion as well as of superimposed spasms. The recovery period was characterized by an initial muscle hyperexcitability, a slowly increasing reflexive EMG and a delayed hyperexcitability. Histological data from the supraspinous ligament demonstrate significant increase (x 10) in neutrophil density in the ligament 2 h into the recovery and even larger increase (x 100) 6 h into the recovery from the 20 min flexion, indicating an acute soft tissue inflammation. It was concluded that sustained static or cyclic loading of lumbar viscoelastic tissues may cause micro-damage in the collagen structure, which in turn reflexively elicit spasms in the multifidus as well as hyperexcitability early in the recovery when the majority of the creep recovers. The micro-damage, however, results in the time dependent development of inflammation. In all cases, the spasms, initial and delayed hyperexcitabilities represent increased muscular forces applied across the intervertebral joints in an attempt to limit the range of motion and unload the viscoelastic tissues in order to prevent further damage and to promote healing. It is suggested that a significant insight is gained as to the development and implications of a common idiopathic low back disorder as well as to the development of cumulative trauma disorders.     

Intramuscular pressure and EMG relate during static contractions but dissociate with movement and fatigue.

Intramuscular pressure (IMP) and electromyography (EMG) mirror muscle force in the nonfatigued muscle during static contractions. The present study explores whether the constant IMP-EMG relationship with increased force may be extended to dynamic contractions and to fatigued muscle. IMP and EMG were recorded from shoulder muscles in three sessions: 1). brief static arm abductions at angles from 0 to 90 degrees, with and without 1 kg in the hands; 2). dynamic arm abductions at angular velocities from 9 to 90 degrees /s, with and without 1 kg in the hands; and 3). prolonged static arm abduction at 30 degrees for 30 min followed by recovery. IMP and EMG increased in parallel with increasing shoulder torque during brief static tasks. During dynamic contractions, peak IMP and EMG increased to values higher than those during static contractions, and EMG, but not IMP, increased significantly with speed of abduction. In the nonfatigued supraspinatus muscle, a linear relationship was found between IMP and EMG; in contrast, during fatigue and recovery, significant timewise changes of the IMP-to-EMG ratio occurred. The results indicate that IMP should be included along with EMG when mechanical load sharing between muscles is evaluated during dynamic and fatiguing contractions.     

Electromyographic evaluation of masseter and anterior temporalis muscles in rest position of edentulous patients with temporomandibular disorders, before and after using complete dentures with sliding plates

Objectives: This study was performed with the purpose of investigating electromyographic (EMG) activity of the anterior temporalis and masseter muscles in edentulous individuals with temporomandibular disorder (TMD), before and after using sliding plates on complete dentures in the mandibular rest position. Background: Edentulous patients may present TMD, which is characterised by pain in temporomandibular joints, masticatory and neck muscles, uncoordinated and limited mandible movements, joint sounds and an altered occlusal relationship. It is imperative to offer treatment in order to re‐establish stomatognathic system structures before submitting the individual to any definitive restorative treatment. Materials and methods: The patients were edentulous for at least 10 years. EMG recordings were made before the insertion of the dentures (0 months) and also after using the sliding plates at the fourth month, 9th month and 12th month, using computerised electromyography K6‐I/ EMG Light Channel Surface. EMG evaluations of the muscles were performed under the following clinical conditions: rest position with dentures (R1), rest position without dentures (R2), rest position with dentures post‐activity (chewing) (R3), rest position without dentures post‐activity (chewing) (R4). Results: All patients obtained remission of muscular fatigue and reduced pain in stomatognathic system structures. Temporalis muscle showed significant increase in EMG activity compared with initial values (p < 0.01). Masseter muscles showed significantly lower mean values (p < 0.01) compared with initial values. Conclusion: The sliding plates allowed the process of neuromuscular deprogramming, contributing to muscular balance of the masticatory system, and are therefore indicated to be used before the fabrication of definitive complete dentures in patients with TMD.     

Frequency-dependent changes in neuromuscular responses to cyclic lumbar flexion

Repetitive lifting in the workplace has been identified to be a cause of low back disorders. Epidemiologic data further supports an hypothesis that higher repetition rate (i.e. frequency) is an added risk factor. The objective of this study was to provide experimental data testing the above hypothesis. An in vivo feline model was subjected to 20-min of cyclic lumbar loading at frequencies of 0.1 Hz and 0.5 Hz while monitoring the EMG from the L-3/4-L-5/6 multifidus muscles and the creep at the L-4/5 level. Seven hours of rest were allowed after the cyclic flexion/extension was terminated. During this rest period, a single test cycle was performed every hour to assess recovery of EMG and lumbar creep. The results demonstrate that cyclic lumbar flexion elicits a transient neuromuscular disorder consisting of EMG spasms during the cyclic loading and initial and delayed muscular hyperexcitabilities during the rest period. Cyclic loading at 0.5 Hz resulted in significant (p<0.05) increase in the hyperexcitability magnitude and duration during the recovery period. It was concluded that repetitive lumbar loading at fast rates is indeed a risk factor as it induces larger creep in the lumbar viscoelastic tissues which in turn intensify the resulting neuromuscular disorder.     

Impact of the contraction of medial forearm muscles during grip tasks in different forearm positions on medial support at the elbow joint

We measured the medial joint distance (MJD), activity of muscles involved in medial elbow-joint support, and grip strength, aiming to examine the supportfunction of muscles.MJD was measured in supinated and pronated positions of the forearm of 10 participants under three conditions: at rest (R), under valgus load on the elbow joint (L), and under valgus load on the elbow joint during the grip task (L-grip). Under the L-grip condition, electromyography was performed on flexor digitorum superficialis (FDS), pronator teres (PT), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU); subsequently, normalized integrated electromyograms (NIEMG) were calculated. Under the L-grip condition, MJD was shorter in the pronated position than in the supinated position (p < 0.001); however, grip strength was lower in the pronated position. NIEMG of FDS was 90% in both positions, and those of FCR and FCU were low at 10%. However, that of PT was 3.6% in the supinated position and 40.9% in the pronated position, showing higher NIEMG in the pronated position (p < 0.001). Medial support during grip tasks was higher in the pronated position probably because PT activity compensates for the decreased FDS activity.     

Biofeedback modification of frontal EMG in normal subjects

We carried out a controlled study on the voluntary control of the frontalis muscle by biofeedback procedures employing 20 normal subjects. Subjects were randomly divided into two groups of 10: (1) the biofeedback group and (2) the control group. Each of the two groups received five training sessions of about 40 minutes' duration each on different days. The results obtained are as follows: (1) In the biofeedback group, mean EMG levels decreased progressively and markedly from 2.16µVp-p min in the first session to 1.54µVp-p min in the last session. On the contrary, the control group did not show constant decreases in EMG levels over sessions. (2) The changes in the heart rate did not correlate with the changes in EMG activity. (3) The changes in the respiratory rate correlated with the changes in EMG activity.     

The effect of reward duration on demand functions for rest in dairy heifers and lying requirements as measured by demand functions

The present experiment investigated the effect of reward duration on elasticity of the demand function for rest, and assessed for how long dairy heifers are motivated to lie on a 24-h basis. In Part 1, eight heifers housed in tether stalls had free access to lie down for 9 h daily, while they could work for access to lie during 6-h daily test sessions. The price of access to lie down was varied by varying the fixed ratio of pressing a panel per reward (FR) from 10 to 50, and the reward duration was 20, 30, 50 or 80 min. The reward duration affected the elasticity of the demand function. Based on the number of rewards earned, the elasticity of the demand function for 20-min rewards was more elastic (elasticity −0.33) than the demand functions for the longer reward duration periods (elasticity −0.07 to −0.08; P < 0.001). Also based on the lying time during test sessions, the elasticity of demand function for 20-min rewards was more elastic (elasticity −0.32) than demand functions for the longer reward duration periods (elasticity −0.01 to −0.10; P < 0.001). Heifers interrupted lying during the reward periods more often, the longer the reward duration (P < 0.001). With a reward duration of 50 min, heifers earned approx. 4 h of lying in addition to the 9 h where they were free to rest, i.e. they could lie down for a total of 13 h. In Part 2, the same dairy heifers were free to rest for 6 h and could work for access to lie during 12 h daily. Reward duration was always 50 min. Here heifers earned approx. 7 h of lying during the test sessions (elasticity −0.07). During the 6 h where the heifers were free to rest they lay down during on average 5.5 h, i.e. total lying time was on average 12.5 h. This suggests that heifers of this age and stage of pregnancy have an inelastic demand for rest of about 12–13 h per 24 h.     

Effect of ischemia-reperfusion on the post-rest inotropy of isolated perfused rat heart

This paper aims to study of the effects of ischemia‐reperfusion on the post‐rest inotropy and to characterize post‐rest B1:B2 ratio as an index of intracellular Ca²⁺ overload. When the rest interval between the cardiac beats is increased, the magnitude of the post‐rest beats is increased. First beat (B1) is maximally potentiated with exponental decline of the second (B2) and subsequent beats, thereby establishing a normal B1:B2 ratio of post‐ rest inotropy of the cardiac muscle. The rest potentiation of B1 and subsequent decay in the magnitude B2 is thought to develop from the time‐dependent changes in the Ca²⁺‐uptake and release from the sarcoplasmic reticulum (SR). Ca²⁺‐kinetics of SR can be modulated by a variety of interventions which produce Ca²⁺ loading of the SR. Methods: Isolated perfused (K‐H buffer, 34°C) rat hearts were paced at 1 Hz (steady state frequency). Interbeat intervals between 1s and 10s were introduced and the recovery in the left ventricular contractile force (Pmax) of post‐rest B1 and B2 for each interval was recorded. Their relative relationship was computed and compared under control and experimental conditions. Results: High extracellular Ca²⁺ (2.50 to 7.0 mM) or low extracellular Na⁺ (50% of control), and ischemia (60 min, 34°C) ‐ reperfusion (30 min, 34°C) caused the reversal of the control relationship of the B1 to B2, with B2 being more potentiated than B1, accompanied by the appearance of after‐contractions during the rest intervals of 4s or more. The mean (± SE) control B1:B2 ratio (at 4s interval) of 1.12 ± 0.05 was significantly (P<0.001) reduced to 0.93 ± 0.07; 0.89 ± 0.01; and 0.96 ± 0.02 after high Ca²⁺ (6 mM) perfusion, low Na⁺(50%) perfusion and ischemia‐reperfusion respectively. Simultaneous perfusion with ryanodine (1 μM) abolished the after‐contractions and significantly increased the reduced ratios. The time course of changes in B1:B2 ratio after graded ischemia‐reperfusion showed a significant fall in the ratio between 30 and 60 min of ischemia. A parallel change in Pmax and a significant rise in the left ventricular end‐diastolic pressure, indicating an irreversible phase of the injury was recorded. No significant changes in B1:B2 ratio were detected during the reversible phase (<30 min) of the ischemia‐reperfusion injury. Conclusions: Ischemia‐reperfusion induces significant alterations in the relative ratio of the post‐rest contractions of the left ventricle in isolated perfused rat heart. The altered ratios were characterized to predict the irreversibility of the reperfusion injury and to index the extent of Ca²⁺‐loading of the sarcoplasmic reticulum.     

Biofeedback modification of frontal EMG in normal subjects

We carried out a controlled study on the voluntary control of the frontalis muscle by biofeedback procedures employing 20 normal subjects. Subjects were randomly divided into two groups of 10: (1) the biofeedback group and (2) the control group. Each of the two groups received five training sessions of about 40 minutes' duration each on different days. The results obtained are as follows: (1) In the biofeedback group, mean EMG levels decreased progressively and markedly from 2. 16 muVp-p min the first session to 1.54 muVp-p min in the last session. On the contrary, the control group did not show constant decreases in EMG levels over sessions. (2) The changes in the heart rate did not correlate with the changes in EMG activity. (3) The changes in the respiratory rate correlated with the changes in EMG activity.     

Post‐Exercise Blood Pressure Reduction Is Greater Following Intermittent Than Continuous Exercise and Is Influenced Less by Diurnal Variation

Recently, we reported that circadian variation exists in the response of blood pressure (BP) following a bout of uninterrupted exercise. The usual phenomenon of post‐exercise hypotension was absent or reversed when such exercise was performed between 04:00–08:00 h. Nevertheless, research examining BP changes following bouts of intermittent exercise at different times of the day is scarce, even though this type of activity is probably more popular. Therefore, we aimed to compare post‐exercise BP reductions of continuous (CONT) and intermittent (INT) exercise protocols performed at 08:00 h and 16:00 h. At both of these times of day, eight normotensive males completed 30 min of continuous cycling in the CONT and three 10 min bouts of cycling separated by 10 min of rest in the INT protocol. The exercise intensity was set at 70% V˙O_2peak during both protocols. Heart rate, systolic (S) and diastolic (D) BP, and mean arterial pressure (MAP) were measured 5 min before and 20 min after exercise. Changes from pre‐exercise baseline were analyzed using linear mixed modeling. MAP was 8±1 mm Hg lower following INT compared with CONT exercise (p<0.05). SBP and DBP were also significantly lower following INT compared with CONT exercise (p<0.05). Diurnal variation in MAP was evident, with attenuated hypotension being observed after morning exercise (p<0.05), although this diurnal variation was less marked following INT compared with CONT exercise (p<0.05). We conclude that intermittent exercise mediates greater post‐exercise hypotension compared with a single continuous bout of equivalent work and that this protocol‐dependent difference is greatest in the afternoon. Therefore, a bout of afternoon exercise that is occasionally interrupted with short rest periods is recommended for lowering BP acutely.     

Diurnal Variations in Cycling Kinematics

Physiological and biomechanical constraints as well as their fluctuations throughout the day must be considered when studying determinant factors in the preferred pedaling rate of elite cyclists. The aim of this study was to monitor the diurnal variation of spontaneous pedaling rate and movement kinematics over the crank cycle. Twelve male competitive cyclists performed a submaximal exercise on a cycle ergometer for 15 min at 50% of their W_max. Two test sessions were performed at 06:00 and 18:00 h on two separate days to assess diurnal variation in the study variables. For each test session, the exercise bout was divided into three equivalent 5‐min periods during which subjects were requested to use different pedal rates (spontaneous cadence, 70 and 90 rev min^?1). Pedal rate and kinematics data (instantaneous pedal velocity and angle of the ankle) were collected. The results show a higher spontaneous pedal rate in the late afternoon than in the early morning (p < 0.001). For a given pedal rate condition, there was a less variation in pedal velocity during a crank cycle in the morning than in the late afternoon. Moreover, diurnal variations were observed in ankle mobility across the crank cycle, the mean plantar flexion observed throughout the crank cycle being greater in the 18:00 h test session (p < 0.001). These results suggest that muscular activation patterns during a cyclical movement could be under the influence of circadian fluctuations.     

Adaptations during familiarization to resistive exercise.

This study focused on adaptations during familiarization to resistive exercise. It was also determined if familiarization requires one or more sessions. Twenty-six sedentary, college-aged females were matched and randomly assigned to one of two groups. Measurements were obtained during the initial familiarization period (Group 1: 15 trials on 1 day, Group 2: 5 trials on each of three consecutive days), and during retention tests scheduled two weeks and 3 months after the first test session. Elbow flexion torque and surface electromyography (SEMG) of the biceps and triceps were monitored concurrently. There were no significant differences between groups for any of the criterion measures. There was a significant (p<0.05) increase (12.4 Nm, or 38.8%) in maximal isometric elbow flexion torque. Biceps (agonist) root-mean-square (RMS) SEMG exhibited a significant (p<0.05) increase of 95 microV (29%). Triceps (antagonist) RMS SEMG underwent alternating decreases then increases, and each change was significant (p<0.05). The ratio of biceps to triceps RMS SEMG was used to assess cocontraction, and it followed the same pattern of change as triceps RMS SEMG. We concluded that both groups responded in the same way to testing, regardless of the pattern of the first 15 contractions. The increase in maximal isometric elbow flexion torque was due to neural drive to the bicep (agonist). There was a low level of triceps (antagonist) cocontraction to provide joint stability, and it was adjusted throughout the duration of testing.     

Yawning: A possible confounding variable in EMG biofeedback studies

An elderly hemiplegic patient participating in an EMG biofeedback training program was observed to produce a synergistic flexion movement of the plegic (determined by functional evaluations) upper limb while yawning. In the course of the training sessions the electrical activity of the anterior deltoid (the target muscle) was recorded during yawning. These peak EMG values were greatly facilitated in comparison with the session mean peak values obtained during an attempted maximum voluntary isometric contraction (shoulder flexion) of the same limb (e.g., Trial 1: 85.00 vs. 4.33 µV). The possibility of yawning as a confounding variable in EMG biofeedback studies is presented and discussed.This research was supported in part by Health and Welfare Canada (NHRDP).     

Assessment of work-related muscle strain by using surface EMG during test contractions interposed between work periods of simulateted mushroom picking

Surface electromyograms(EMG) during test contractions (TCs) were studied to assess the muscle strain in simulated mushroom picking. Additionally, the duration of the TC for the effective assessment was investigated. Nine female subjects performed standardized shoulder abduction and a stooped posture for one minute as TCs. Each experiment consisted of a 60-min rest, three work periods (W1-W3), a 30-min rest, and two work periods (W4 and W5) separated by a 30-min rest period. The duration of each work period was about 20 min. A total of 18 TCs was performed between the work periods and every 10 minutes in the rest periods. EMGs were recorded from the trapezius, infraspinatus, deltoid, and erector spinae muscles. The amplitude of EMG (AEMG) and mean power frequency (MPF) of EMG were calculated. Each TC was divided equally into three parts. Ratings of perceived exertion (RPE) in the neck, shoulder and low-back were reported during TCs. The work increased RPE of all the parts. AEMG and RPE were increased and MPF was decreased by W1, W2 and W3 in the neck and shoulder muscles. MPF of the erector spinae was increased by the work. The results were not affected by the duration of TCs and the parts during the TCs. AEMG and MPF fluctuated before W1 although the changes of RPE were small. Averaging several TCs was recommended to get stable results from TCs. EMG changes and appropriate TC conditions were discussed in relation to the adaptation in fatiguing contractions.     

An assessment of the reliability and standardisation of tests used to elicit reference muscular actions for electromyographical normalisation

Prior to implementing a normalisation method, the standardisation and reliability of the method needs to be examined. This investigation aimed to assess the reliability of EMG amplitudes and test outputs from proposed normalisation methods for the triceps surae. Sixteen participants completed isometric (maximum and sub-maximum); isokinetic (1.05 rad/s, 1.31 rad/s and 1.83 rad/s) squat jump and 20 m sprint conditions, on 3 separate occasions over 1 week. The EMG data was collected from the medial and lateral gastrocnemius (MG and LG) and soleus (SOL). Log transformed typical error measurements (TEM_CV%) assessed EMG signal and test output reliability across the three sessions. Only the squat jump provided acceptable EMG reliability for all muscles both between days (SOL: 13%; MG: 14.5%; LG: 11.8%) and between weeks (SOL: 14.5%; MG: 12.9%; LG: 8.9%), with the sprint only showing poor reliability in the LG between days (16.3%). Acceptable reliability for the isometric and isokinetic conditions were muscle and re-test period dependant. Reliable output was found for the squat jump (4.1% and 3.6%), sprint (0.8% and 0.6%) and 1RM plantar flexion test (2.8% and 3.5%) between days and weeks, respectively. Isokinetic plantar flexion displayed poor reliability at all velocities between days and weeks. It was concluded that the squat jump provides a standardised and reproducible reference EMG value for the triceps surae for use as a normalisation method.     

Yawning: a possible confounding variable in EMG biofeedback studies

An elderly hemiplegic patient participating in an EMG biofeedback training program was observed to produce a synergistic flexion movement of the plegic (determined by functional evaluations) upper limb while yawning. In the course of the training sessions the electrical activity of the anterior deltoid (the target muscle) was recorded during yawning. These peak EMG values were greatly facilitated in comparison with the session mean peak values obtained during an attempted maximum voluntary isometric contraction (shoulder flexion) of the same limb (e.g., Trial 1: 85.00 vs. 4.33 microV). The possibility of yawning as a confounding variable in EMG biofeedback studies is presented and discussed.     

Caffeine supplementation improves physical performance without affecting fatigue level: a double-blind crossover study

This study examined the effect of caffeine supplementation (CAFF) in a Wingate test (WT), and the behaviour of blood lactate concentrations (BLa) and neuromuscular fatigue (NMF), measured as reduced countermovement jump (CMJ) performance, in response to the WT. In a double-blind crossover study, 16 participants attended the laboratory twice, separated by a 72-hour window. In the sessions, participants first ingested 6 mg·kg^-1 of either CAFF or placebo (PLAC), and then performed a WT. BLa was measured before (L-pre), and 0.5 min (L-post-0.5) and 3.5 min (L-post-3.5) after conducting the WT. The CMJ test was conducted before (CMJ pre), after (CMJ post), and 3 min after completing (CMJ post-3) the WT. The results indicated that CAFF enhanced peak power (Wpeak: + 3.22%; p = 0.040), time taken to reach Wpeak (T_Wpeak: -18.76%; p = 0.001) and mean power (Wmean: + 2.7%; p = 0.020). A higher BLa was recorded for CAFF at L-post-0.5 (+ 13.29%; p = 0.009) and L-post-3.5 (+ 10.51%; p = 0.044) compared to PLAC. CAFF improved peak power (PP; + 3.44%; p = 0.003) and mean power (MP; + 4.78%; p = 0.006) at CMJ pre, compared to PLAC, whereas PP and MP were significantly diminished at CMJ post and CMJ post-3 compared to pre (p < 0.001 for all comparisons) under both the CAFF and PLAC conditions. PP and MP were increased at post-3 compared to post (p < 0.001 for all comparisons) for both conditions. In conclusion, CAFF increased WT performance and BLa without affecting NMF measured by CMJ. Thus, CAFF may allow athletes to train with higher workloads and enhance the supercompensation effects after an adequate recovery period.