Handgrip Contractions Research Articles

Single-Trial EEG-EMG Coherence Analysis Reveals Muscle Fatigue-Related Progressive Alterations in Corticomuscular Coupling

Voluntary muscle fatigue is a progressive process. A recent study demonstrated muscle fatigue-induced weakening of functional corticomuscular coupling measured by coherence between the brain [electroencephalogram (EEG)] and muscle [electromyogram (EMG)] signals after a relatively long-duration muscle contraction. Comparing the EEG-EMG coherence before versus after fatigue or between data of two long-duration time blocks is not adequate to reveal the dynamic nature of the fatigue process. The purpose of this study was to address this issue by quantifying single-trial EEG-EMG coherence and EEG, EMG power based on wavelet transform. Eight healthy subjects performed 200 maximal intermittent handgrip contractions in a single session with handgrip force, EEG and EMG signals acquired simultaneously. The EEG and EMG data during each 2-s handgrip was subjected to single trial EEG-EMG wavelet energy spectrum and coherence computation. The EEG-EMG coherence and energy spectrum at beta (15 ~ 35 Hz) and gamma (35-50 Hz) frequency bands were statistically analyzed in 2-block (75 trials per block), 5-block (30 trials/block), and 10-block (15 trials/block) data settings. The energy of both the EEG and EMG signals decreased significantly with muscle fatigue. The EEG-EMG coherence had a significant reduction for the 2-block comparison. More detailed dynamical changing and inter-subject variation of the EEG-EMG coherence and energy were revealed by 5- and 10-block comparisons. These results show feasibility of wavelet transform-based measurement of the EEG-EMG coherence and corresponding energy based on single-trial data, which provides extra information to demonstrate a time course of dynamic adaptations of the functional corticomuscular coupling, as well as brain and muscle signals during muscle fatigue.

Impact of increased muscle sympathetic nerve activity on conduit artery shear rate patterns

Disturbed flow patterns induce a proatherogenic endothelial cell phenotype; however, the factors accountable for the oscillatory shear profiles in peripheral conduit arteries are not fully understood. We tested the hypothesis that acute elevations in muscle sympathetic nerve activity (MSNA) are accompanied by increases in conduit artery retrograde shear and oscillatory shear index (OSI). Fourteen healthy men (25±1 yrs) performed three sympatho‐excitatory maneuvers: progressive lower body negative pressure (LBNP) from −10 to −40 Torr, cold pressor test (CPT), and 35% maximal voluntary contraction handgrip followed by postexercise ischemia (PEI). MSNA, arterial blood pressure (BP), and brachial artery velocity and diameter were recorded continuously. All maneuvers elicited significant increases in MSNA total activity from baseline; whereas mean BP was only increased during CPT and PEI (p<0.05). Interestingly, retrograde shear (−3.96±1.2 baseline vs. −8.15±1.8 s−1 −40 LBNP, p<0.05) and OSI (0.09±0.02 baseline vs. 0.20±0.02 a.u. −40 LBNP, p<0.05) were primarily increased during LBNP in which elevated MSNA was not accompanied by increases in BP. Together, these data suggest that elevated MSNA may promote an increase in retrograde and oscillatory shear; however, these effects may be influenced by concomitant changes in BP. Support: NIH RO1HL093167, HL52490

Increased muscle sympathetic nerve activity acutely alters conduit artery shear rate patterns

Escalating evidence indicates that disturbed flow patterns, characterized by the presence of retrograde and oscillatory shear stress, induce a proatherogenic endothelial cell phenotype; however, the mechanisms underlying oscillatory shear profiles in peripheral conduit arteries are not fully understood. We tested the hypothesis that acute elevations in muscle sympathetic nerve activity (MSNA) are accompanied by increases in conduit artery retrograde and oscillatory shear. Fourteen healthy men (25 +/- 1 yr) performed three sympathoexcitatory maneuvers: graded lower body negative pressure (LBNP) from 0 to -40 Torr, cold pressor test (CPT), and 35% maximal voluntary contraction handgrip followed by postexercise ischemia (PEI). MSNA (microneurography; peroneal nerve), arterial blood pressure (finger photoplethysmography), and brachial artery velocity and diameter (duplex Doppler ultrasound) in the contralateral arm were recorded continuously. All maneuvers elicited significant increases in MSNA total activity from baseline (P < 0.05). Retrograde shear (-3.96 +/- 1.2 baseline vs. -8.15 +/- 1.8 s(-1), -40 LBNP, P < 0.05) and oscillatory shear index (0.09 +/- 0.02 baseline vs. 0.20 +/- 0.02 arbitrary units, -40 LBNP, P < 0.05) were progressively augmented during graded LBNP. In contrast, during CPT and PEI, in which MSNA and blood pressure were concomitantly increased (P < 0.05), minimal or no changes in retrograde and oscillatory shear were noted. These data suggest that acute elevations in MSNA are associated with an increase in conduit artery retrograde and oscillatory shear, an effect that may be influenced by concurrent increases in arterial blood pressure. Future studies should examine the complex interaction between MSNA, arterial blood pressure, and other potential modulatory factors of shear rate patterns.

Evoked myotonia can be “dialed‐up” by increasing stimulus train length in myotonic dystrophy type 1

It is unknown how evoked myotonia varies with stimulus frequency or train length, or how it compares to voluntary myotonia in myotonic dystrophy type 1 (DM1). First dorsal interosseous (FDI) tetanic contractions evoked by trains of 10-20 ulnar nerve stimuli at 10-50 HZ were recorded in 10 DM1 patients and 10 normals. For comparison, maximum voluntary handgrip contractions were also recorded. An automated computer program placed cursors along the declining (relaxation) phase of the force recordings at 90% and 5% of peak force (PF) and calculated relaxation times (RTs) between these points. For all stimulus frequencies and train lengths, evoked RTs were much shorter, and evoked PFs were much greater in normals than in DM1. In normals, evoked RT was independent of stimulus frequency and train length, while in DM1 RT was longer for train lengths of 20 stimuli (mean: 9 s in DM1; 0.20 in normals) than for 10 stimuli (mean: 3 s in DM1, 0.19 in normals), but it did not change with stimulus frequency. In both groups PF increased greatly as stimulus frequency rose from 10-50 HZ but only slightly as train length rose from 10-20 stimuli. Voluntary handgrip RT (mean: 1.9 s) was less than evoked FDI RT (mean: 9 s). In DM1, evoked RT can be "dialed up" by increasing stimulus train length. Evoked myotonia testing utilizing a stimulus paradigm of at least 20 stimuli at 30-50 HZ may be useful in antimyotonic drug trials, particularly when grip RT is normal or equivocal.

Nonlinear features of surface EEG showing systematic brain signal adaptations with muscle force and fatigue

Nonlinear dynamics has been introduced to the analysis of biological data and increasingly recognized to be functionally relevant. The purpose of this study was to examine chaotic properties of human scalp EEG signals associated with voluntary motor tasks using the largest Lyapunov exponent (L1). 64-channel scalp EEG data were recorded from eight healthy subjects in two tasks: (1) intermittent handgrip contractions at 20, 40, 60, and 80% of maximal voluntary contraction (MVC) with 20 trials at each level. No significant fatigue were induced; (2) intermittent handgrip MVCs (100 trials) that resulted in significant fatigue. The L1 values of all EEG channels were calculated in each trial first then averaged across the 20 trials at each force level (Task 1) or over each of the 5-trial blocks (Task 2) before the group means were obtained. A multivariate statistical model was used to examine the effect of force and fatigue on L1. L1 values were greater with higher force (Task 1), and decreased significantly with fatigue (Task 2). The L1 of the EEG signals changes systematically and correlates significantly with muscle force and fatigue. The results suggest that nonlinear chaotic index L1 may serve as a quantitative measure for motor control-related cortical signal adaptations.

Immobilization‐induced increase in fatigue resistance is not explained by changes in the muscle metaboreflex

Immobilization has been reported to enhance fatigability, which is paradoxical in light of the metabolic and molecular alterations that occur in atrophied muscles. We examined whether the immobilization-induced enhancement in fatigability was associated with attenuation in the muscle metaboreflex response. Ten subjects were examined after 3 weeks of hand-forearm immobilization. The time to task failure of a handgrip contraction (20% intensity) was determined along with heart rate (HR) and mean arterial pressure (MAP) at rest, during the task and during a 2-min postexercise muscle ischemia (PEMI) test that continues to stimulate the metaboreflex. Immobilization decreased strength by 25% (P<0.01) and increased the time to task failure by 21% (P=0.03). However, no changes were observed for the HR and MAP responses to the exercise task or during PEMI (P>0.05). These findings indicate that the augmentation of time to task failure with immobilization is not associated with changes in the pressor or metaboreflex responses.

Assessing time-dependent association between scalp EEG and muscle activation: A functional random-effects model approach

This study investigates time-dependent associations between source strength estimated from high-density scalp electroencephalogram (EEG) and force of voluntary handgrip contraction at different intensity levels. We first estimate source strength from raw EEG signals collected during voluntary muscle contractions at different levels and then propose a functional random-effects model approach in which both functional fixed effects and functional random-effects are considered for the data. Two estimation procedures for the functional model are discussed. The first estimation procedure is a two-step method which involves no iterations. It can flexibly use different smoothing methods and smoothing parameters. The second estimation procedure benefits from the connection between linear mixed models and regression splines and can be fitted using existing software. Functional ANOVA is then suggested to assess the experimental effects from the functional point of view. The statistical analysis shows that the time-dependent source strength function exhibits a nonlinear feature, where a bump is detected around the force onset time. However, there is the lack of significant variations in source strength on different force levels and different cortical areas. The proposed functional random-effects model procedure can be applied to other types of functional data in neuroscience.

Heart Rate Responses at Onset of Contraction

We tested the hypothesis that the initial heart rate (HR) response at the onset of maximal handgrip contraction is altered after training. 17 volunteers (nine trained and eight controls) performed ten intermittent static handgrip contractions with maximal effort, alternating between 15-s contractions and 15-s pauses. High-intensity static handgrip training was performed using the nondominant arm alone for 4 weeks. Handgrip force (HGF) and HR were analyzed for the initial 7 s of every static handgrip exercise. Peak HR (pre-training: 94.5 +/- 12.8 beats/min; post-training: 89.7 +/- 10.2 beats/min, p < 0.05) decreased. However, the magnitude of HR change at the onset of contraction remained constant (pre-training: 23.0 +/- 7.7 beats/min; post-training: 25.7 +/- 6.5 beats/min, p = 0.0767), while the HR responses in the subsequent bouts increased after training (p < 0.001). The resting HR decreased (pre-training: 71.5 +/- 9.3 beats/min; post-training 64.1 +/- 5.7 beats/min, p < 0.05). Maximal HGF increased by 11.1 % in trained arms and by 8.7 % in untrained arms, although an increase in maximal forearm girth was only observed in the trained arm (2.0 %, p < 0.0001). Although high-intensity training modulated the abrupt HR responses, the magnitude of the response remained unchanged at the onset of maximal forearm contraction and the resting HR significantly decreased.

Wave reflection and central aortic pressure are increased in response to static and dynamic muscle contraction at comparable workloads

We determined the effects of static and dynamic muscle contraction at equivalent workloads on central aortic pressure and wave reflection. At random, 14 healthy men and women (23 +/- 5 yr of age) performed a static handgrip forearm contraction [90 s at 30% of maximal voluntary contraction (MVC)], dynamic handgrip contractions (1 contraction/s for 180 s at 30% MVC), and a control trial. During static and dynamic trials, tension-time index was controlled by holding peak tension constant. Measurements of brachial artery blood pressure and the synthesis of a central aortic pressure waveform (by radial artery applanation tonometry and generalized transfer function) were conducted at baseline, during each trial, and during 1 min of postexercise ischemia (PEI). Aortic augmentation index (AI), an index of wave reflection, was calculated from the aortic pressure waveform. AI increased during both static and dynamic trials (static, 5.2 +/- 3.1 to 11.8 +/- 3.4%; dynamic, 5.8 +/- 3.0 to 13.3 +/- 3.4%; P < 0.05) and further increased during PEI (static, 18.5 +/- 3.1%; dynamic, 18.6 +/- 2.9%; P < 0.05). Peripheral and central systolic and diastolic pressures increased (P < 0.05) during both static and dynamic trials and remained elevated during PEI. AI and pressure responses did not differ between static and dynamic trials. Peripheral and central pressures increased similarly during static and dynamic contraction; however, the rise in central systolic pressure during both conditions was augmented by increased wave reflection. The present data suggest that wave reflection is an important determinant of the central blood pressure response during forearm muscle contractions.

Functional changes in cerebral and paraspinal muscle physiology of healthy women during exposure to whole-body vibration

The objective of this study was to investigate the effects of whole-body vibration on multiple tissues simultaneously in fourteen healthy women. On three separate days, participants were exposed to frequencies, 3, 4.5, or 6 Hz (at 0.9 g r.m.s acceleration in vertical direction) per day on a simulator for 16 min. While sitting ‘with’ and ‘without’ backrest support, participants also performed handgrip contractions for 1 min. Cerebral and lumbar muscle oxygenation and blood volume responses were measured using near-infrared spectroscopy. Cardiorespiratory responses were collected using a metabolic cart. In general, cerebral and cardiorespiratory responses increased with vibration compared to without vibration, whereas in the lumbar region oxygenation and blood volume responses decreased. Greatest cerebral responses were observed at 6 Hz ( P < 0.05). When compared to exposure to vibration without performing work, significant decrease in lumbar responses was observed during handgrip contractions in both conditions of sitting ‘with’ and ‘without’ a backrest ( P < 0.05). Such decreases in the lumbar responses suggest postural load due to prolonged sitting combined with physical activity during vibration, might reduce vascular supply to the paraspinal muscles. This study reiterates the importance of understanding the physiological basis for various health disorders in women due to exposure to whole-body vibration.

Do vasoregulatory mechanisms in exercising human muscle compensate for changes in arterial perfusion pressure?

We tested the hypothesis that vasoregulatory mechanisms completely counteract the effects of sudden changes in arterial perfusion pressure on exercising muscle blood flow. Twelve healthy young subjects (7 female, 5 male) lay supine and performed rhythmic isometric handgrip contractions (2 s contraction/ 2 s relaxation 30% maximal voluntary contraction). Forearm blood flow (FBF; echo and Doppler ultrasound), mean arterial blood pressure (arterial tonometry), and heart rate (ECG) were measured. Moving the arm between above the heart (AH) and below the heart (BH) level during contraction in steady-state exercise achieved sudden approximately 30 mmHg changes in forearm arterial perfusion pressure (FAPP). We analyzed cardiac cycles during relaxation (FBF(relax)). In an AH-to-BH transition, FBF(relax) increased immediately, in excess of the increase in FAPP (approximately 69% vs. approximately 41%). This was accounted for by pressure-related distension of forearm resistance vasculature [forearm vascular conductance (FVC(relax)) increased by approximately 19%]. FVC(relax) was restored by the second relaxation. Continued slow decreases in FVC(relax) stabilized by 2 min without restoring FBF(relax). In a BH-to-AH transition, FBF(relax) decreased immediately, in excess of the decrease in FAPP (approximately 37% vs. approximately 29%). FVC(relax) decreased by approximately 14%, suggesting pressure-related passive recoil of resistance vessels. The pattern of FVC(relax) was similar to that in the AH-to-BH transition, and FBF(relax) was not restored. These data support rapid myogenic regulation of vascular conductance in exercising human muscle but incomplete flow restoration via slower-acting mechanisms. Local arterial perfusion pressure is an important determinant of steady-state blood flow in the exercising human forearm.

The effect of changing limb position on the validity of venous occlusion plethysmography

Venous occlusion plethysmography (VOP) is a technique used for the non-invasive measurement of limb blood flow. A fundamental technical consideration of venous occlusion plethysmography is that the limb in question must be placed above heart level. However, in light of advances in technology and methodology, the necessity of this has been questioned. We investigated the validity of the VOP technique with the forearm approximately 10 cm above and below the level of the heart in both resting and dynamic conditions. Nine healthy male participants performed four bouts of handgrip exercise, two at each of 15% and 30% maximum voluntary contraction (MVC) (one above and one below the heart). As hypothesized, resting forearm blood flow (FBF) measured below the level of the heart was significantly lower than for above the heart (p = 0.046). However, the opposite occurred during exercise, where FBF measured after the fifth minute of handgrip contractions was significantly higher below the level of the heart (p = 0.013). Furthermore, the ability to accurately quantify FBF below the level of the heart was severely impeded by artifact, and as such VOP appears to remain constricted to use above the phlebostatic level.

Sex differences in forebrain and cardiovagal responses at the onset of isometric handgrip exercise: a retrospective fMRI study

In general, cardiac regulation is dominated by the sympathetic and parasympathetic nervous systems in men and women, respectively. Our recent study had revealed sex differences in the forebrain network associated with sympathoexcitatory response to baroreceptor unloading. The present study further examined the sex differences in forebrain modulation of cardiovagal response at the onset of isometric exercise. Forebrain activity in healthy men (n = 8) and women (n = 9) was measured using functional magnetic resonance imaging during 5 and 35% maximal voluntary contraction handgrip exercise. Heart rate (HR), mean arterial pressure (MAP), and muscle sympathetic nerve activity (MSNA) were collected in a separate recording session. During the exercise, HR and MAP increased progressively, while MSNA was suppressed (P < 0.05). Relative to men, women demonstrated smaller HR (8 +/- 2 vs. 18 +/- 3 beats/min) and MAP (3 +/- 2 vs. 11 +/- 2 mmHg) responses to the 35% maximal voluntary contraction trials (P < 0.05). Although a similar forebrain network was activated in both groups, the smaller cardiovascular response in women was reflected in a weaker insular cortex activation. Nevertheless, men did not show a stronger deactivation at the ventral medial prefrontal cortex, which has been associated with modulating cardiovagal activity. In contrast, the smaller cardiovascular response in women related to their stronger suppression of the dorsal anterior cingulate cortex activity, which has been associated with sympathetic control of the heart. Our findings revealed sex differences in both the physiological and forebrain responses to isometric exercise.

False activation in the brain ventricles related to task-correlated breathing in fMRI speech and motor paradigms

We demonstrate, and show how to eliminate, a task-correlated breathing activation artefact created while performing exercise tasks during a gap in functional magnetic resonance imaging (fMRI). Two studies are presented. The first was intended to isolate a reliable fMRI paradigm for intense handgrip contractions. A gapped acquisition was used to reduce motion artefact, where the contraction was performed during a gap, and image acquisition was between contractions. The second study involved naming regular words (REGs) and nonwords (NWs), where a gap is required for the analysis of participants' overt speech. For study 1, brain ventricle activation was present when breathing responses were task-correlated, and was only eliminated by removing the gap from the sequence. For study 2, NWs were associated with activation artefact in the ventricles, and slower reaction time (RT), reflecting a strategy whereby breathing falls in synchrony with image acquisition. REGs showed the expected RT distribution and frequency effect (reflecting lexical access), with no ventricle activation, and consequently no synchrony with image acquisition. The gapped paradigm increased the likelihood of breathing correlated T2* signal changes in brain ventricles. FMRI researchers should examine the brain ventricles for activation artefact as they are likely associated with false activations in other brain regions.

In vivo lumbar erector spinae oxygenation and blood volume measurements in healthy men during seated whole‐body vibration

Exposure to whole-body vibration is implicated as one of the occupational risk factors for lower back disorders; however, its influence on the lumbar muscle physiology is still poorly understood. The objective of this study was to investigate the effects of backrest support and hand grip contractions on lumbar muscle oxygenation and blood volume responses during seated whole-body vibration using continuous dual-wave near-infrared spectroscopy. Thirteen healthy men were exposed to frequencies of 3, 4.5 and 6 Hz on a vibration simulator, in randomized order on separate days. Each day the duration of the protocol was 30 min. During the fifth minute of vibration 'with' and 'without' backrest support, participants performed maximal rhythmic hand grip contractions for 1 min. In general, erector spinae oxygenation and blood volume showed a trend to decrease with vibration exposure compared to the control condition. However, these responses were not influenced by the change in vibration frequency (P > 0.05). Sitting without backrest resulted in a greater decrease in oxygenation (by 27%, P = 0.02) and blood volume (by 11%, P = 0.05) than with backrest, implying a deficiency in oxygen supply owing to the sitting posture. Compared to the vibration-only condition, hand grip work decreased both oxygenation (by 22%, P = 0.003) and blood volume responses (by 13%, P = 0.04), suggesting that postural load due to prolonged sitting combined with physical activity during vibration might further burden paraspinal muscles. The influence of adipose tissue thickness of the lumbar muscle on optically derived oxygenation and blood volume changes was inconclusive.

Rapid vasoregulatory mechanisms in exercising human skeletal muscle: dynamic response to repeated changes in contraction intensity

We tested the hypothesis that vasoregulatory mechanisms exist in humans that can rapidly adjust muscle blood flow to repeated increases and decreases in exercise intensity. Six men and seven women (age, 24.4+/-1.3 yr) performed continuous dynamic forearm handgrip contractions (1- to 2-s contraction-to-relaxation duty cycle) during repeated step increases and decreases in contraction intensity. Three step change oscillation protocols were examined: Slow (7 contractions per contraction intensityx10 steps); Fast (2 contractions per contraction intensityx15 steps); and Very Fast (1 contraction per contraction intensityx15 steps). Forearm blood flow (FBF; Doppler and echo ultrasonography), heart rate (ECG), and mean arterial pressure (arterial tonometry) were examined for the equivalent of a cardiac cycle during each relaxation phase (FBFrelax). Mean arterial pressure and heart rate did not change during repeated step changes (P=0.352 and P=0.190). For both Slow and Fast conditions, relaxation phase FBFrelax adjusted immediately and repeatedly to both increases and decreases in contraction intensity, and the magnitude and time course of FBFrelax changes were virtually identical. For the Very Fast condition, FBFrelax increased with the first contraction and thereafter slowly increased over the course of repeated contraction intensity oscillations. We conclude that vasoregulatory mechanisms exist in human skeletal muscle that are capable of rapidly and repeatedly adjusting muscle blood flow with ongoing step changes in contraction intensity. Importantly, they demonstrate symmetry in response magnitude and time course with increasing versus decreasing contraction intensity but cannot adjust to very fast exercise intensity oscillations.

Muscle Blood Flow and Exercise Tolerance during Isometric Handgrip Contractions in Males and Females

Previous investigations have demonstrated that time to exhaustion (TTE) is shorter in males compared to females during sustained submaximal isometric contractions. This discrepancy has been attributed to males generating a greater absolute force at the same relative intensity causing occlusion of the vessel supplying blood to the working muscle. PURPOSE: To determine if differences in muscle blood flow (MBF) between sexes is associated with a reduced TTE in males compared to females during isometric handgrip exercise. METHODS: Eighteen subjects (9 male and 9 female, 24 ± 1 yrs (± SE)) performed isometric handgrip exercise at a target force requiring 20%, 50%, or 80% maximal voluntary contraction (MVC) on 3 separate occasions. TTE was determined when the subject could no longer maintain force within 5% of the target value. Forearm flexor muscle activity was measured using surface electromyography (EMG) and analyzed in the time domain (integrated EMG). Brachial artery blood velocity was measured continuously during exercise using Doppler ultrasonography and corrected to MBF by obtaining brachial artery diameter at rest. Forearm volume was determined by water displacement and MBF was expressed both in absolute and relative values. To analyze MBF responses between subjects, TTE was normalized to 100% and compared at 10% intervals. RESULTS: Males (401 ± 62 N) generated greater absolute force compared to females (215 ± 45 N). Although TTE decreased (p<0.05) with an increase in contraction intensity, there was no significant difference in TTE between sexes. At 20% MVC, absolute MBF increased (p<0.05) from rest to TTE in both males (80.3 ± 42.6 ml/min) and females (50.3 ± 22.3 ml/min). At TTE >60% absolute MBF was higher (p<0.05) in males compared to females. There was no difference in absolute MBF between sexes at 50% MVC. However, absolute MBF was reduced in males only during 50% MVC compared to 20% MVC. Expressed relative to forearm volume, MBF was similar between sexes at 20% and 50% MVC. MBF could not be measured at 80% MVC for either sex. EMG increased (p<0.05) with contraction intensity, and time, but no difference between the sexes was observed. CONCLUSIONS: Despite females generating less absolute force, the similar TTE between sexes and the lower absolute MBF in females then males at 20% MVC suggests that factors other then MBF and/or vascular occlusion explain the longer exercise times for females observed in other studies. When MBF was adjusted for forearm volume, differences in MBF between the sexes were eliminated suggesting that MBF was appropriate for the exercising muscle mass, independent of sex. Supported by deArce Memorial Endowment Fund, The University of Toledo

Reliability of Cerebral and Muscle Oxygenation/Blood Volume During Hand-Gripping Measured Simultaneously by Near Infrared Spectroscopy

The stimulus to contract a muscle originates in the cerebral motor cortex and is transmitted to the motor unit via the spinal cord. Near infrared spectroscopy (NIRS) has been used to measure both cerebral and muscle oxygenation (Cox, Mox) /blood volume (Cbv, Mbv) changes during exercise. Although the reliability of these measurements has been reported separately in both tissues, their reproducibility in these two tissues has not been examined simultaneously. PURPOSE: To examine the reliability of the Cox/Mox/Cbv/Mbv measurements during handgrip exercise in healthy subjects. METHODS: Informed consent was obtained from six volunteers (mean age = 28 ± 13 yr, grip strength = 30.4 ± 10.8 kg) who completed two identical testing sessions 24h apart. After a 2min baseline, each subject performed maximal handgrip contractions on a dynamometer (Takei, Japan) with their dominant hand continuously for 15sec, and intermittently for 60sec at a contraction rate of 10/min without hyperventilation during the same session. A 2min rest was allowed between the two protocols. NIRS responses were recorded simultaneously from the extensor carpi ulnaris of the dominant forearm and the contralateral frontal lobe using a dual wavelength spectrometer (MicroRunman, NIM Inc., PA). Delta values were calculated as the difference between the resting value just before initiation of contraction and the maximal value observed during the test and/or recovery period. Values for the two protocols were pooled (N = 12) for statistical analysis. RESULTS: During trials 1 and 2, Mox and Mbv showed a consistent decrease (−0.306±0.069, −0.270±0.082, −0.300±0.086 and −0.315±0.054 respectively), whereas Cox and Cbv demonstrated a continual increase (0.069±0.027, 0.076±0.036, 0.076±0.036 and 0.033±0.015 respectively). The intraclass correlation coefficients for the Cox, Cbv, Mox and Mbv were 0.86 (p=0.001), 0.82 (p=0.004), 0.60 (p=0.085) and 0.32 (p=0.281) respectively. No significant differences were observed between the mean values of the two sessions for each of the variables (p=0.106, 0.372, 0.639, and 0.216 for Cox, Cbv, Mox, and Mbv). Bland-Altman analysis indicated no outliers for the cerebral measurements, whereas there was one outlier for the Mbv. The half recovery time for Mox (an index of muscle aerobic capacity) demonstrated moderate reliability (ICC = 0.72, p=0.023). CONCLUSIONS: The evidence indicates that: (1) the Cox, Cbv and Mox responses, as well as half recovery time for Mox, measured by NIRS are reproducible, and (2) the consistency of the measurements is higher in cerebral compared to muscle tissue. Funding: First author was supported by Ministério da Educação / CAPES, Brazil.

Reliability of near-infrared spectroscopy measures of cerebral oxygenation and blood volume during handgrip exercise in nondisabled and traumatic brain-injured subjects

We compared the test-retest reliability of near-infrared spectroscopy (NIRS) measures of cerebral oxygenation and blood volume during a rhythmic handgrip exercise in 13 nondisabled subjects and 25 subjects with moderate to severe traumatic brain injury (TBI). Subjects with TBI (average Glasgow Coma Scale score = 4.2, average time since injury = 21 mo) had completed an acute brain injury rehabilitation program. After 2 min of rest, each subject performed 60 s of maximal rhythmic handgrip contractions with the right hand in two trials 24 to 48 h apart. We used NIRS to measure cerebral oxygenation and blood volume responses from the left prefrontal lobe. Both groups' cerebral oxygenation and blood volume increased during handgrip contractions. The change in cerebral oxygenation was significantly lower in subjects with TBI compared with nondisabled subjects. Intraclass correlations between the two trials for cerebral oxygenation and blood volume were 0.83 and 0.80, respectively, in nondisabled subjects and 0.70 and 0.64, respectively, in subjects with TBI. The findings indicate that NIRS is a reliable noninvasive technique for evaluating cerebral oxygenation and blood volume changes during motor function. NIRS can be useful in monitoring recovery of cerebral oxygenation during rehabilitation of patients with TBI.

α2B-Adrenergic receptor deletion polymorphism and cardiac autonomic nervous system responses to exercise in obese women

To investigate the association of short form (Glu9/Glu9) of the 12Glu9 deletion polymorphism of the alpha2B-adrenergic receptor (alpha2B-AR) gene polymorphism with the cardiac autonomic responsiveness during sustained isometric handgrip exercise. Cross-sectional clinical study. In all, 97 normotensive obese women (body mass index (BMI) = 33.2 kg/m2). Of these, 78 (80.41%) were genotyped as Glu12/Glu12, 13 (13.40%) as Glu12/Glu9 and six (6.19%) as Glu9/Glu9 form. The sympathovagal balance was assessed by means of power spectral analysis of heart rate variability at rest and during sustained isometric handgrip exercise at 30% of maximal voluntary handgrip contraction for 3 min. Two spectral components were analysed: low-frequency component reflecting sympathetic efferent activity and high-frequency power (HFnu) reflecting parasympathetic modulation. In addition, a normalized low-frequency power (LFnu) and HFnu were analysed. Genotypes were determined by polymerase chain reaction followed by agarose gel electrophoresis. There were no differences in baseline measurements among groups. The absolute level of LFnu throughout handgrip exercise was significantly lower in Glu9/Glu9 subjects compared with other genotypes, while the decline of absolute HFnu was significantly smaller compared with Glu12/Glu12 genotype. These findings suggest that 12Glu9 deletion polymorphism of the alpha2B-AR gene (Glu9/Glu9 genotype) might result in reduced autonomic responsiveness by altering cardiac sympathetic and vagal function during sustained handgrip exercise in normotensive obese women.