Biomedical subjects

J P Weir

Publications and source records attributed to J P Weir.

At least 19 recordsLinked to original sources

PubMed2006

Is fatigue all in your head? A critical review of the central governor model.

The central governor model has recently been proposed as a general model to explain the phenomenon of fatigue. It proposes that the subconscious brain regulates power output (pacing strategy) by modulating motor unit recruitment to preserve whole body homoeostasis and prevent catastrophic physiological failure such as rigor. In this model, the word fatigue is redefined from a term that describes an exercise decline in the ability to produce force and power to one of sensation or emotion. The underpinnings of the central governor model are the refutation of what is described variously as peripheral fatigue, limitations models, and the cardiovascular/anaerobic/catastrophe model. This argument centres on the inability of lactic acid models of fatigue to adequately explain fatigue. In this review, it is argued that a variety of peripheral factors other than lactic acid are known to compromise muscle force and power and that these effects may protect against "catastrophe". Further, it is shown that a variety of studies indicate that fatigue induced decreases in performance cannot be adequately explained by the central governor model. Instead, it is suggested that the concept of task dependency, in which the mechanisms of fatigue vary depending on the specific exercise stressor, is a more comprehensive and defensible model of fatigue. This model includes aspects of both central and peripheral contributions to fatigue, and the relative importance of each probably varies with the type of exercise.

Energy Metabolism↗

PubMed2005

Comparison of Fourier and wavelet transform procedures for examining mechanomyographic and electromyographic frequency versus isokinetic torque relationships.

The purpose of this study was to compare the isokinetic torque-related patterns for mechanomyographic (MMG) and electromyographic (EMG) center frequency [wavelet center frequency (CF), mean power frequency (MPF), and median frequency (MDF)] determined by the fast Fourier transform (FFT) and discrete wavelet transform (DWT). Ten adults [mean +/- SD age = 22.0 +/- 3.4 yrs] performed submaximal to maximal, isokinetic muscle actions of the biceps brachii on a Cybex II dynamometer. For both MMG and EMG, the CF, MPF, and MDF values were intercorrelated at (r = 0.91-0.98). Quadratic models provided the best fit for the absolute and normalized CF, MPF, and MDF versus isokinetic torque relationships for MMG (R2 = 0.67-0.83) and EMG (R2 = 0.72-0.90). The similarities among the CF, MPF, and MDF patterns suggested that Fourier or wavelet transform procedures can be used to examine the patterns of MMG and EMG responses during dynamic muscle actions.

Adult↗

PubMed2004

The acute effects of static stretching on peak torque, mean power output, electromyography, and mechanomyography.

The purpose of this study was to examine the acute effects of static stretching on peak torque (PT), the joint angle at PT, mean power output (MP), electromyographic (EMG) amplitude, and mechanomyographic (MMG) amplitude of the vastus lateralis (VL) and rectus femoris (RF) muscles during maximal, voluntary concentric isokinetic leg extensions at 60 and 240 degrees x s(-1) of the stretched and unstretched limbs. Twenty-one volunteers [mean age (SD) 21.5 (1.3) years] performed maximal, voluntary concentric isokinetic leg extensions for the dominant and non-dominant limbs at 60 and 240 degrees x s(-1). Surface EMG (muVrms) and MMG (mVrms) signals were recorded from the VL and RF muscles during the isokinetic tests. PT (Nm), the joint angle at PT, and MP (W) were calculated by a dynamometer. Following the initial isokinetic tests, the dominant leg extensors were stretched using four static stretching exercises. After the stretching, the isokinetic tests were repeated. PT decreased (P< or =0.05) from pre- to post-stretching for the stretched limb at 60 and 240 degrees x s(-1) and for the unstretched limb at 60 degrees x s(-1). EMG amplitude of the VL and RF also decreased (P< or =0.05) from pre- to post-stretching for the stretched and unstretched limbs. There were no stretching-induced changes (P>0.05) for the joint angle at PT, MP, or MMG amplitude. These findings indicated stretching-induced decreases in force production and muscle activation. The decreases in PT and EMG amplitude for the unstretched limb suggested that the stretching-induced decreases may be due to a central nervous system inhibitory mechanism.

Adult↗

PubMed2004

Mechanomyographic time and frequency domain responses of the vastus medialis muscle during submaximal to maximal isometric and isokinetic muscle actions.

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) amplitude and mean power frequency (MPF) versus torque relationships during isometric and isokinetic muscle actions. Ten adults (mean age +/- SD = 22 +/- 1 y) volunteered to perform isometric and isokinetic leg extension muscle actions at 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100% of peak torque on a Cybex II dynamometer. A piezoelectric crystal contact sensor was placed on the vastus medialis to detect the MMG signal. Regression analyses indicated that for the isometric muscle actions, the relationships for MMG amplitude (R2 = 0.998) and MPF (R2 = 0.987) versus torque were cubic. For the isokinetic muscle actions, the relationships for MMG amplitude (r2 = 0.927) and MPF (r2 = 0.769) versus torque were linear. The different patterns for MMG amplitude and frequency may reflect differences in the motor control strategies that modulate torque production for isometric versus dynamic muscle actions.

Adult↗

PubMed2002

Mechanomyographic and electromyographic amplitude and frequency responses from the superficial quadriceps femoris muscles during maximal, eccentric isokinetic muscle actions.

The purposes of this study were to examine the effects of gender and muscle (vastus lateralis = VL, rectus femoris = RF, and vastus medialis = VM) on the velocity-related patterns for peak torque (PT), mean power output (MP), mechanomyographic (MMG) amplitude, electromyographic (EMG) amplitude, MMG mean power frequency (MPF), and EMG MPF during maximal, eccentric isokinetic muscle actions. Thirteen females (mean +/- SD age = 21 +/- 1 years) and eleven males (mean +/- SD age = 21 +/- 2 years) volunteered for this investigation. PT and MP were measured on a calibrated Cybex 6000 dynamometer at randomly ordered velocities of 60, 120, and 180 degrees.s-1, while MMG and EMG signals were recorded simultaneously from the VL, RF, and VM muscles. The results indicated no gender-related differences for the patterns of PT, MP, MMG amplitude, EMG amplitude, MMG MPF, or EMG MPF. Furthermore, no muscle-related differences were found for the patterns of MMG amplitude, EMG amplitude, or MMG MPF. The normalized values for MP and MMG amplitude increased from 60 to 180 degrees.s-1 (60 degrees.s-1 < 120 degrees.s-1 < 180 degrees.s-1). PT and EMG MPF remained unchanged across velocity, while EMG amplitude remained unchanged from 60 to 120 degrees.s-1, but decreased (approximately 10%) from 120 to 180 degrees.s-1. The findings indicated a close association between the patterns for MP and MMG amplitude, and a similarity between the patterns for PT, EMG amplitude, and EMG MPF across velocity. Therefore, the present findings suggested that motor unit recruitment (EMG amplitude), firing rate (MMG MPF), and muscle fiber action potential conduction velocity (EMG MPF) exhibited velocity-related patterns that were similar to PT production, while MMG amplitude was more closely associated with MP.

Adult↗

PubMed2001

Regulation of herpes simplex virus gene expression.

Expression of the more than 80 individual genes of herpes simplex virus 1 (HSV-1) takes place in a tightly regulated sequential manner that was first described over 20 years ago. Investigations since that time have focused on understanding the mechanisms that regulate this orderly and efficient expression of viral genes. This review examines recent findings that have shed light on how this process is regulated during productive infection of the cell. Although the story is still not complete, several aspects of HSV gene expression are now clearer as a result of these findings. In particular, several new functions have recently been ascribed to some of the known viral regulatory proteins. The results indicate that the viral gene expression is regulated through transcriptional as well as post-transcriptional mechanisms. In addition, it has become increasingly clear that the virus has evolved specific functions to interact with the host cell in order to divert and redirect critical host functions for its own needs. Understanding the interactions of HSV and the host cell during infection will be essential for a complete understanding of how viral gene expression is regulated. Future challenges in the field will be to develop a complete understanding of the mechanisms that temporally regulate virus gene expression, and to identify and characterize the relevant interactions between the virus and the distinctive cell types normally infected by the virus.

Gene Expression Regulation, Viral↗

PubMed2001

Protective immunity against herpes simplex virus generated by DNA vaccination compared to natural infection.

To evaluate the utility of plasmid DNA vaccination against disease caused by herpes simplex virus (HSV), we compared the strength of protection against lethal challenge following natural virus infection with that following vaccination with a plasmid encoding HSV glycoprotein gD (gD-DNA). We further determined the cellular basis of each type of protection using lymphocyte deficient knockout mice. Establishment of immunity to HSV using live virus immunization required CD8+ T cells and B cells, but not CD4+ or gamma/delta+ T cells, and was related to specific antibody levels; surprisingly, CD4 knockout mice had large quantities of IgG anti-HSV serum antibodies. Establishment of immunity to HSV using gD-DNA immunization approached the strength of that generated following sublethal infection, but was dependent on alpha/beta+ CD4+ T cells, CD8+ T cells, B cells, and even partially on gamma/delta+ T cells, and not strictly correlated with antibody levels.

Animals↗

PubMed2001

The effects of autonomic dysfunction and endurance training on cardiovascular control.

The effects of autonomic dysfunction and regular activity on the cardiovascular system were investigated. The 48 participants included 12 subjects with tetraplegia, 12 subjects with paraplegia, 12 sedentary subjects, and 12 endurance-trained able-bodied controls. Central and peripheral autonomic data were obtained at rest to estimate efferent cardiac vagal output and sympathetic vasomotor control, and plasma norepinephrine concentration was determined as a marker of peripheral sympathetic activity. Cardiovascular parameters were obtained using a noninvasive cardiac output maneuver. The group with paraplegia did not differ from the sedentary group for efferent cardiac vagal output, but all other group comparisons were different (p <0.05). Sympathetic vasomotor control and stroke index were also similar between the paraplegia and sedentary groups, whereas both were increased in the endurance-trained group and were significantly reduced in the tetraplegia group. A strong relation between efferent cardiac vagal output and stroke index was established for the total group (r = 0.78, p <0.01), and analysis of covariance determined that the slope of this relation was similar among the groups. Sympathetic vasomotor control correlated significantly with plasma norepinephrine (r = 0.57, p <0.01), and a relation between sympathetic vasomotor control and stroke index was identified for the total group (r = 0.40, p <0.01). These results suggest that vagal control of resting central cardiac function is maintained despite autonomic dysfunction. The comparable findings in the paraplegia and sedentary groups suggest that regardless of peripheral autonomic dysfunction, the absence of regular physical activity has a similar effect on the resting vagal modulation and stroke index.

Adult↗

PubMed2001

Mean power frequency and amplitude of the mechanomyographic and electromyographic signals during incremental cycle ergometry.

The purpose of this investigation was to determine the relationships for mechanomyographic (MMG) amplitude, MMG mean power frequency (MPF), electromyographic (EMG) amplitude, and EMG MPF versus power output during incremental cycle ergometry. Seventeen adults volunteered to perform an incremental test to exhaustion on a cycle ergometer. The test began at 50 W and the power output was increased by 30 W every 2 min until the subject could no longer maintain 70 rev min(-1). The MMG and EMG signals were recorded simultaneously from the vastus lateralis during the final 10 s of each power output and analyzed. MMG amplitude, MMG MPF, EMG amplitude, EMG MPF, and power output were normalized as a percentage of the maximal value from the cycle ergometer test. Polynomial regression analyses indicated that MMG amplitude increased (P<0.05) linearly across power output, but there was no change (P>0.05) in MMG MPF. EMG amplitude and MPF were fit best (P<0.05) with quadratic models. These results demonstrated dissociations among the time and frequency domains of MMG and EMG signals, which may provide information about motor control strategies during incremental cycle ergometry. The patterns for amplitude and frequency of the MMG signal may be useful for examining the relationship between motor-unit recruitment and firing rate during dynamic tasks.

Adult↗

PubMed2001

Infection of human NT2 cells and differentiated NT-neurons with herpes simplex virus and replication-incompetent herpes simplex virus vectors.

The human embryonal carcinoma cell line NT2 differentiates irreversibly into postmitotic neuron-like cells following treatment with retinoic acid. These differentiated NT-neurons resemble central nervous system (CNS) neurons and are characterized by development of dendrites and axons and the expression of neuron-specific markers. Because of their unique biological characteristics, NT-neurons were investigated for their utility as a system for studying the replication of herpes simplex virus (HSV) in the neuron and for evaluating characteristics of HSV vectors designed for gene delivery to the neuron. Virus replication in differentiated NT-neurons was significantly reduced and delayed relative to replication in undifferentiated NT2 cells. Replication of thymidine-kinase (tk) deficient HSV was further impaired in NT-neurons, reflecting the behavior of tk-negative virus in primary neurons in vitro and ganglia in vivo. Furthermore, replication-incompetent HSV vectors were capable of infecting NT-neurons, expressing a foreign gene, and persisting in a recoverable state for at least 2 weeks following delivery. These results suggest that differentiated NT-neurons can provide a continuous source of human, post-mitotic neurons-like cells for the study of HSV biology and HSV vector development.

Cell Differentiation↗

PubMed2001

The effect of hand-grip stabilization on isokinetic torque at the knee.

Isokinetic dynamometers commonly measure muscle strength during conditioning and rehabilitation. Previous studies have shown that stabilization can affect isokinetic torque production. However, the effect of stabilization with hand-grip use has not been examined, and there are inconsistencies in the literature regarding its use. Fifteen men (mean age +/- SD = 24 +/- 3 years, height = 187 +/- 6 cm, mass = 84 +/- 10 kg) and 15 women (age = 25 +/- 4 years, height = 167 +/- 8 cm, mass = 62 +/- 6 kg) were tested on a Cybex 6000 dynamometer. Torque and joint angle signals, as well as quadriceps and hamstrings electromyographic (EMG) signals, were recorded during maximal knee flexion/extension at speeds of 60, 180, and 300 degrees x s(-1). Subjects performed the testing both with (stabilized) and without (nonstabilized with arms folded across the chest) hand-grip use. Repeated-measures analysis of variance (ANOVA) revealed a significant-motion (flexion, extension), X-condition (stabilized, non-stabilized) interaction in the data from the men in which hand-grip stabilization resulted in an 8.4% increase in knee extensor torque vs. a 0.2% increase in knee flexor torque. Stabilization did not significantly affect torque in women. The EMG analyses did not indicate a significant change in either agonist drive or antagonist cocontraction that accounted for the enhanced torque output with hand-grip use. This study found that hand-grip use enhanced knee extension maximal torque in men, but did not affect torque in women. The EMG data did not account for the changes in the torque data. Differences between men and women may have been due to mechanical factors and grip strength. These results indicate that hand-grip use needs to be considered when examining gender differences in knee strength and when studying knee flexion-extension ratios.

Adult↗

PubMed2000

Herpes simplex virus 1 late gene expression is preferentially inhibited during infection of the TAF250 mutant ts13 cell line.

A key component of the polymerase II transcription machinery is the transcription factor TFIID, a complex that contains the TATA-box binding protein and several (10-12) associated factors designated as TAFs (TBP-associated factors). ts13 cells, which contain a temperature-sensitive mutant in TAF250, the largest subunit of TFIID, exhibit promoter-specific defects in gene expression at the nonpermissive temperature, suggesting that individual TAFs are required for transcription of specific subsets of eukaryotic genes. Herpes simplex virus 1 (HSV-1) does not replicate in ts13 cells at the nonpermissive temperature, but the point at which the replicative process is blocked is not known. We used the TAF250 defect in ts13 cells to investigate the role of TAF250 in the expression of HSV-1 genes of each temporal class. At a low m.o.i., expression of most immediate-early mRNAs is reduced at the nonpermissive temperature, and consequently, there is little expression of early genes and no viral DNA replication. In contrast, at high m.o.i., expression of immediate-early genes is unaffected by the TAF250 defect and is not dependent on de novo viral protein synthesis. Early genes and early proteins are produced under these conditions, and viral DNA replication ensues, albeit at somewhat reduced levels. In contrast, late gene expression and late protein synthesis are severely restricted, even in the presence of appreciable viral DNA replication. Thus the lack of late protein synthesis is responsible for the inability of HSV-1 to replicate in ts13 cells at the nonpermissive temperature. Further, it appears that late viral gene expression may be preferentially inhibited by the TAF250 mutation in ts13 cells.

Animals↗

PubMed2000

The effect of leg extension training on the mean power frequency of the mechanomyographic signal.

The purpose of the present investigation was to examine the effect of concentric isokinetic leg extension training on the mean power frequency (MPF) of the mechanomyographic (MMG) signal. Twenty-one men were assigned into a training (TRN; n = 12) or control (CTL; n = 9) group. The TRN group performed six sets of leg extensions 3 days per week for 12 weeks at a velocity of 90 degrees /s. All subjects were tested every 4 weeks for peak torque (PT), while MMG was recorded from the vastus lateralis. PT increased, but there was no significant (P > 0.05) change in the MMG MPF over the 12-week training period. These results indicate that MMG MPF, measured from the vastus lateralis, was not sensitive to training-induced increases in leg-extension strength, possibly due to competing influences of hypertrophy on the MMG signal and/or training-induced adaptations in muscles other than the vastus lateralis.

Electromyography↗

PubMed2000

Mechanomyographic and electromyographic responses during fatigue in humans: influence of muscle length.

Mechanomyography (MMG) provides a measure of muscle mechanical changes during contractions. The purpose of this study was to quantify alterations in MMG signals during fatigue at two muscle lengths. Comparisons with electromyographic (EMG) recordings were made. A group of 13 subjects performed isometric dorsiflexions (50% of maximum for 60 s) at 40 degrees of plantarflexion (long, l(l)) and 5 degrees of dorsiflexion (short, l(s)). The mean power frequency of the EMG (f(EMG)) and MMG (f(MMG)) signals and the mean rectified MMG (rMMG) and EMG (rEMG) were determined over each 1-s period, normalized to the respective maximal value, regressed against time, and the resulting slopes (units = %max. s(-1)) were analyzed. The slopes were larger (P = 0.007) at l(l) compared to l(s) [mean l(l) 0.50 (SD 0.26), mean l(s) 0.27 (SD 0.16)], however there were no differences (P = 0.24) between mean fMMG slopes [l(l) -0.10 (SD 0.16), l(s) -0.16 (SD 0.11)]. Similarly, slopes were larger (P = 0.001) at l(l) versus l(s) [l(l) 0.26 (SD 0.13), l(s) 0.08 (SD 0.15)] and there were no differences (P=0.89) between mean fEMG slopes [l(l) -0.15 (SD 0.14), l(s) -0.14 (SD 0.12)]. At 5 s following the exercise to fatigue mean MVC (units = %max) were not significantly different between l(l) and l(s) [P = 0.08; l(l) 78.8 (SD 9.1), l(s) 85.2 (SD 6. 0)]. These results showed that during fatiguing contractions, MMG and EMG amplitudes increased while frequency characteristics decreased at both muscle lengths. The change in and was greater at l(l) but no differences in fMMG or fEMG slopes occurred between lengths. These results would suggest that larger increases in motor unit recruitment occur with time during fatigue at l(l) compared to l(s).

Adult↗

PubMed2000

Effects of autonomic disruption and inactivity on venous vascular function.

The effects of autonomic disruption and inactivity were studied on the venous vascular system. Forty-eight subjects, 24 with spinal cord injury (SCI) and 12 sedentary and 12 active able-bodied controls, participated in this study. Peripheral autonomic data were obtained to estimate sympathetic vasomotor control [low-frequency component of systolic blood pressure (LF(SBP))]. Vascular parameters were determined using strain-gauge venous occlusion plethysmography: venous capacitance (VC), venous emptying rate (VER), and total venous outflow (VO(t)). An additional vascular parameter was calculated: venous compliance [(VC/occlusion pressure) x 100]. VC and VO(t) were significantly different (SCI < sedentary < active). VER adjusted for VC was not different for any group comparison, whereas venous compliance was significantly lower in the SCI group than in the able-bodied groups and in the sedentary group compared with the active group. Regression analysis for the total group revealed a significant relationship between LF(SBP) and venous compliance (r = 0.64, P < 0.0001). After controlling for LF(SBP) through analysis of covariance, we found that mean differences for all venous vascular parameters did not change from unadjusted mean values. Our findings suggest that in subjects with SCI, the loss of sympathetic vasomotor tone contributes more than inactivity to reductions in venous vascular function. Heightened VC, VO(t), vasomotor tone, and venous compliance in the active group compared with the sedentary group imply that regular endurance training contributes to optimal venous vascular function and peripheral autonomic integrity.

Adult↗

PubMed1999

Allometric scaling of isokinetic peak torque: the Nebraska Wrestling Study.

Allometric scaling has been used increasingly in the exercise sciences to control statistically for body size differences in physical performance variables. The purpose of this study was to use multivariate allometric scaling to examine the influence of fat-free mass (FFM) on age-related differences in strength in young club (8-13 years) and high-school (14-18 years) wrestlers. The dependent variables were log-transformed values of isokinetic peak torque for leg extension and flexion at 0.52, 3.14, and 5.24 rad x s(-1)(30, 180, and 300 x s(-1)). The independent variables used in the multiple regression analyses were log-transformed values for FFM, age, and the FFM versus age interaction. The resulting regression equations were of the form: log Y = log a + b1 log X1 + b2 log X2 + bn log Xn. The initial multiple regression analyses showed significant interaction effects (P < 0.05) for all dependent variables, therefore separate regression analyses were performed for the younger and older groups of wrestlers. The results indicate that for the younger wrestlers there were increases in isokinetic peak torque at all velocities across age after controlling for FFM. The FFM scaling exponents ranged from 0.94 to 1.31. All exponents included 1.0 in the 95% confidence interval, except for extension at 3.14 rad x s(-1). For the high-school wrestlers, both FFM and age were significant for the extension data, but only FFM was significant for the flexion data. All FFM exponents included 1.0 in the 95% confidence interval. These results indicate that the relationship between FFM and peak torque differed across age. In addition, with the exception of the flexion data for the high-school wrestlers, within each group increases in isokinetic peak torque occurred across age, independent of increases in FFM. The causes of the age effect for strength are speculative, but it may be due to developmental changes in neuromuscular function, alterations in the distribution of muscle mass as a percentage of FFM and/or the distribution of FFM across body segments.

Adolescent↗

PubMed1999

Mean power frequency and amplitude of the mechanomyographic signal during maximal eccentric isokinetic muscle actions.

The purpose of the present investigation was to examine the effects of knee angular velocity on the mean power frequency (MPF) and amplitude of the mechanomyographic (MMG) signal during maximal eccentric (ECC) isokinetic muscle actions. Eleven adult subjects performed maximal ECC muscle actions of the leg extensors on a calibrated Cybex 6000 dynamometer at knee angular velocities of 60, 120, and 180 degrees.s-1. MMG was detected by a piezoelectric crystal contact sensor placed over the vastus lateralis muscle. There were no significant (p > 0.05) velocity-related changes in ECC peak torque (PT) or MMG MPF, however, the mean MMG amplitude value at 60 degrees.s-1 was significantly less (p < 0.05) than that at 180 degrees.s-1. These results did not support our previous hypothesis that the velocity-related increase in MMG amplitude for maximal ECC isokinetic muscle actions was due to selective recruitment of fast twitch fibers and derecruitment of slow twitch fibers with increasing velocity.

Adult↗

PubMed1998

Characterization of the initiator and downstream promoter elements of herpes simplex virus 1 late genes.

Previously identified cis-acting regulatory elements of herpes simplex virus (HSV) 1 late promoters include a TATA element upstream from the start of transcription, an initiator-like element at the start of transcription, and sequences downstream from the start of transcription. To determine whether these elements are functionally equivalent to similar elements from other eukaryotic genes, model late promoters were constructed using well-characterized regulatory elements from non-HSV genes. These modular promoters were then inserted into the viral genome upstream from a lacZ marker gene. Results showed that a eukaryotic initiator element, along with a TATA element, can function as a late HSV promoter. Several initiator sequences from both viral and nonviral genes were functionally similar to the initiator-like element in HSV-1 late promoters; however, a random sequence of the same size and a similarly located sequence from the HSV-1 early thymidine kinase promoter could not substitute for the initiator element. These results indicate that eukaryotic initiator elements are functionally equivalent to HSV-1 late promoter initiator elements. In addition, the downstream element of the late glycoprotein C promoter was further analyzed by construction of a series of small deletions and insertions. The presence of the downstream glycoprotein C region in a promoter consisting of a strong TATA and initiator element increased mRNA expression by a modest amount; this effect appeared to be sequence specific and dependent on its exact alignment with the upstream elements of the promoter.

Animals↗