PubMed Health⌕ Search

Biomedical subjects

L Griffin

Publications and source records attributed to L Griffin.

At least 19 recordsLinked to original sources
PubMed2007

Force output during fatigue with progressively increasing stimulation frequency.

There is currently a controversy over whether stimulation frequencies should increase or decrease to optimize force output over time. This study compared changes in thenar muscle force and M-wave amplitude during progressively increasing (20-40 Hz), decreasing (40-20 Hz) and constant (20 Hz) frequency stimulation of the median nerve continuously for 3 min. Twenty-three individuals participated in three sets of experiments. There was no significant difference in the force-time integrals between the three fatigue tasks. The rate of fatigue was not correlated to the number of stimulation pulses delivered (20 Hz: 3,600, 20-40 and 40-20 Hz: 5,400). All fatigue tasks caused a significant reduction in M-wave amplitude and the reduction was largest for the 20-40 Hz protocol. However, multiple linear regression analysis revealed that the M-wave amplitude could not predict the changes in force over time for the 20 Hz or 20-40 Hz protocols. Thus during sustained evoked contractions with stimulation frequencies within the physiological range, frequencies can vary significantly without changing the overall force-time integral.

Adult↗
PubMed2006

Reflex gain of muscle spindle pathways during fatigue.

There are conflicting observations of the effects of fatigue on the sensitivity of large diameter Ia afferents. Our goal was to characterize any fatigue-related changes in the spinal reflex pathways during fatigue. Manipulation of the Ia afferent response by vibration and tendon tap, in which the motor neuron pool is modulated by both short- and long-loop activation from muscle spindles, were elicited before and after a fatigue task. The fatigue task consisted of intermittent submaximal and maximal voluntary contractions (MVCs). Percent voluntary activation fell from 98.75% MVC to 80.92% MVC following the fatigue task as measured by the twitch interpolation technique. Voluntary contractions of the same force profile as the force produced by 30 s of vibration were produced by having participants (n = 10) follow the trajectory on a computer monitor, before and after the fatigue task. Recruitment thresholds (RTs) of voluntarily activated units showed no change during fatigue; however, units activated via the reflex pathway were recruited approximately 30% sooner during fatigue (P < 0.05). The ratio of the electrical-to-mechanical response of the tendon tap increased significantly with fatigue. Our findings of decreased RTs in response to vibration and increased EMG activity during the tendon tap following the fatigue task indicate that Ia afferent input to the motoneuron pool was increased. The decrease in MVC force indicates that during this time the descending drive was compromised. These results provide evidence that the gain of the gamma loop is increased during fatigue, indicating possible peripheral neural compensation to the motor neuron pool in order to preserve force output.

Action Potentials↗
PubMed2006

Transcranial magnetic stimulation during resistance training of the tibialis anterior muscle.

During the first few weeks of resistance training, maximal voluntary contraction (MVC) force increases at a faster rate than can be accounted for by increases in protein synthesis. This early increase in MVC force has been attributed to neural mechanisms but the sources have not been identified. The purpose of this study was to measure changes in cortical excitability with transcranial magnetic stimulation during 4 weeks of resistance training of the tibialis anterior muscle. Ten individuals performed 6 sets of 10 MVCs 3 times per week for 4 weeks and ten participated as a control group. There were no changes in any parameters tested in the control group over the 4 weeks. In the training group, TA muscle strength increased significantly by 10% at week 2 and by 18% at week 4. As hypothesized, cortical excitability during resistance training also increased. The amplitude of the TA surface EMG motor evoked potential elicited by TMS during a low-level contraction increased by 32% after training with no change in the M-wave. These data indicate that there may be an increase in cortical excitability during the first few weeks of resistance training of the TA muscle.

Adolescent↗
PubMed2005

Maximal motor unit firing rates during isometric resistance training in men.

This study measured changes in maximal voluntary contraction (MVC) force, percentage maximal activation, maximal surface EMG, M-wave amplitude and average motor unit firing rates during the initial 3 weeks of isometric resistance training of the quadriceps muscle. Ten men participated in a resistance training programme three times a week for 3 weeks and 10 men participated as a control group. In the training group, MVC increased by 35% (from 761 +/- 77 to 1031 +/- 78 N) by the end of the 3 weeks. There were no changes in mean motor unit firing rates during submaximal or maximal voluntary contractions of 50 (15.51 +/- 1.48 Hz), 75 (20.23 +/- 1.85 Hz) or 100% MVC (42.25 +/- 2.72 Hz) with isometric resistance training. There was also no change in maximal surface EMG relative to the M-wave amplitude. However, there was a small increase in maximal activation (from 95.7 +/- 1.83 to 98.44 +/- 0.66%) as measured by the twitch interpolation technique. There were no changes in any of the parameters measured in the control group. It is suggested that mechanisms other than increases in average motor unit firing rates contributed to the increase in maximal force output with resistance training. Such mechanisms may include a combination of increased motor unit recruitment, enhanced protein synthesis, and changes in motor unit synchronization and muscle activation patterns across the quadriceps synergy.

Adult↗
PubMed2001

Muscle vibration sustains motor unit firing rate during submaximal isometric fatigue in humans.

1. In keeping with the 'muscular wisdom hypothesis', many studies have documented that the firing rate of the majority of motor units decreased during fatiguing isometric contractions. The present study investigated whether the application of periodic muscle vibration, which strongly activates muscle spindles, would alter the modulation of motor unit firing rate during submaximal fatiguing isometric contractions. 2. Thirty-three motor units from the lateral head of the triceps brachii muscle were recorded from 10 subjects during a sustained isometric 20 % maximal voluntary contraction (MVC) of the elbow extensors. Vibration was interposed on the contraction for 2 s every 10 s. Twenty-two motor units were recorded from the beginning of the fatigue task. The discharge rate of the majority of motor units remained constant (12/22) or increased (4/22) with fatigue. Six motor units demonstrated a reduction in discharge rate that later returned toward initial values; these motor units had higher initial discharge rates than the other 16 motor units. 3. In a second series of experiments, four subjects held a sustained isometric 20 % MVC for 2 min and then vibration was applied as above for the remainder of the contraction. In this case, motor units initially demonstrated a decrease in firing rate that increased after the vibration was applied. Thus muscle spindle disfacilitation of the motoneurone pool may be associated with the decline of motor unit discharge rate observed during the first 2 min of the contraction. 4. In a third set of experiments, seven subjects performed the main experiment on one occasion and repeated the fatigue task without vibration on a second occasion. Neither the endurance time of the fatiguing contraction nor the MVC torque following fatigue was affected by the application of vibration. This finding calls into question the applicability of the muscular wisdom hypothesis to submaximal contractions.

Adult↗
PubMed2001

Blood flow in the triceps brachii muscle in humans during sustained submaximal isometric contractions.

The main purpose of this study was to determine the extent to which blood flow through the profunda artery within the triceps brachii muscle may be compromised during maintained low-force isometric fatiguing contractions. Doppler ultrasound techniques were used to record mean blood velocity and arterial diameter of the profunda brachii artery during sustained isometric contractions of 20% maximal voluntary contraction. The arterial diameter did not change throughout the contraction. Thus, blood velocity was considered to be an indicator of blood flow. The mean blood velocity increased initially and then remained constant during the contraction period. When compared to rest [0.06 (SD 0.03) m s-1] mean blood velocity was significantly larger at the start of the contraction [0.13 (SD 0.07) m s-1] and larger yet during recovery following the contraction [0.30 (SD 0.14) m s-1]. Although blood flow through the conduit artery did not drop during the contraction, the post-contraction hyperaemia suggested that circulatory compromise might have occurred at the level of the capillary beds.

Adult↗
PubMed2001

Preferences for quality of life or survival expressed by patients with heart failure.

BACKGROUND: As many patients with heart failure develop symptoms limiting daily life, newer therapies may be found to improve functional status without concomitant survival benefit. As some of these therapies may actually increase mortality, it is increasingly relevant to assess patients' preferences for survival vs improvement in symptoms. METHODS: We enrolled 99 patients with advanced heart failure (ejection fraction 24 +/- 10, duration 6 +/- 5 years). Each patient completed time trade-off and standard gamble instruments, Minnesota Living with Heart Failure questionnaires and visual analog scales for dyspnea and overall health. Jugular venous pressure was assessed in all patients and peak oxygen consumption was measured during bicycle exercise in 60 patients. RESULTS: Strong polarity of preference toward either survival or quality of life was expressed by 60% of patients. There was good correlation between time trade-off and standard gamble utility scores (r = 0.64), and between preference and functional class (r = 0.60). Higher jugular venous pressure and lower peak oxygen consumption were associated with poorer utility scores (p <.05). Higher dyspnea scores and worse Living with Heart Failure scores were also associated with preference to trade time or take risks for better health. CONCLUSIONS: These findings suggest that heart failure patients express meaningful preferences about quality vs length of life. High jugular venous pressure, low peak oxygen consumption and poor Living with Heart Failure scores were related to low utility scores. These cannot be assumed, however, to predict the intensity of individual preference to trade nothing or virtually everything for better health.

Adult↗
PubMed2000

Role of limb movement in the modulation of motor unit discharge rate during fatiguing contractions.

Motor unit firing rates of the triceps brachii muscle have been shown to decline during sustained isometric contractions, but not if the fatiguing contraction incorporates arm movements. The purpose of this study was to determine the impact of the actual physical displacement of the limb on the maintenance of motor unit discharge rate during dynamic muscle fatigue. An isometric force pulse paradigm was used to recreate the motor unit activity patterns that occur during a dynamic contraction. With this paradigm, the variable force output that would occur during a dynamic contraction remained intact, but the movement of the limb was eliminated. Motor unit firing rates declined in the isometric force pulse protocol. Thus, factors related to the actual movement of the limb appear to enable the maintenance of motor unit discharge rates during fatigue.

Adult↗
PubMed2000

Application of a polymerase chain reaction-ELISA to detect Wuchereria bancrofti in pools of wild-caught Anopheles punctulatus in a filariasis control area in Papua New Guinea.

Chemotherapy-based eradication programs are aimed at stopping transmission of Wuchereria bancrofti by its obligatory mosquito vector. This study compares one year post-treatment W. bancrofti infection rates of Anopheles punctulatus, the main vector of lymphatic filariasis in Papua New Guinea, using traditional dissection techniques and a polymerase chain reaction (PCR)-based ELISA of a parasite-specific Ssp I repeat. A total of 633 mosquitoes in 35 batches were dissected. Six batches contained W. bancrofti-infected mosquitoes, giving a minimum infection rate of 0.9%. This value was not different than the actual infection rate, which was 9 (1.4%) of 633 mosquitoes (P = 0.48). The DNA was extracted from 47 pools containing a mean of 13.2 mosquitoes per pool. A total of 621 mosquitoes were processed for the PCR-ELISA, including 486 caught by human bait and 135 by light trap, which included both dead and live mosquitoes. Of 23 pools of alcohol-preserved human-bait mosquitoes, seven were positive by the PCR-ELISA, giving an infection rate identical to that obtained by dissection of individual mosquitoes (1.4%). The minimum infection rates for pools of light-trap mosquitoes found dead and alive were 2.7% (2 of 74) and 4.9% (3 of 61), respectively. These values did not differ from each other (P = 0.84), but the overall infection rate of light-trap mosquitoes was greater than that of mosquitoes captured by human bait (3.7% versus 1.4%; P = 0.09). These data indicate that the PCR-ELISA of a W. bancrofti Ssp I repeat using pools of mosquitoes is comparable to traditional dissection techniques for monitoring transmission intensity following introduction of mass chemotherapy. This approach may also be useful for rapid and cost-effective assessment of transmission in endemic areas where the frequency of overt lymphatic pathology is low.

Animals↗
PubMed2000

Laser tattoo removal.

The availability of Q-switched ruby Nd:YAG and alexandrite lasers has revolutionized the treatment of tattoos. These modalities offer significant advantages over all previously available treatments and are currently the standard of care for the cosmetic removal of unwanted tattoos.

History, 20th Century↗
PubMed1999

Impact of community health workers on access, use of services, and patient knowledge and behavior.

Community health workers have been used in health centers for several decades. Known by various names (e.g., community health advisors, outreach workers, promotoras), they are trusted community members providing informal community-based health-related services and establishing vital links between health providers and the community. They perform many functions, including outreach and case finding, health education, translation, patient transportation, and case management (under the supervision of a nurse or social worker). This article, based on a study of community health worker programs at seven sites, is intended to share the lessons learned that could be helpful to other program managers and to examine the outcomes of such programs.

Community Health Centers↗
PubMed1999

Motor unit double discharges: statistical anomaly or functional entity?

Motor unit double discharges, or doublets, have been described as two consecutive motor unit discharges that occur with a short interspike interval of 2.5 - 20 ms (Simpson, 1969). Double discharges have been reported in the literature for over 70 years. For instance, Eccles and Hoff (1932) found that double discharges were elicited occasionally at the onset of a crossed extension reflex in the soleus muscle of the anaesthetized cat. With the use of electrical stimulation protocols, short interspike intervals inserted at the beginning of a stimulation train have been shown to increase both the peak force and rate of rise of force production, and also decrease the range of fatigue. The extent to which double discharges occur in naturally-occurring voluntary behaviours remains relatively unexplored. This review examines the issue of whether double discharges occur solely because of an intrinsic property of motoneurones, thereby representing a "statistical anomaly," or whether they may result from a neural control strategy to augment force production, i.e., a "functional entity."

Animals↗
PubMed1998

Pharmacokinetics and bioavailability of the anti-human immunodeficiency virus nucleotide analog 9-[(R)-2-(phosphonomethoxy)propyl]adenine (PMPA) in dogs.

The pharmacokinetics, bioavailability, and metabolism of the anti-human immunodeficiency virus nucleotide analog 9[(R)-2-(phosphonomethoxy)propyl]adenine (PMPA) were determined in beagle dogs following intravenous, intraperitoneal, and oral administration. Fasted male beagle dogs (n = 5) were pretreated with pentagastrin and received PMPA (10 mg/kg of body weight) by the intravenous and oral routes with a washout period of 1 week between doses. A further group of male dogs received PMPA as a single dose via the intravenous (1 mg/kg; n = 5) and the intraperitoneal (10 mg/kg; n = 3) routes, with 1-week washout period between doses. The concentrations of PMPA in plasma and urine were determined over 48 h postdosing by fluorescence derivatization and high-performance liquid chromatography (HPLC). The potential for metabolism or biliary excretion of PMPA was evaluated in a dog with a chronic indwelling bile cannula. Urine, feces, and bile were collected at intervals over 48 h following the intravenous administration of [14C]PMPA (10 mg/kg; 55 microCi/kg). The concentrations of PMPA in plasma after intravenous injection were best described by an open two-compartment model with a terminal half-life of approximately 10 h. PMPA was excreted unchanged in urine (70%); recovery in feces (0.42%) or bile (0.26%) was negligible. The plasma clearance of PMPA (0.28+/-0.05 liter/h/kg) was substantially greater than the glomerular filtration rate in this species, suggesting active tubular secretion of PMPA. No metabolites of [14C]PMPA were observed in urine, feces, or bile on the basis of HPLC with radioactive flow detection. The remainder of the dose was probably excreted unchanged in urine beyond 48 h postdosing. The mean+/-standard deviation observed bioavailabilities of PMPA following oral and intraperitoneal administration at 10 mg/kg were 17.1%+/-1.88% and 73.5%+/-10.5%, respectively.

Adenine↗
PubMed1998

Discharge patterns in human motor units during fatiguing arm movements.

The purpose of this study was to determine whether short interspike intervals (ISIs of <20 ms) would occur naturally during voluntary movement and would increase in number with fatigue. Thirty-four triceps brachii motor units from nine subjects were assessed during a fatigue task consisting of fifty extension and fifty flexion elbow movements against a constant-load opposing extension. Nineteen motor units were recorded from the beginning of the fatigue task; the number of short ISIs was 7.1 +/- 4.1% of the total number of ISIs in the first one-third of the task (unfatigued state). This value increased to 11.8 +/- 5.9% for the last one-third of the task (fatigued state). Fifteen motor units were recruited during the fatigue task and discharged, with 16.4 +/- 6.0% of short ISIs in the fatigued state. For all motor units, the number of short ISIs was positively correlated (r2 = 0.85) with the recruitment threshold torque. Short ISIs occurred most frequently at movement initiation but also occurred throughout the movement. These results document the presence of short ISIs during voluntary movement and their increase in number during fatigue.

Adult↗