No abstract

No abstract

Previous work has shown that variable-frequency trains (VFTs) that use an initial doublet to take advantage of the catch-like property of muscle produce more force in fatigued muscle than constant-frequency trains (CFTs); however, it is unclear whether repetitive activation with VFTs is more or less fatiguing than repetitive activation with CFTs. The purpose of this research was to investigate the forces and fatigue produced by various stimulation trains during repetitive isometric muscle contractions. Two force measurements, peak force and force-time integral, were used to measure the performance of the human quadriceps muscle. Three fatiguing protocols, each consisting exclusively of either CFTs, trains with an initial doublet (VFTs), or trains with doublets separated by longer intervals [doublet-frequency trains (DFTs)], were tested. In addition, force responses to each of the three train types were tested before and immediately following each fatiguing protocol. Regardless of the fatiguing protocol, the doublet-frequency testing trains produced the greatest peak forces and force-time integrals before and immediately following the fatiguing protocols. Repetitive activation with exclusively DFTs produced greater attenuation of the testing trains than repetitive activation with CFTs or VFTs. These results suggest that clinical applications of electrical stimulation to activate skeletal muscle may need to contain a combination of train types to optimize performance.

Thinning of the RNFL associated with elevated IOP was demonstrated with OCT in a group of experimentally glaucomatous monkey eyes over a period. OCT measurements corresponded with histomorphometric measurements of the same tissues.

The ideal functional electrical stimulation (FES) system requires a mathematical model to provide feedforward control of the stimulation parameters such that they are optimal for different individuals across a range of physiological conditions, muscles, and tasks. Recently we tested and validated such a model using able-bodied subjects. The purpose of this study was to determine whether this model applied to persons with spinal cord injuries (SCI). To this end, the isometric force responses of the paralyzed quadriceps femoris muscles of 14 adolescents and young adults were tested. For each subject, the force responses to two six-pulse stimulation trains were used to identify the parameter values of the model and then the model was used to predict the force responses to three train patterns across a range of frequencies in both a nonfatigued and fatigued condition. The intraclass correlation coefficients (ICCs) between the experimental and predicted force-time integrals and peak forces were above 0.90 for 12 of the 13 stimulation trains tested in the nonfatigued condition and all 13 trains tested in the fatigued condition. The success of our model with SCI subjects leads us to believe that our model may be useful for designing optimal stimulation parameters for standing and ambulation in patients who use FES.

Loss of muscle mass and limitations in activity have been reported in persons infected with human immunodeficiency virus (HIV), even those who are otherwise asymptomatic. The extent to which factors other than muscle atrophy impair muscle performance has not been addressed in depth. The purpose of this study was to determine the extent of neuromuscular activation of the knee extensors and ankle dorsiflexors of 27 men infected with HIV receiving antiretroviral therapy and its relationship to muscle performance. The central activation ratio (CAR) was determined using superimposed electrical stimulation during maximum voluntary contractions. In addition to force and power measurements, muscle cross-sectional area and composition was evaluated using computed tomography. Aerobic capacity was determined from treadmill exercise testing. Eleven of the subjects had an impaired ability to activate the knee extensors (CAR = 0.72 ± 0.12) that was associated with weakness and decreased specific force. The reduced central activation was not associated with muscle area, body composition, aerobic capacity, CD4 count, or medication regimen. Those individuals with low central activation had higher HIV-1 viral loads and were more likely to have a history of AIDS-defining illness. These results suggest the possibility of a different mechanism contributing to muscle impairment in the current treatment era that is associated with impairment of central motor function rather than atrophy. Further investigation is warranted in a larger, more diverse population before more definitive claims are made.Keywords central activation; electrical stimulation; HIV infection; muscle cross-sectional area Loss of skeletal muscle mass has a significant impact on functional performance, independent function, and associated quality of life in persons infected with the human immunodeficiency virus (HIV).1 , 36 Since highly active antiretroviral therapy (HAART) became the standard of care for HIV infection in 1995, the incidence of wasting (involuntary weight loss >10% ideal body weight) has declined, although it is still common.29 , 49 In the era of HAART, recent work indicates that HIV-associated weight loss is primarily due to fat 26,32 Identifying the factors beyond the reduction of muscle mass that contribute to impaired muscle function is required for optimizing the design of rehabilitation strategies to improve muscle strength and functional capacity in this chronically ill patient population.One factor that can reduce muscle force production in the absence of muscle atrophy is an impairment of central activation (i.e., the ability to activate the available muscle mass). Central activation failure has been shown to contribute significantly to muscle weakness in other clinical populations, e.g., in persons with osteoarthritis, cerebral palsy, or previous knee arthroplasty,14 , 31 , 39 and to negatively influence the relationship between muscle strength and physical function.14 The role of central activation in muscle performance in persons...

OBJECTIVES-To investigate factors related to cardiorespiratory fitness in older human immunodeficiency virus (HIV)-infected patients and to explore the utility of 6-minute walk distance (6-MWD) in measuring fitness. DESIGN-Cross-sectional study in clinic-based cohort.SETTING-Veterans Affairs Medical Center, Baltimore, Maryland.PARTICIPANTS-Forty-three HIV-infected men, median age 57 (range 50-82), without recent acquired immunodeficiency syndrome-related illness and receiving antiretroviral (ARV) therapy.MEASUREMENTS-Peak oxygen utilization (VO 2 peak) according to treadmill graded exercise testing, 6-MWD, grip strength, quadriceps maximum voluntary isometric contraction, crosssectional area, muscle quality, and muscle adiposity.RESULTS-There was a moderate correlation between VO 2 peak (mean ± SD; 18.4 ± 5.6 mL/kg per minute) and 6-MWD (514 ± 91 m) (r = 0.60, P<.001). VO 2 peak was lower in subjects with hypertension (16%, P<.01) and moderate anemia (hemoglobin 10-13 gm/dL; 15%, P = .09) than in subjects without these conditions. CD4 cell count (median 356 cells/mL, range 20-1,401) and HIV-1 viral load (84% nondetectable) were not related to VO 2 peak. Among muscle parameters, only grip strength was an independent predictor of VO 2 peak. Estimation of VO 2 peak using linear Address correspondence to Krisann K. Oursler, Baltimore VA Medical Center, GRECC/BT 18, 10 North Greene Street, Baltimore, MD 21201. koursler@grecc.umaryland.edu. Data presented in part at the Gerontological Society of America Annual Scientific Meeting, San Francisco, November 16 to 20, 2007.Author Contributions: All authors were involved with the study concept and design, acquisition of subjects and data, analysis and interpretation of data, and preparation of manuscript. Conflict of Interest:The editor in chief has reviewed the conflict of interest checklist provided by the authors and has determined that the authors have no financial or any other kind of personal conflicts with this paper. Dr. Oursler's effort on this research was supported by National Institutes of Health (NIH) grant K23 AG024896. Dr. Scott's effort on this research was supported by a NIH T32 grant. CONCLUSION-Non-AIDS-related comorbidity predicts cardiorespiratory fitness in older HIVinfected men receiving ARV therapy. The 6-MWD is a valuable measure of fitness in this patient population, but a larger study with diverse subjects is needed. KeywordsHIV; AIDS; cardiorespiratory fitness; physical functionHuman immunodeficiency virus (HIV)-infected patients who receive antiretroviral (ARV) therapy are surviving into older age and are more likely to die from age-associated comorbid conditions than acquired immunodeficiency syndrome (AIDS). 1-3 Deterioration in physical function is a consequence of chronic infection and sarcopenia in older adults.4 Aging, HIV infection, and ARV therapy have an additive effect on metabolism, pharmacokinetics, and immune function in older HIV-infected patients,5 but the combined effect on physical function is unknown.Physical function e...

It has been suggested that short pulse durations are most appropriate for NMES because they are less likely to recruit nociceptors. The results of this study, however, support the use of a medium pulse duration rather than a short pulse duration when the goal is to produce a maximum torque response from a muscle. These observations may be related to the currents and phase charges for the pain thresholds for the 2 pulse duration conditions.