The mechanisms of central fatigue are largely unexplored, but the central fatigue hypothesis suggests that increased brain serotonin (5-HT) can cause a deterioration in sport and exercise performance. There is now convincing evidence that exercise-induced increases in the plasma free tryptophan (f-TRP)/branched-chain amino acids (BCCA) ratio are associated with increased brain 5-HT and the onset of fatigue during prolonged exercise. Furthermore, when drugs are administered to alter brain 5-HT, they have the predicted effects on exercise performance. The influence of nutritional manipulations of f-TRP/BCCA on performance is less well established. The effects of BCCA supplementation on exercise performance are mixed, and the published studies often suffer from methodological flaws. Alternatively, dramatic reductions in f-TRP/BCCA and enhanced performance accompany carbohydrate feedings during prolonged exercise. However, it is difficult to distinguish between the effects of carbohydrate feedings on mechanisms that reside in the brain versus the muscles themselves.
Brian L. Davis and Mark D. Grabiner
Measurement of postural sway is a valuable research and clinical tool that can provide information related to various central and peripheral elements of the nervous system. The present study involved modeling single-limb standing as an inverted pendulum tethered to a supporting surface by two sets of springs that simulated the stiffness of muscles spanning the joint and the inherent stiffness of the joint itself. There are four key elements of this model: (a) joint stiffness is greater in the frontal plane compared to the sagittal plane (neither being affected by fatigue), (b) muscle stiffness is exponentially related to its extension from a resting position, (c) muscle stiffness is reduced by fatigue, and (d) an "ankle strategy" is used to maintain upright single-limb posture. It is concluded that an inverted pendulum model can be used to adequately predict sway frequencies and amplitudes in the mediolateral (ML) and anteroposterior (AP) directions for single-limb stance pre- and postfatigue. In particular, it is possible for acute muscle fatigue to increase sway in the ML direction but not necessarily in the AP direction.
Mark Swanson, Nancy E. Schoenberg, Heather Erwin and Rian E. Davis
Most children in the United States receive far less physical activity (PA) than is optimal. In rural, under resourced areas of Appalachian Kentucky, physical inactivity rates are significantly higher than national levels. We sought to understand children’s perceptions of PA, with the goal of developing culturally appropriate programming to increase PA.
During 11 focus groups, we explored perspectives on PA among 63 Appalachian children, ages 8−17. Sessions were tape recorded, transcribed, content analyzed, and subjected to verification procedures.
Several perspectives on PA emerged among these rural Appalachian youth, including the clear distinction between PA (viewed as positive) and exercise (viewed as negative) and an emphasis on time and resource factors as barriers to adequate PA. Additional PA determinants expressed in the focus groups are similar to those of other populations. We include children’s recommendations for appealing PA programs.
Appalachian and other rural residents contend with the loss of rural health advantages (due to declines in farming/other occupational and avocational transitions). At the same time, Appalachian residents have not benefitted from urban PA facilitators (sidewalks, recreational facilities, clubs and organized leisure activities). Addressing low PA levels requires extensive community input and creative programming.
J. Mark Davis, Ralph S. Welsh and Nathan A. Alderson
This study was designed to test the hypothesis that addition of chromium (Cr) to a carbohydrate-electrolyte drink would enhance the reported benefits of carbohydrate on exercise capacity during intermittent high-intensity shuttle running.
Eight physically active men performed 3 exercise trials while ingesting 6% carbohydrate-electrolyte (CHO), CHO plus chromium picolinate (400 μg) (CHO + Cr+3). or placebo (P) using a double-blind, counterbalanced design. Each trial consisted of 5 × 15 min bouts of shuttle running (walk, sprint, and run at 95 and 55% of estimated V̇O2max, separated by 3-min rest). This was followed by a fatigue test (running alternating 20-m lengths at 55 and 95% of estimated V̇O2, until fatigue).
During the standardized shuttle running, blood glucose was higher with both CHO and CHO + Cr+3 than P. Plasma free fatty acid was higher in P than both CHO and CHO + Cr+3 at 75 min of exercise and at fatigue. In the fatigue test, subjects ran longer with both CHO and CHO + Cr+3 than P.
The data confirm an ergogenic benefit of ingesting CHO during exercise designed to imitate sports like basketball, soccer, and hockey, but do not support the hypothesis that the addition of Cr would enhance this effect.
J. Mark Davis, Catherine J. Carlstedt, Stephen Chen, Martin D. Carmichael and E. Angela Murphy
Quercetin, a natural polyphenolic flavonoid substance present in a variety of food plants, has been shown in vitro and in animal studies to have widespread health and performance benefits resulting from a combination of biological properties, including antioxidant and anti-inflammatory activity, as well as the ability to increase mitochondrial biogenesis. Little is known about these effects in humans, however, especially with respect to exercise performance. The authors determined whether quercetin ingestion would enhance maximal aerobic capacity and delay fatigue during prolonged exercise in healthy but untrained participants. Twelve volunteers were randomly assigned to 1 of 2 treatments: (a) 500 mg of quercetin twice daily dissolved in vitamin-enriched Tang or (b) a nondistinguishable placebo (Tang). Baseline VO2max and bike-ride times to fatigue were established. Treatments were administered for a period of 7 days using a randomized, double-blind, placebo-controlled, crossover study design. After treatment both VO2max and ride time to fatigue were determined. Seven days of quercetin feedings were associated with a modest increase in VO2max (3.9% vs. placebo; p < .05) along with a substantial (13.2%) increase in ride time to fatigue (p < .05). These data suggest that as little as 7 days of quercetin supplementation can increase endurance without exercise training in untrained participants. These benefits of quercetin may have important implications for enhancement of athletic and military performance. This apparent increase in fitness without exercise training may have implications beyond that of performance enhancement to health promotion and disease prevention.
Russell R. Pate, Bonnie J. Miller, J. Mark Davis, Chris A. Slentz and Lisa A. Klingshirn
This study was designed to observe iron status and prevalence of iron deficient conditions in adult female habitual runners (n=111) and inactive females of comparable age (n=65). The runners were significantly lower (p<.05) than the reference group in mean serum ferritin (SF), total iron binding capacity, and red blood cell count, but significantly higher (p<.05) in mean corpuscular hemoglobin. The groups did not differ significantly in hemoglobin concentration, hematocrit, serum iron, percent saturation of transfenin, or red cell protoporphyrin. Chi square analysis indicated that iron depletion (SF <20 ng·ml-1) was significantly more prevalent (p<.005) in the runners than in the controls. Anemia was extremely rare in both groups. A multiple regression analysis revealed significant negative associations between serum ferritin and coffeeltea intake (p<.001) and running activity (p<.05). These results indicate that habitual runners, as compared with inactive women, are at increased risk for iron deficient states but that full-blown anemia is a rare consequence of this deficient iron status.
William A. Burgess, J. Mark Davis, William P. Bartoli and Jeffrey A. Woods
The effects of ingesting a low dose of CHO on plasma glucose, glucoregulatory hormone responses, and performance during prolonged cycling were investigated. Nine male subjects cycled for 165 min at ≈67% peak
Afroditi Stathi, Holly Gilbert, Kenneth R. Fox, Jo Coulson, Mark Davis and Janice L. Thompson
This mixed-methods study investigated personal, interpersonal, and environmental factors salient to decisions about being active in neighborhoods of different levels of deprivation.
Twenty-five participants age 70 years and older (10 women) with diverse physical activity levels provided data on their weekly activity patterns (using accelerometry) and their perceived barriers to exercise (questionnaire). They also participated in semistructured individual interviews exploring the barriers and facilitators influencing neighborhood activity.
Functional limitations, lack of intrinsic motivation, and not having an activity companion were the highest impact barriers. Walkable access to amenities, positive physical activity perceptions, and existing habit of being active were the highest impact facilitators.
The perceived quality and accessibility of the built and natural environments influence neighborhood activity in older adults. However, this relationship might be altered through the influence of personal and interpersonal determinants such as maintenance of good health and functional ability and supportive social networks.
Tanya Trayers, Debbie A. Lawlor, Kenneth R. Fox, Jo Coulson, Mark Davis, Afroditi Stathi and Tim Peters
Associations of objectively measured physical activity (PA) with objectively measured lower limb function in adults age 70 and older were studied. Lower limb function was assessed using the Short Physical Performance Battery (SPPB) and PA by an accelerometer providing mean daily counts per minute (CPM), mean daily steps and minutes of moderate or vigorous PA (MVPA) per day. A minority (32 [13%]) scored low (≤6 out of a maximum of 12) on the SPPB, but only 3 (1%) achieved recommended PA levels. Adjusting for confounders, the odds ratio of low SPPB (≤6) comparing those in the lowest one third to highest two thirds of mean CPM was 55 (95%CI: 6, 520); for mean steps per day it was 23 (95%CI: 4, 137) and for minutes of MVPA per day 56 (95%CI: 6, 530). Low levels of PA are common and are associated with poor levels of lower limb function in older adults.
Mark G. Davis, Kenneth R. Fox, Afroditi Stathi, Tanya Trayers, Janice L. Thompson and Ashley R. Cooper
The relationship of objectively measured sedentary time (ST), frequency of breaks in ST, and lower extremity function (LEF) was investigated in a diverse sample aged ≥ 70 years (n = 217). Physical activity (PA) was assessed by accelerometry deriving moderate-vigorous PA (MVPA) minutes per registered hour (MVPA min · hr−1), registered ST (ST min · hr−1), and breaks in ST min · hr−1 (breaks · hr−1). LEF was assessed by the Short Physical Performance Battery. Univariate associations with overall LEF were MVPA (r = .523), ST (r = −.499), and breaks (r = .389). Adjusted linear regression including MVPA min · hr−1, ST min · hr−1, and breaks · hr−1 explained 41.5% of LEF variance. Each additional break · hr−1 was associated with 0.58 point increase in LEF. Breaks and MVPA had strongest independent associations with LEF. Promoting regular breaks might be useful in maintaining or increasing LEF and later life independence. This novel finding is important for the design of effective lifestyle interventions targeting older adults.