J. R. Sutton

Effects of electrical stimulation-induced leg training on skeletal muscle adaptability in spinal cord injury

Neuromuscular electrical stimulation has grown in popularity as a therapeutic device for training and an ambulation aid to human paralyzed muscle. Despite its current clinical use, few studies have attempted to concurrently investigate the functional and intramuscular adaptations which occur after electrical stimulation training. Six individuals with a spinal cord injury performed 10 weeks of electrical stimulation leg cycle training (30u2003minu2003d−1, 3u2003d week−1). The paralyzed vastus lateralis muscle showed significant alterations in skeletal muscle characteristics after the training, indicated by an improvement in total work output (52–112u2003kJ; Pu2003<u20030.05), an increase in fiber cross‐sectional area (18 to 41u2003×u2003102u2003µm2; Pu2003<u20030.05), a reduction in the percentage of type IIX fibers (75% to 12%; Pu2003<u20030.05), a decrease in myosin heavy chain IIx (68% to 44%; Pu2003<u20030.05), an increase in capillary density (2–3.5 capillaries around fiber; Pu2003<u20030.05) and increases in activity levels of citrate synthase (7–16u2003mUu2003mg−1 protein) and hexokinase (1.2–2.4u2003mUu2003mg−1 protein). This study showed that 10 weeks of electrical stimulation training of human paralyzed muscle induces concurrent improvements in functional capacity and oxidative metabolism.

DYNAMICS OF PERIODIC BREATHING AND AROUSAL DURING SLEEP AT EXTREME ALTITUDE

To determine whether nocturnal periodic breathing (PB) at altitude is due primarily to unstable control of ventilation or the inability to maintain stable sleep states, we performed visual and computer analyses of the electroencephalographic and respiratory records of healthy volunteers at simulated altitudes of 4572, 6100 and 7620 m. Transient arousals were associated with < 52% of the apneas identified; thus, the PB cycle was not always associated with transient arousal. Following the termination of oxygen breathing, the reinitiation of PB was not dependent on the occurrence of arousal as the primary event. The transition from apnea to breathing preceded the appearance of arousal by approximately 1 to 4 sec. Ventilatory drive in the breaths immediately following arousal was significantly larger than corresponding control breaths, matched for SaO2. Our findings suggest that altitude-induced PB is unlikely to result from primary fluctuations in state. Arousals promote the development of PB with apnea and help to sustain these episodes, but are not necessary for their initiation.

Smaller alveolar-arterial O2 gradients in Tibetan than Han residents of Lhasa (3658 m)

Previous studies have indicated that native Tibetans have a larger lung capacity and better maintain arterial O2 saturation during exercise than Han (Chinese) acclimatized lowlanders. To test if differences in ventilation or alveolar-arterial O2 gradient (A-aDO2) were responsible, we compared 10 lifelong Tibetan and 9 Han acclimatized newcomer residents of Lhasa (3658 m) at rest and during progressive exercise. Resting blood gas tensions and arterial O2 saturation in the two groups were similar. During exercise the Tibetans had lower total ventilation and higher arterial CO2 tensions than the Han (both P < 0.01) and markedly lower A-aDO2 (7 +/- 1 vs. 11 +/- 1, 13 +/- 1 vs. 18 +/- 1, and 14 +/- 1 vs. 20 +/- 1 mmHg at light, medium, and heavy workloads respectively, all P < 0.01). The Tibetans narrower A-aDO2 compensated for their lower exercise ventilation such that arterial O2 tension and saturation were raised above acclimatized newcomer values and better maintained during exercise. We concluded that the Tibetans exhibited more efficient pulmonary gas exchange which compensated for reduced ventilation and lessened respiratory effort.

Effects of electrical stimulation leg training during the acute phase of spinal cord injury: a pilot study

Abstract Four individuals with a spinal cord injury underwent 16u2009weeks of isometric electrical stimulation training to both legs for 60u2009min, five times per week during the first 5u2009months after injury, while two SCI individuals remained untrained. A baseline biopsy sample of the vastus lateralis muscle was obtained within 1u2009month of injury, and another biopsy sample was taken after a further 16u2009weeks. The untrained, paralyzed skeletal muscle displayed a reduction in (1) type I fibers (from 50% to 9%), (2) myosin heavy chain (MHC) I (from 27% to 6%), and (3) fiber cross-sectional area of type I, type IIA and type IIX fibers (−62%, −68%, and −55%, respectively) when compared to the baseline sample of muscle taken within 1u2009month of injury. In contrast, the trained group showed smaller alterations in type I fibers (from 49% to 40%) and MHC I composition (from 39% to 25%), while fiber cross-sectional area was similar to baseline levels for type I, type IIA and type IIX fibers (−3%, −8%, and −4%, respectively). In conclusion, electrical stimulation training can largely prevent the adverse effects of a spinal cord injury upon paralyzed human skeletal muscle if applied soon after the injury.

Cardiorespiratory responses to arm cranking and electrical stimulation leg cycling in people with paraplegia

PURPOSEnThe purpose of this study was to assess the cardiorespiratory responses during arm exercise with and without concurrent electrical stimulation-induced leg cycling in people with paraplegia.nnnMETHODSnOn separate days, 10 subjects with spinal cord injuries (T5-T12) performed either arm cranking (ACE), or simultaneous arm cranking + electrical stimulation-induced leg cycling (ACE+ES-LCE) graded exercise tests.nnnRESULTSnDuring submaximal, steady-state exercise, ACE+ES-LCE elicited significantly higher VO2, (by 0.25-0.28 L x min(-1)) stroke volume (by 13 mL), and VE(BTPS) (by 9.4 L x min(-1)) compared with ACE alone. In contrast, there were no significant differences of submaximal HR, cardiac output, or power output between the exercise modes. At maximal exercise, ACE+ES-LCE elicited significantly higher VO2 (by 0.23 L x min(-1)) compared with ACE alone, but there were no differences in power output, HR, or VE(BTPS).nnnCONCLUSIONSnThese results demonstrate that during submaximal or maximal exercise there was a greater metabolic stress elicited during ACE+ES-LCE compared with during ACE alone. The higher stroke volume observed during submaximal ACE+ES-LCE, in the absence of any difference in HR, implied a reduced venous pooling and higher cardiac volume loading during ACE+ES-LCE. These results suggest that training incorporating ACE+ES-LCE may be more effective in improving aerobic fitness in people with paraplegia than ACE alone.

Operation Everest II: An indication of deterministic chaos in human heart rate variability at simulated extreme altitude

It has been shown that fluctuation of human heartbeat intervals (heart rate variability, HRV) reflects variations in autonomic nervous system activity. We studied HRV at simulated altitudes of over 6000 m from Holter electrocardiograms recorded during the Operation Everest II study (Houston et al. 1987). Stationary, ∼30-min segments of HRV data from six subjects at sea level and over 6000m were supplied to (1) spectral analysis to evaluate sympathetic and parasympathetic nervous system (SNS, PNS) activity, (2) the analysis of Poincaré section of the phase space trajectory reconstructed on a delayed coordinate system to evaluate whether there was fluctuation with deterministic dynamics, (3) the estimation of the correlation dimension to evaluate a static property of putative attractors, and (4) the analysis of nonlinear predictability of HRV time series which could reflect a dynamic property of the attractor. Unlike HRV at sea level, the recordings at over 6000 m showed a strong periodicity (period of about 20 s) with small cycle-to-cycle perturbation. When this perturbation was expressed on a Poincaré section, it seemed to be likely that the perturbation itself obeyed a deterministic law. The correlation dimensions of these recordings showed low dimensional values (3.5 ± 0.4, mean±SD), whereas those of the isospectral surrogates showed significantly (P < 0.05) higher values (5.3 ±0.5) with embedding dimensions of 5.6 ± 0.9. At over 6000 m, the correlation coefficients between the observed and the predicted time series with the prediction time of < 4 beats were significantly (P < 0.01) higher than those for the surrogate data, whereas there was no significant difference in the nonlinear predictability between the observed and the surrogate data at sea level. The results of the spectral analyses showed that, at over 6000 m, there was hardly any power > 0.15 Hz in the HRV spectra possibly due to PNS withdrawal. Hence, these deterministic and/or chaotic dynamics might be mediated by variations in SNS activity at over 6000 m.

Limitations to Maximal Oxygen Uptake

SummaryAn increase in exercise capacity depends on the magnitude of increase in maximum aerobic capacity. Central and peripheral factors may limit oxygen uptake. Central oxygen delivery depends on cardiac output and maximal arterial oxygen content. Peripheral extraction of the delivered oxygen is expressed as a-v̅ O2. With increasing intensities of exercise, the respiratory system may become limiting in some trained individuals. Most studies have shown a higher stroke volume in maximal as well as submaximal exercise in the trained vs untrained individuals A variety of peripheral factors determine vascular tone. Maximal oxygen uptake depends on all components of the oxygen transporting system, but stroke volume appears to be the prime determinant in the trained subject. At maximum exercise the capacity of the muscle capillary network is never reached.

Oxygen uptake and heart rate responses during arm vs combined arm/ electrically stimulated leg exercise in people with paraplegia

The purpose of this study was to compare the oxygen uptake and heart rate responses during submaximal arm cranking to combined arm cranking+electrical stimulation (ES)-induced leg cycling in individuals with spinal cord injury (SCI). Seven subjects with paraplegia (T4u2009–u2009T12) performed combined arm and leg cycling exercise for 5u2009min, followed by arm cranking alone at the same power output for a further 5u2009min. During both exercise conditions, steady state oxygen consumption (VO2), carbon dioxide output (VCO2), expired ventilation (VE) and heart rate (HR) were determined. The respiratory exchange ratio (RER) and oxygen pulse were calculated from the measured variables. During combined arm+electrical stimulation-induced leg cycling exercise, the VO2 was 25% higher (1.58u2009lu2009min−1 vs 1.26u2009lu2009min−1), but the HR was 13% lower (132u2009bu2009min−1 vs 149u2009bu2009min−1), than during arm cranking exercise alone. Oxygen pulse and VCO2 were also significantly higher (by 42% and 25%, respectively) during combined arm+ES-induced leg exercise, but there were no differences between the two exercise conditions for VE or RER. These data suggest that the absence of the leg `muscle pump and a reduced venous return of blood to the heart elevate exercise heart rates during submaximal arm cranking. Conversely, combined arm cranking+ES-induced leg cycling exercise provides the body with a greater metabolic stress than arm cranking alone, while reducing the cardiac stress. The mechanism explaining the heart rate response, however, remains unclear, but may have been influenced by the blood pressure variations across the range of lesions. The findings from this study may have implications for the relative benefit of combined arm+ES-induced leg cycling training for people with paraplegia.

Effects of acute expansion of plasma volume on cardiovascular and thermal function during prolonged exercise

Abstract To investigate the hypothesis that an increase in plasma volume (PV) is obligatory in reducing the cardiovascular drift that is associated with prolonged exercise following training, a plasma expander (Macrodex) was used to acutely elevate PV. Eight untrained volunteers [maximal oxygen consumption; V˙O2max 45.2 (2.2)u2009mlu2009·u2009kg−1u2009·u2009min−1, mean (SE)] cycled for 2u2009h [at 46 (4)% V˙O2max] in ambient conditions either with no PV expansion (CON) or following PV expansions of either 14% (LOW) or 21% (HIGH). During CON, heart rate (HR) increased (P<0.05) from 147 (2.4)u2009beatsu2009·u2009min−1 to 173 (3.6)u2009beatsu2009·u2009min−1 from 15 to 120u2009min of exercise. Both LOW and HIGH conditions depressed (P<0.05) HR, an effect that was manifested following 15u2009min of exercise. In contrast, stroke volume (SV) was elevated following PV expansion, with values (ml) of 89.6 (6.8), 97.8 (5.9) and 104 (4.6) noted by 15u2009min of exercise for CON, LOW and HIGH conditions, respectively. Acute PV expansion, regardless of magnitude, also resulted in elevations in cardiac output (Q˙c). These differences between conditions persisted throughout the exercise, as did the elevation in Q˙c that was noted with LOW and HIGH conditions. No difference between Q˙c, HR or SV was found between LOW and HIGH. In addition, neither LOW nor HIGH conditions altered the change in rectal temperature that was observed during exercise. These results demonstrate that, at least for moderate exercise performed in ambient conditions, PV expansion serves only to alter cardiac function (Q˙c, HR, SV) early in exercise, and not to attenuate the drift that occurs as the exercise is prolonged.

Blood metabolite and catecholamine responses to prolonged exercise following either acute plasma volume expansion or short-term training

Abstractu2002To determine the effect of acute plasma volume (PV) expansion on substrate utilization, blood metabolites and catecholamines to prolonged, moderate intensity cycle exercise, eight untrained men mean maximal oxygen uptake,O2max 4.10 (SEM 0.32) lu2009·u2009min−1 were infused (10 ml·kg−1) with a 6% dextran (DEX) solution. These responses were also compared to those elicited using a short-term training (TR) protocol involving cycling for 90 to 120 minu200a·u200aday−1 at 60% O2max for 3 consecutive days. In general DEX, which resulted in a calculated expansion of PV by 23.9%, was without effect in modifying exercise oxygen uptake or the reduction in the respiratory exchange ratio (R) observed during prolonged exercise. In addition, the concentrations of blood glucose, glycerol, alanine and serum free fatty acids, although altered (Pu2009<u20090.05) by exercise, were not altered by DEX. Blood lactate concentration was only higher (Pu2009<u20090.05) at 30 min of exercise during DEX compared to the control. With the exception of blood lactate concentration, which was reduced (Pu2009<u20090.05), TR did not change R or the concentrations of other blood metabolites. The concentrations of nonadrenaline and adrenaline, were depressed (Pu2009<u20090.05) by DEX and TR at 60 and 90 min of exercise. These results would suggest that mechanisms as yet undefined can compensate for the estimated 10% reduction in arterial oxygen content mediated by acute PV expansion and enable prolonged exercise to be performed without adjustments in substrate selection and substrate mobilization.