Dana K. Townsend

Effects of inspiratory muscle training on exercise responses in normoxia and hypoxia

The purpose of this study was to determine the effects of inspiratory muscle training (IMT) on exercise in hypoxia (H) and normoxia (N). A 4-week IMT program was implemented with 12 healthy subjects using an inspiratory muscle trainer set at either 15% (C; n=5) or 50% (IMT; n=7) maximal inspiratory mouth pressure (PImax). Two treadmill tests (85% VO2max) to exhaustion and measures of diaphragm thickness (Tdi) and function were completed before and after training in H and N. Significant increases of 8-12% and 24.5+/-3.1% in Tdi and PImax, respectively, were seen in the IMT group. Time to exhaustion remained unchanged in all conditions. Inspiratory muscle fatigue (downward arrowPImax) following exercise was reduced approximately 10% (P<0.05) in IMT after both N and H. During H, IMT reduced (P<0.05) VO2 by 8-12%, cardiac output by 14+/-2%, ventilation by 25+/-3%; and increased arterial oxygen saturation by 4+/-1% and lung diffusing capacity by 22+/-3%. Ratings of perceived exertion and dyspnea were also significantly reduced. These data suggest that IMT significantly improves structural and functional physiologic measures in hypoxic exercise.

Human femoral artery and estimated muscle capillary blood flow kinetics following the onset of exercise.

The purpose of this study was to compare the kinetics of estimated capillary blood flow to those of femoral artery blood flow and estimated muscle oxygen uptake . Nine healthy subjects performed a series of transitions from rest to moderate (below estimated lactate threshold, 6 min bouts) knee extension exercise. Pulmonary oxygen uptake was measured breath by breath, was measured continuously using Doppler ultrasound, and deoxyhaemoglobin ([HHb]) was estimated by near‐infrared spectroscopy over the rectus femoris throughout the tests. The time course of was estimated by rearranging the Fick equation (i.e. ), (arterio – venous O2 difference) using the primary component of to represent and [HHb] as a surrogate for (a−v)O2. The overall kinetics of (mean response time, MRT, 13.7 ± 7.0 s), (τ, 27.8 ± 9.0 s) and (MRT, 41.4 ± 19.0 s) were significantly (P < 0.05) different from each other. We conclude that for moderate intensity knee extension exercise, conduit artery blood flow kinetics may not be a reasonable approximation of blood flow kinetics in the microcirculation , the site of gas exchange. This temporal dissociation suggests that blood flow may be controlled differently at the conduit artery level than in the microcirculation.

Kinetics of estimated human muscle capillary blood flow during recovery from exercise.

The kinetic characteristics of muscle capillary blood flow during recovery from exercise are controversial (e.g. one versus two phases). Furthermore, it is not clear how the overall kinetics are temporally associated with muscle oxygen uptake kinetics. To address these issues, we examined the kinetics of estimated from the rearrangement of the Fick equation using the kinetics of pulmonary ( , primary component) and deoxy‐haemoglobin concentration ([HHb]) as indices of andC  (a − v)O 2(arterio‐venous oxygen difference) kinetics, respectively. (l min−1) was measured breath by breath and [HHb] (μm) was measured by near infrared spectroscopy during moderate (M; below lactate threshold, LT) and heavy exercise (H, above LT) in nine subjects. The kinetics of were biphasic, with an initial fast phase (τI; M = 9.3 ± 4.9 s and H = 6.0 ± 3.8 s) followed by a slower phase 2 (τP; M = 29.9 ± 8.6 s and H = 47.7 ± 26.0 s). For moderate exercise, the overall kinetics of (mean response time [MRT], 36.1 ± 8.6 s) were significantly slower than the kinetics of (τP; 27.8 ± 5.3 s) and [HHb] (MRT for [HHb]; 16.2 ± 6.3 s). However, for heavy exercise, there was no significant difference between MRT‐[HHb] (34.7 ± 10.4 s) and τP for (32.3 ± 6.7 s), while MRT for (48.7 ± 21.8 s) was significantly slower than MRT for [HHb] and τP for . In conclusion, during recovery from exercise the estimated kinetics were biphasic, showing an early rapid decrease in blood flow. In addition, the overall kinetics of were slower than the estimated kinetics.

Dynamics of skeletal muscle oxygenation during sequential bouts of moderate exercise

In rat muscle, faster dynamics of microvascular PO2 (approximately blood flow to O2 uptake ratio) after prior contractions that did not alter blood [lactate] have been considered to be a consequence of faster kinetics. However, in humans, prior exercise below the lactate threshold does not affect the pulmonary kinetics. To clarify this apparent discrepancy, we examined the effects of prior moderate exercise on the kinetics of muscle oxygenation (deoxyhaemoglobin, [HHb]α ) and pulmonary in humans. Eight subjects performed two bouts (6 min each) of moderate‐intensity cycling separated by 6 min of baseline pedalling. Muscle (vastus lateralis) oxygenation was evaluated by near‐infrared spectroscopy and was measured breath‐by‐breath. The time constant (τ) of the primary component of was not significantly affected by prior exercise (21.5 ± 9.2 versus 25.6 ± 9.7 s; Bout 1 versus 2, P= 0.49). The time delay (TD) of [HHb] decreased (11.6 ± 2.6 versus 7.7 ± 1.5 s; Bout 1 versus 2, P < 0.05) and τ[HHb] increased (7.0 ± 3.5 versus 10.2 ± 4.6 s; Bout 1 versus 2, P < 0.05), while the mean response time (TD +τ) did not change (18.6 ± 2.7 versus 17.9 ± 3.9 s) after prior moderate exercise. Thus, prior moderate exercise resulted in shorter onset and slower rate of increase in [HHb] during subsequent exercise. These data suggest that prior exercise altered the dynamic interaction between and following the onset of exercise.

Muscle microvascular hemoglobin concentration and oxygenation within the contraction-relaxation cycle.

Inability to directly measure microvascular oxygen distribution and extraction in striated muscle during a contraction/relaxation cycle limits our understanding of oxygen transport to and utilization by contracting muscle. We examined muscle microvascular hemoglobin concentration (total [Hb/Mb]) and oxygenation within the contraction-relaxation cycle to determine if microvascular RBC volume would be preserved and if oxygen extraction continued during the actual contraction phase. Eight subjects performed dynamic knee extension exercise (40 contractions/min) at moderate ( approximately 30% of peak work rate) and heavy ( approximately 80% of peak) work rates. Total hemoglobin/myoglobin (total [Hb/Mb]) and deoxy-hemoglobin/myoglobin (deoxy-[Hb/Mb]) were measured in the rectus femoris using NIRS to determine if microvascular total [Hb/Mb] would be preserved during the contraction, and to estimate microvascular oxygen extraction, respectively. Mean values during the relaxation (RP) and contractile phases and the peak values during the contractile phase for both moderate and heavy exercise were calculated. Total [Hb/Mb] increased from rest to steady-state exercise (6.36+/-5.08 microM moderate; 5.72+/-4.46 microM heavy exercise, both P<0.05), but did not change significantly within the contraction/relaxation cycle. Muscle contractions were associated with a significant (1.29+/-0.98 microM moderate; 2.16+/-2.12 microM heavy exercise, P<0.05) increase in deoxy-[Hb/Mb] relative to RP. We conclude that (a) microvascular RBC volume is preserved during muscle contractions (i.e., RBCs are present in the capillaries), and (b) the cyclical pattern of deoxygenation/oxygenation during the respective contraction/relaxation phases of the contraction cycle suggests that oxygen extraction is not restricted to the relaxation phase but continues to occur during muscle contractions.

Matching of blood flow to metabolic rate during recovery from moderate exercise in humans

It is unclear whether measurement of limb or conduit artery blood flow during recovery from exercise provides an accurate representation of flow to the muscle capillaries where gas exchange occurs. To investigate this, we: (a) examined the kinetic responses of femoral artery blood flow ( ), estimated muscle capillary blood flow ( ) and estimated muscle oxygen uptake ( ) following cessation of exercise; and (b) compared these responses to verify the adequacy of O2 delivery during recovery. Pulmonary ( ) was measured breath by breath, was measured using Doppler ultrasonography, and deoxy‐haemoglobin/myoglobin (deoxy‐[Hb/Mb]) was estimated by near‐infrared spectroscopy over the rectus femoris in nine healthy subjects during a series of transitions from moderate knee‐extension exercise to rest. The time course of was estimated by rearranging the Fick equation [i.e. ], using the primary component of to represent and deoxy‐[Hb/Mb] as a surrogate for arteriovenous O2 difference. There were no significant differences among the overall kinetics of (τ, 31.4 ± 8.2 s), [mean response time (MRT), 34.5 ± 20.4 s] and (MRT, 31.7 ± 14.7 s). The kinetics were also significantly correlated (P < 0.05) with those of both and . Both and appear to be coupled with during recovery from moderate knee‐extension exercise, such that extraction falls (thus cellular energetic state is not further compromised) throughout recovery.