Effects of the menstrual cycle on muscle recruitment and oxidative fuel selection during cold exposure.

Differences in core temperature and body heat content, generally observed between the luteal and follicular phase of the menstrual cycle, have been reported to modulate the thermogenic activity of cold-exposed women. However, it is unclear how this change in whole body shivering activity will influence fuel selection. The goal of this study was to quantify the effects of the menstrual cycle on muscle recruitment and oxidative fuel selection during low-intensity shivering. Electromyographic activity of eight large muscles was monitored while carbohydrate, lipid, and protein utilization was simultaneously quantified in the follicular and luteal phases of the menstrual cycle in nonacclimatized women shivering at a low intensity. The onset (∼25 min), intensity (∼15% of maximal voluntary contraction), and pattern (∼6 shivering bursts/min) of the shivering response did not differ between menstrual cycle phases, regardless of differences in core temperature and hormone levels. This resulted in lipids remaining the predominant substrate, contributing 75% of total heat production, independent of menstrual phase. We conclude that hormone fluctuations inherent in the menstrual cycle do not affect mechanisms of substrate utilization in the cold. Whether the large contribution of lipids to total heat production in fuel selection confers a survival advantage remains to be established.

Metabolic requirements of shivering humans.

This review examines the heat production component of thermoregulation in adult humans. It describes the energy requirements of shivering muscles as they attempt to provide sufficient heat to counterbalance increases in heat loss in cold environments. Emphasis is placed on types of metabolic substrates used under various shivering conditions as well as on the effects energy deficit and food consumption. During shivering, muscle recruitment intensity and pattern of fiber recruitment are highly variable between muscles and individuals. In addition, a number of studies have indicated that shivering can be sustained with different fuels for several hours under variable conditions of cold stress and CHO availability. However, little is still known on the effects of prolonged fasting and energy deficit in the cold on energy metabolism. Even though it is clear that food consumption increases the odds for survival, the metabolic fate of ingested substrates remains highly uncertain. Combining fundamental principles surrounding metabolic fuel selection with applied knowledge of human performance in the cold may allow important breakthroughs in this field of research.

Effects of two glucose ingestion rates on substrate utilization during moderate-intensity shivering.

Although the importance of food consumption to survive in the cold is well established, most shivering studies have focused on fuel selection in fasting subjects. Therefore, the aim of the present study was to provide the first estimates of exogenous glucose as well as liver and muscle glycogen oxidation rates of non-cold acclimatized men (n = 6) ingesting glucose in trace amounts (Control; C), and at rates of 400 mg min(-1) (Low Glucose; LG), and 800 mg min(-1) (High Glucose; HG) during moderate-intensity shivering (~3 times resting metabolic rate or ~20% VO(2max)) using indirect calorimetry and stable isotope methodologies. Exogenous glucose oxidation peaked at ~200 mg min(-1) at the lowest glucose ingestion rate (~400 mg min(-1)). In addition, glucose ingestion increased the contribution of plasma glucose to total heat production by ~50% but did not change the role played by muscle glycogen (~27% of heat production for control condition and ~23-28% for LG and HG). Instead, the contribution of liver-derived glucose to total heat production was reduced by 40-60% in LG and HG, respectively. In conclusion, glucose ingestion even at low rates contributes a significant proportion of total heat production during moderate intensity shivering and reduces the utilization of liver-derived glucose but not muscle glycogen.

On the potential role of the corticospinal tract in the control and progressive adaptation of the soleus h-reflex during backward walking.

When untrained subjects walk backward on a treadmill, an unexpectedly large amplitude soleus H-reflex occurs in the midswing phase of backward walking. We hypothesized that activity in the corticospinal tract (CST) during midswing depolarizes the soleus alpha-motoneurons subliminally and thus brings them closer to threshold. To test this hypothesis, transcranial magnetic stimulation (TMS) was applied to the leg area of the motor cortex (MCx) during backward walking. Motor-evoked potentials (MEPs) were recorded from the soleus and tibialis anterior (TA) muscles in untrained subjects at different phases of the backward walking cycle. We reasoned that if soleus MEPs could be elicited in midswing, while the soleus is inactive, this would be strong evidence for increased postsynaptic excitability of the alpha-motoneurons. In the event, we found that in untrained subjects, despite the presence of an unexpectedly large H-reflex in midswing, no soleus MEPs were observed at that time. The soleus MEPs were in phase with the soleus electromyographic (EMG) activity during backward walking. Soleus MEPs increased more rapidly as a function of the EMG activity during voluntary activity than during backward walking. Furthermore, a conditioning stimulus to the motor cortex facilitated the soleus H-reflex at rest and during voluntary plantarflexion but not in the midswing phase of backward walking. With daily training at walking backward, the time at which the H-reflex began to increase was progressively delayed until it coincided with the onset of soleus EMG activity, and its amplitude was considerably reduced compared with its value on the first experimental day. By contrast, no changes were observed in the timing or amplitude of soleus MEPs with training. Taken together, these observations make it unlikely that the motor cortex via the CST is involved in control of the H-reflex during the backward step cycle of untrained subjects nor in its progressive adaptation with training. Our observations raise the possibility that the large amplitude of H-reflex in untrained subjects and its adaptation with training are mainly due to control of presynaptic inhibition of Ia-afferents by other descending tracts.

On the soleus H-reflex modulation pattern during walking.

In a recent paper it was claimed that in the majority (9/15) of subjects studied the soleus H-reflex increases progressively during the swing phase of walking. This pattern was at odds with our numerous observations made since 1986, as was the very large proportion of subjects reported to exhibit this pattern. We therefore reinvestigated the issue in an extensive series of experiments and detailed subsequent analysis on 21 subjects. In most subjects (13/21) the soleus H-reflex was completely inhibited during most or all of the swing phase (group A). In 8/21 subjects (group B) there was a small H-reflex mean 16% (SD=10.6%) of the value in quiet standing present during most or all of swing, but there was no systematic modulation pattern; the reflex amplitude fluctuated in a seemingly random manner. The difference between the two somewhat arbitrary groups could not be explained on the basis of greater electromyographic activity in the tibialis anterior (TA) during the swing phase or at the time of heel contact. However, by normalizing the mean level of TA activity to the peak level, the ratio was significantly greater for the group A subjects. This highlights the importance of reciprocal inhibition in accounting for the suppression of the soleus H-reflex in swing. In the discussion we emphasize that the presence of a small H-reflex during swing in the group B subjects is unlikely to have any functional role. What is of functional importance is the strong inhibition of the H-reflex during swing which reflects the ensemble of neural mechanisms at play to prevent the unwanted activation of the powerful ankle extensor muscles.