Relationship between disturbances of CO2 homeostasis and force output characteristics during isometric knee extension.

To determine whether disturbances of CO2 homeostasis alter force output characteristics of lower limb muscles, participants performed four isometric knee extension trials (MVC30 %, 10 s each with 20-s rest intervals) in three CO2 conditions (normocapnia [NORM], hypercapnia [HYPER], and hypocapnia [HYPO]). Respiratory frequency and tidal volume were matched between CO2 conditions. In each MVC30 %, the participants exerted a constant force (30 % of maximum voluntary contraction [MVC]). The force coefficient of variation (Fcv) during each MVC30 % and MVC before and after the four MVC30 % trials were measured. For the means of the four trials, Fcv was significantly lower in HYPER than in HYPO. However, within HYPER, a significant positive correlation was found between the increase in end-tidal CO2 partial pressure and the increase in Fcv. MVCs in NORM and HYPO decreased significantly over the four trials, while no such reduction was observed in HYPER. These results suggest that perturbed CO2 homeostasis influences the force output characteristics independently of breathing pattern variables.

Enhancement of self-sustained muscle activity through external dead space ventilation appears to be associated with hypercapnia.

We reported that external dead space ventilation (EDSV) enhanced self-sustained muscle activity (SSMA) of the human soleus muscle, which is an indirect observation of plateau potentials. However, the main factor for EDSV to enhance SSMA remains unclear. The purpose of the present study was to examine the effects of EDSV-induced hypercapnia, hypoxia, and hyperventilation on SSMA. In Experiment 1 (n = 11; normal breathing [NB], EDSV, hypoxia, and voluntary hyperventilation conditions) and Experiment 2 (n = 9; NB and normoxic hypercapnia [NH] conditions), SSMA was evoked by electrical train stimulations of the right tibial nerve and measured using surface electromyography under each respiratory condition. In Experiment 1, SSMA was significantly higher than that in the NB condition only in the EDSV condition (P < 0.05). In Experiment 2, SSMA was higher in the NH condition than in the NB condition (P < 0.05). These results suggest that the EDSV-enhanced SSMA is due to hypercapnia, not hypoxia or increased ventilation.

Effects of acute static stretching on exercise efficiency during high-intensity cycling exercise.

BACKGROUND: Static stretching has been shown to reduce exercise efficiency and performance of aerobic exercises as well as decreasing the ability to generate maximum muscle force in momentary movements such as jumping. As a mechanism for the reduced efficiency, the possibility that changes in muscle fiber recruitment occur to compensate for the decrease in maximum voluntary contraction (MVC) has been suggested. We examined whether the stretching-induced decrease in exercise efficiency is accompanied by a decrease in MVC and changes in electromyographic (EMG) activities of exercising muscles. METHODS: Eight healthy subjects performed 6 min of cycling exercise at 70% of maximal oxygen uptake in the conditions with and without pre-exercise static stretching (stretching condition [SC] and control condition [CC]). MVC was measured before stretching maneuvers (or the resting period in CC) and after the end of exercise. In the 6-min exercise test, respiratory parameters and EMG activities of knee extensors were measured. RESULTS: Oxygen uptake at 3 min and 6 min during the cycling exercise were higher in the SC than in the CC (P<0.05). There was no difference in MVC at the end of 6 min exercise between the two conditions. Similarly, there were no differences in EMG activities during the 6-min exercise between the two conditions. CONCLUSIONS: The results suggest that pre-exercise static stretching reduces exercise efficiency independent of its negative effect on MVC exertional capacity.

Effect of hypercapnia on self-sustained muscle activity.

The aim of the present study was to determine the effect of hypercapnia on motor neuromuscular activity of the human triceps surae muscle. Nine subjects participated in trials in a normal breathing condition and a CO2 rebreathing condition. In both conditions, in order to provoke self-sustained muscle activity, percutaneous electrical train stimulation was applied to the tibial nerve while each subject lay on a bed. Self-sustained muscle activity, which is an indirect observation of plateau potentials in spinal motoneurons, was measured for 30 s after the train stimulation by using surface electromyography. The sustained muscle activity was increased by CO2 rebreathing (P < 0.05). This finding suggests that motor neuromuscular activity may be linked to the respiratory system that is activated during hypercapnia.