Substrate profile in rat soleus muscle fibers after hindlimb unloading and fatigue.
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Limb muscles from rats flown in space and after hindlimb unloading (HU) show an increased fatigability, and spaceflight has been shown to result in a reduced ability to oxidize fatty acids. The purpose of this investigation was to determine the effects of HU on the substrate content in fast- and slow-twitch fibers and to assess the substrate utilization patterns in single slow type I fibers isolated from control and HU animals. A second objective was to assess whether HU altered the ability of the heart or limb muscle to oxidize pyruvate or palmitate. After 2 wk of HU, single fibers were isolated from the freeze-dried soleus and gastrocnemius muscles. HU increased the glycogen content in all fiber types, and it increased lactate, ATP, and phosphocreatine in the slow type I fiber. After HU, the type I fiber substrate profile was shifted toward that observed in fast fibers. For example, fiber glycogen increased from 179 +/- 16 to 285 +/- 25 mmol/kg dry wt, which approached the 308 +/- 23 mmol/kg dry wt content observed in the post-HU type IIa fiber. With contractile activity, the type I fiber from the HU animal showed a greater utilization of glycogen and accumulation of lactate compared with the control type I fiber. HU had no effect on the ability of crude homogenate or mitochondria fractions from the soleus or gastrocnemius to oxidize pyruvate or palmitate. The increased fatigability after HU may have resulted from an elevated glycolysis producing an increased cell lactate and a decreased pH. (+info)
Contractile and fatigue properties of the rat diaphragm musculature during the perinatal period.
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The following two hypotheses regarding diaphragm contractile properties in the perinatal rat were tested. First, there is a major transformation of contractile and fatigue properties during the period between the inception of inspiratory drive transmission in utero and birth. Second, the diaphragm muscle properties develop to functionally match changes occurring in phrenic motoneuron electrophysiological properties. Muscle force recordings and intracellular recordings of end-plate potentials were measured by using phrenic nerve-diaphragm muscle in vitro preparations isolated from rats on embryonic day 18 and postnatal days 0-1. The following age-dependent changes occurred: 1) twitch contraction and half relaxation times decreased approximately two- and threefold, respectively; 2) the tetanic force levels increased approximately fivefold; 3) the ratio of peak twitch force to maximum tetanic force decreased 2.3-fold; 4) the range of forces generated by the diaphragm in response to graded nerve stimulation increased approximately twofold; 5) the force-frequency curve was shifted to the right; and 6) the propensity for neuromuscular transmission failure decreased. In conclusion, the diaphragm contractile and phrenic motoneuron repetitive firing properties develop in concert so that the full range of potential diaphragm force recruitment can be utilized and problems associated with diaphragm fatigue are minimized. (+info)
Human muscle power generating capability during cycling at different pedalling rates.
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The effect of different pedalling rates (40, 60, 80, 100 and 120 rev min-1) on power generating capability, oxygen uptake (O2) and blood lactate concentration [La]b during incremental tests was studied in seven subjects. No significant differences in O2,max were found (mean +/- S.D., 5.31 +/- 0.13 l min-1). The final external power output delivered to the ergometer during incremental tests (PI,max) was not significantly different when cycling at 60, 80 or 100 rev min-1 (366 +/- 5 W). A significant decrease in PI,max of 60 W was observed at 40 and 120 rev min-1 compared with 60 and 100 rev min-1, respectively (P < 0.01). At 120 rev min-1 there was also a pronounced upward shift of the O2-power output (O2-P) relationship. At 50 W O2 between 80 and 100 rev min-1 amounted to +0.43 l min-1 but to +0.87 l min-1 between 100 and 120 rev min-1. The power output corresponding to 2 and 4 mmol l-1 blood lactate concentration (P[La]2 and P[La]4 ) was also significantly lower (> 50 W) at 120 rev min-1 (P < 0.01) while pedalling at 40, 60, 80 and 100 rev min-1 showed no significant difference. The maximal peak power output (PM, max) during 10 s sprints increased with pedalling rate up to 100 rev min-1. Our study indicates that with increasing pedalling rate the reserves in power generating capability increase, as illustrated by the PI,max/PM,max ratio (54.8, 44.8, 38.1, 34.6, 29.2%), the P[La]4/PM,max ratio (50.4, 38.9, 31.0, 27.7, 22.9%) and the P[La]2/PM,max ratio (42.8, 33.5, 25.6, 23.1, 15.6%) increases. Taking into consideration the O2,max, the PI,max and the reserve in power generating capability we concluded that choosing a high pedalling rate when performing high intensity cycling exercise may be beneficial since it provides greater reserve in power generating capability and this may be advantageous to the muscle in terms of resisting fatigue. However, beyond 100 rev min-1 there is a decrease in external power that can be delivered for an given O2 with an associated earlier onset of metabolic acidosis and clearly this will be disadvantageous for sustained high intensity exercise. (+info)
Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during muscle fatigue.
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The effects of 10 microM glibenclamide, an ATP-sensitive K(+) (K(ATP)) channel blocker, and 100 microM pinacidil, a channel opener, were studied to determine how the K(ATP) channel affects mouse extensor digitorum longus (EDL) and soleus muscle during fatigue. Fatigue was elicited with 200-ms-long tetanic contractions every second. Glibenclamide did not affect rate and extent of fatigue, force recovery, or (86)Rb(+) fractional loss. The only effects of glibenclamide during fatigue were: an increase in resting tension (EDL and soleus), a depolarization of the cell membrane, a prolongation of the repolarization phase of action potential, and a greater ATP depletion in soleus. Pinacidil, on the other hand, increased the rate but not the extent of fatigue, abolished the normal increase in resting tension during fatigue, enhanced force recovery, and increased (86)Rb(+) fractional loss in both the EDL and soleus. During fatigue, the decreases in ATP and phosphocreatine of soleus muscle were less in the presence of pinacidil. The glibenclamide effects suggest that fatigue, elicited with intermittent contractions, activates few K(ATP) channels that affect resting tension and membrane potentials but not tetanic force, whereas opening the channel with pinacidil causes a faster decrease in tetanic force, improves force recovery, and helps in preserving energy. (+info)
The effect of fatigue on multifinger co-ordination in force production tasks in humans.
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1. This study investigated the effects of fatigue, induced by production of maximal isometric force for 60 s with four fingers, upon indices of multifinger co-ordination. 2. Measurements of individual finger forces were performed during single- and multifinger maximal force production (maximal voluntary contraction, MVC) for two sites of force application, the middle of the distal or the middle of the proximal phalanxes. Two fatiguing exercises were used, involving force production at the distal phalanxes and at the proximal phalanxes. Fourteen subjects were tested. 3. The total force in four-finger tasks dropped by about 43 % when it was produced at the site involved in the fatiguing exercise. During force production at the other site, MVC dropped by 23 %. During single-finger MVC tests, force drop with fatigue was similar across all four fingers (about -25 % of their corresponding MVCs). 4. Force production by one finger was accompanied by involuntary force production by other fingers (enslaving). Enslaving remained unchanged by fatigue when measured during force generation at the site involved in the fatiguing exercise, but increased during force production at the other site. 5. The total MVC of four fingers acting in parallel was smaller than the sum of the MVCs of these fingers in single-finger tasks (force deficit). The force deficit increased with fatigue. Force-sharing patterns during four-finger tasks showed only minor changes under fatigue. 6. These results indicate that the effects of fatigue were not limited to changes in the force-generating capabilities of the muscles. In particular, fatigue could lead to a reorganisation at a neural level that defines commands to individual fingers. (+info)
Isotonic mechanics of a pharyngeal dilator muscle and diaphragm in the rat before and after fatigue.
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Pharyngeal and diaphragm muscles contract and relax in synergy, which is why it was decided to compare their mechanical performance throughout the overall load continuum. The effects of fatigue were also studied. The isotonic mechanics of rat sternohyoid (SH; n=10) and diaphragm (D; n=10) were investigated in vitro. Force and length were measured in muscles contracting from zero load up to isometry. Maximum isometric tension (Pmax), peak mechanical work (Wmax), maximum unloaded shortening velocity (vzL) and mechanical efficiency (eff(max)) were recorded. Data were obtained both at baseline and after fatigue. SH muscles had a lower Pmax (96.0+/-13.7 versus 119.5+/-22.7 mN x mm(-2); p<0.05), a lower Wmax (5.5+/-1.2 versus 8.0+/-2.1 mJ x g(-1); p<0.01), a lower eff(max) (56.0+/-6.9 versus 62.6+/-5.8%; p<0.05) and a higher vzL (4.8+/-0.4 versus 3.4+/-0.4 initial length (L0) x s(-1); p<0.001) than D muscles. Wmax occurred at a higher relative load in SH (40% Pmax) than in D (30% Pmax). Fatigue did not modify eff(max) in SH muscles, whereas it significantly improved eff(max) in D muscles. These findings suggest that under control conditions, economy of force generation was less efficient in sternohyoid than in diaphragm muscles. Fatigue in sternohyoid muscles induced unfavourable mechanical behaviour. This may partly explain pharyngeal dilator muscle failure in the presence of increased loads. Whether these findings are relevant to human sleep apnoea syndrome has yet to be determined. (+info)
Blunted pressor and intramuscular metabolic responses to voluntary isometric exercise in multiple sclerosis.
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To test the hypothesis that a lower mean arterial pressure (MAP) response during voluntary isometric exercise in multiple sclerosis (MS) is related to a dampened muscle metabolic signal, 9 MS and 11 control subjects performed an isometric dorsiflexor contraction at 30% maximal voluntary contraction until target failure (endurance time). We made continuous and noninvasive measurements of heart rate and MAP (Finapres) and of intramuscular pH and P(i) (phosphorus magnetic resonance spectroscopy) in a subset of 6 MS and 10 control subjects. Endurance times and change in heart rate were similar in MS and control subjects. The decrease in pH and increase in P(i) were less throughout exercise in MS compared with control subjects, as was the change in MAP response. Differences in muscle strength accounted for some of the difference in MAP response between groups. Cardiovascular responses during Valsalva and cold pressor tests were similar in MS and control subjects, suggesting that the blunted MAP response during exercise in MS was not due to a generalized dysautonomia. The dampened metabolic response in MS subjects was not explained by inadequate central muscle activation. These data suggest that the blunted pressor response to exercise in MS subjects may be largely appropriate to a blunted muscle metabolic response and differences in contracting muscle mass. (+info)
Development of a mathematical model that predicts optimal muscle activation patterns by using brief trains.
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Because muscles must be repetitively activated during functional electrical stimulation, it is desirable to identify the stimulation pattern that produces the most force. Previous experimental work has shown that the optimal pattern contains an initial high-frequency burst of pulses (i.e., an initial doublet or triplet) followed by a low, constant-frequency portion. Pattern optimization is particularly challenging, because a muscle's contractile characteristics and, therefore, the optimal pattern change under different physiological conditions and are different for each person. This work describes the continued development and testing of a mathematical model that predicts isometric forces from fresh and fatigued muscles in response to brief trains of electrical pulses. By use of this model and an optimization algorithm, stimulation patterns that produced maximum forces from each subject were identified. (+info)