Tension-time index, fatigue, and energetics in isolated rat diaphragm: a new experimental model.

The tension-time index (TTI) has been used to estimate mechanical load, energy utilization, blood flow, and susceptibility to fatigue in contracting muscle. The TTI can be defined, for a rhythmically contracting muscle, as the product of average force development divided by maximum tetanic force times duty cycle [contraction time / (contraction + relaxation time)]. In this study, the TTI concept was applied to isolated diaphragm via a method that allowed TTI to be clamped at a predetermined value. The hypothesis tested was that, at constant TTI, muscle energetics and the extent of fatigue would vary with stimulation frequency. Isolated diaphragm strips were stimulated at 25, 50, 75, or 100 Hz for 4 min, one per second. Duty cycle was continuously adjusted to maintain TTI at 0.07, which was near the highest TTI tolerated for 4 min, at 20-Hz stimulation. At the end of the fatigue run, muscles were either immediately frozen for determination ATP, creatine, and creatine phosphate concentrations (n = 6) or stimulated for evaluation of low- and high-frequency fatigue (n = 5). Results demonstrated no difference in the extent of fatigue or in the final ATP and creatine phosphate concentrations between groups. Large within-run increases in duty cycle were required at low stimulation frequencies, but only small increases were required at the highest frequencies. The results demonstrate that, at a constant TTI, similar fatigue properties predominate at all stimulation frequencies with no clear distinction between high- and low-frequency fatigue. The method of clamping TTI during fatigue may be useful for evaluating energetics and contractile function between treatment groups in isolated muscle when treatment influences baseline contractile characteristics.     

Induction of a fatigue-resistant phenotype in rabbit fast muscle by small daily amounts of stimulation.

We have shown that fatigue resistance can be induced in rabbit tibialis anterior (TA) muscles without excessive power loss by continuous stimulation at low frequencies, such as 5 Hz, and that the same result is obtained by delivering a 10-Hz pattern in equal on/off periods. Here we ask whether the same phenotype could be produced with daily amounts of stimulation that would be more appropriate for clinical use. We stimulated rabbit TA muscles for 6 wk, alternating fixed 30-min on periods of stimulation at 10 Hz with off periods of different duration. All patterns transformed fast-glycolytic fibers into fast-oxidative fibers. The muscles had fatigue-resistant properties but retained a higher contractile speed and power production than muscles transformed completely to the slow-oxidative type. We conclude that in the rabbit as little as one 30-min period of stimulation in 24 h can result in a substantial increase in the resistance of the muscle to fatigue.     

The effects of pH on the kinetics of fatigue and recovery in frog sartorius muscle

The effects of pH on the kinetics of fatigue and recovery in frog sartorius muscle were studied to establish whether the pH to which muscles are exposed (extracellular pH) has an effect on both the rate of fatigue development and recovery from fatigue. When frog sartorius muscles were stimulated with short tetanic stimuli at rates varying from 0.2 to 2.0 trains/s, a time- and frequency-dependent decrease in force development was observed, but extracellular pH had comparatively little effect. The recovery of tetanic force was dependent on the extracellular pH. This effect was characterized by a rapid recovery in force at pH 8.0 and an inhibition of recovery at pH 6.4 even when force decreased by only 25% during stimulation. Even when muscles were fatigued at pH 8.0 the rate of force recovery was still very small at pH 6.4. A model is proposed in which a step of the contraction cycle changes from a normal to a fatigued state. The rate of this transition is a function of the stimulation frequency and not pH. The reverse transition, from a fatigued to normal state is pH dependent; i.e., it is inhibited by H+. Measurements of resting and action potentials show that extracellular pH influences these parameters in the fatigue state, but there is no evidence that these changes are directly responsible for the pH-dependent step in the reversal of fatigue.     

Intracellular PO(2) decreases with increasing stimulation frequency in contracting single Xenopus muscle fibers.

There is currently some controversy regarding the manner in which skeletal muscle intracellular PO(2) changes with work intensity. Therefore, this study investigated the relationship between intracellular PO(2) and stimulation frequency in intact, isolated, single skeletal muscle fibers. Single, living muscle fibers (n = 7) were microdissected from the lumbrical muscles of Xenopus and injected with the oxygen-sensitive probe palladium-meso-tetra(4-carboxyphenyl)porphine (0.5 mM). Fibers were mounted with platinum clips to a force transducer in a chamber, which was continuously perfused with Ringer solution (pH = 7.0) at a PO(2) of approximately 30 Torr. Fibers were then stimulated sequentially for 3 min, followed by a 3-min rest, at each of five contraction frequencies (0.15, 0.2, 0.25, 0.33, and 0.5 Hz), in a random order, using tetanic contractions. Resting intracellular PO(2) averaged 31.2 +/- 0.9 Torr. During steady-state stimulation, intracellular PO(2) declined to 21.2 +/- 2.3, 17.1 +/- 2.4, 15.3 +/- 1.9, 9.8 +/- 2.0, and 5.8 +/- 1.4 Torr for 0.15, 0.2, 0.25, 0.33, and 0.5-Hz stimulation, respectively. Significant fatigue, as defined by a decrease in force to <50% of the initial force, occurred only at the highest (0.5 Hz) stimulation frequency in five of the cells and at 0.33 Hz in the other two. Regression analysis demonstrated that there was a significant (P < 0.0001, r = 0.82) negative correlation between intracellular PO(2) and contraction frequency in these isolated, single cells. The linear decrease in intracellular PO(2) with stimulation frequency, and thus energy demand, suggests that a fall in intracellular PO(2) correlates with increased oxygen uptake in these single contracting cells.     

Chronic electrical stimulation of nongrafted and grafted skeletal muscles in rats

Our purpose was to determine the effects of chronic electrical stimulation on the structure and function of neve-intact grafts in rats. Fourteen days after grafting, extensor digitorum longus (EDL) grafts (n = 6) and nongrafted EDL muscles (n = 4) were stimulated 8 h/day at 10 Hz for 26 days. Measurements were made subsequently of cytochrome c concentration, capillary density, contraction and relaxation times, developed tension, and the resistance to fatigue. Compared with contralateral nonstimulated grafts, chronically stimulated grafts demonstrated a 65% greater cytochrome c concentration, 45% greater number of capillaries per millimeter squared, 30% greater resistance to fatigue, 35% longer contraction time, 30% longer relaxation time, and 30% lower maximum tetanic tension. The differences that resulted from the stimulation of nongrafted EDL muscles were significant but of less magnitude. Chronic stimulation of 8 h/day provided a mixed stimulus for adaptation that enhanced the metabolic and endurance characteristics of fibers in muscles and grafts, but decreased the total fiber cross-sectional area and development of force.     

Skeletal muscle hypertrophy in response to isometric, lengthening, and shortening training bouts of equivalent duration.

Movements generated by muscle contraction generally include periods of muscle shortening and lengthening as well as force development in the absence of external length changes (isometric). However, in the specific case of resistance exercise training, exercises are often intentionally designed to emphasize one of these modes. The purpose of the present study was to objectively evaluate the relative effectiveness of each training mode for inducing compensatory hypertrophy. With the use of a rat model with electrically stimulated (sciatic nerve) contractions, groups of rats completed 10 training sessions in 20 days. Within each training session, the duration of the stimulation was equal across the three modes. Although this protocol provided equivalent durations of duty cycle, the torque integral for the individual contractions varied markedly with training mode such that lengthening > isometric > shortening. The results indicate that the hypertrophy response did not track the torque integral with mass increases of isometric by 14%, shortening by 12%, and lengthening by 11%. All three modes of training resulted in similar increases in total muscle DNA and RNA. Isometric and shortening but not lengthening mode training resulted in increased muscle insulin-like growth factor I mRNA levels. These results indicate that relatively pure movement mode exercises result in similar levels of compensatory hypertrophy that do not necessarily track with the total amount of force generated during each contraction.     

The remodelling of skeletal muscle for indefatigable hemodynamic work

Skeletal muscle possesses inherent plasticity of gene expression. Low frequency pulse-train stimulation can remodel the biochemical machinery that confers physiological expression and fatigue resistance approaching that of the myocardium. This fatigue-resistant muscle can generate sufficient force to meet the power requirements for useful cardiac work. This ultimate goal is currently being pursued in models of cardiomyoplasty and muscle-powered cardiac assist devices. In this article, we review the three major subcellular systems subserving canine skeletal muscle transformation and compare them to those of cardiac muscle. The magnitude of the problem of clinical heart failure and the feasibility of fatigue-resistant skeletal muscle joining the therapeutic armamentarium are addressed. The adaptation and transformation of fast-twitch skeletal muscle in response to chronic electrical stimulation augers therapeutic potential as an endogenous, readily available power source for myocardial assistance. The basis mechanisms of skeletal muscle fatigue require elucidation to gain a complete and thorough understanding of how to manipulate this property to provide continuous hemodynamic work.     

Absence of staircase following disuse in rat gastrocnemius muscle

Repetitive stimulation of mammalian fast-twitch skeletal muscles will normally result in a positive staircase response. This phenomenon was investigated in the rat gastrocnemius muscle following a 2-week period of tetrodotoxin-induced disuse. Muscle inactivity was imposed by superfusing tetrodotoxin in saline over the left sciatic nerve via an implanted osmotic pump. In situ isometric contractile responses to double pulse stimulation and repetitive stimulation at 10 Hz were determined the day after removal of the pump. Two weeks of disuse resulted in 40% muscle weight loss. A twitch contraction gave the same force when expressed per gram of wet muscle weight in control muscles, 317 +/- 24.6 (means +/- SE) g/g, as compared with tetrodotoxin-treated muscles, 328 +/- 24.2 g/g. Both contraction time and half-relaxation time were prolonged following treatment with tetrodotoxin. Repetitive stimulation at 10 Hz resulted in a positive staircase response in the control muscles, but not in muscles of the tetrodotoxin-treated rats. The observed changes in the time course of the twitch contraction with repetitive stimulation following tetrodotoxin-induced disuse are consistent with alterations in sarcoplasmic reticulum handling of calcium. It is not certain if there is a change following disuse in the mechanism normally associated with staircase or if this mechanism is merely opposed by an early fatigue.     

Effects of glucose on contractile function, [Ca2+]i, and glycogen in isolated mouse skeletal muscle

Extensor digitorum longus muscles were stimulated to contract to fatigue and allowed to recover for 2 h in the absence or presence of 5.5 or 11 mM extracellular glucose. This was followed by a second fatigue run, which ended when the absolute force was the same as at the end of the first run. During the first fatigue run, the fluorescence ratio for indo 1 increased [reflecting an increase in myoplasmic free Ca2+ concentration ([Ca2+]i)] during the initial tetani, peaking at approximately 115% of the first tetanic value, followed by a continuous decrease to approximately 90% at fatigue. During the first fatigue run, myofibrillar Ca2+ sensitivity was significantly decreased. During the second run, the number of tetani was 57 +/- 6% of initial force in muscles that recovered in the absence of glucose and 110 +/- 6 and 119 +/- 2% of initial force in muscles that recovered in 5.5 and 11 mM glucose, respectively. Fluorescence ratios during the first, peak, and last tetani did not differ significantly between the first and second fatigue runs during any of the three conditions. Glycogen decreased by almost 50% during the first fatigue run and did not change further after recovery in the absence of glucose. After recovery in the presence of 5.5 and 11 mM glucose, glycogen increased 32 and 42% above the nonstimulated control value (P < 0.01). These data demonstrate that extracellular glucose delays the decrease of tetanic force and [Ca2+]i during fatiguing stimulation and that glycogen supercompensation following contraction can occur in the absence of insulin.     

Rate of fatigue during repeated submaximal contractions of human quadriceps muscle.

Our purpose was to determine the effect of eight different combinations of contraction intensity, duration, and rest on the rate of fatigue in vastus lateralis muscle. A single combination consisted of contractions at 30 or 70% maximal voluntary contraction (MVC), held for 3 or 7 s with 3- or 7-s rest intervals. Contractions were repeated until the subject could not hold the force for the requisite duration. At regular intervals during each experiment, a brief MVC, a single twitch, and the response to eight stimulation pulses at 50 Hz were elicited. The rate of fatigue was the rate of decline of MVC calculated from regression analysis. Mean rate of fatigue (n = 8) ranged from 0.3 to 25% MVC/min and was closely related (r = 0.98) to the product of the relative force and the duty cycle. Force from 50 Hz stimulation fell linearly and in parallel with MVC. Twitch force was first potentiated and then fell twice as fast as 50 Hz stimulation and MVC (p less than 0.05). Differentiated twitch contraction and relaxation rates were higher at potentiation and lower at the limit of endurance, compared with control values (p less than 0.05). The maximal electromyogram decreased 25% and the submaximal EMG increased to maximal by the end of the protocol, indicating that the entire motor unit pool had been recruited. The close relation between rate of fatigue and the force x time product probably reflects the off-setting interaction of contraction amplitude, duration, and rest interval. This occurs despite the changes in twitch characteristics and the apparent recruitment of fast fatiguing motor units.     

Skeletal muscle tension, flow, pressure, and EMG during sustained isometric contractions in humans

In five healthy males sustained isometric torques during elbow flexion, knee extension, and plantar flexion correlated positively with intramuscular tissue pressure (MTP) in the range 0-80% of the maximal voluntary contraction (MVC). During passive compression of the muscle at rest 133-Xenon muscle clearance stopped when MTP reached diastolic arterial pressure (DAP) indicating that the muscle vascular bed was occluded. However, during sustained contraction this relation between DAP, flow and MTP was not seen. In two cases 133-Xenon clearance from M. soleus did not stop in spite of an 80% maximal contraction and MTP stayed below DAP. In other cases MTP would reach as high as 240 mm Hg before clearance was zero. In the deeper parts of the muscles MTP during contraction was increased in relation to the more superficial parts. The means values for the % MVC that would stop MBF varied between 50 and 64% MVC for the investigated muscles. Mean rectified EMG (MEMG) showed a high correlation to MTP during sustained exhaustive contractions: When MEMG was kept constant MTP also remained constant while the exerted force decreased; when force was kept constant both MEMG and MTP increased in parallel. This demonstrated that muscle tissue compliance is decreasing during fatigue. Muscle ischemia occurring during sustained isometric contractions is partly due to the developed MTP, where especially the MTP around the veins in the deeper parts of the muscle can be considered of importance. However, ischemia is also affected by muscle fiber texture and anatomical distorsion of tissues.     

Recruitment,force and fatigue characteristics of quadriceps muscles of paraplegics isometrically activated by surface functional electrical stimulation

This study deals with the recruitment characteristics of unfatigued electrically stimulated quadriceps muscles of paraplegic subjects and with the time-dependent force output of these muscles under sustained stimulation conditions. Both these aspects of the performance of paralysed stimulated muscles were studied under isometric conditions and at different muscle lengths. The forces in the knee joint resulting from stimulation of the quadriceps were also calculated. Recruitment force curves due to a ramp-like stimulation function indicated a strong dependence on muscle length and demonstrated a sigmoid-shaped curve with three distinct regions: negligible force up to threshold stimulation intensity; rapid force increase; and levelling-off of the curve after which the force remains constant even though intensity is further increased. When normalized to the maximal force, recruitment was found to be independent of muscle length, generating a typical recruitment curve for every patient, under isometric stimulation. The peak forces were obtained at the same flexion angles previously published for normal subjects, but with much lower values. Muscle fatigue in tetanic isometric conditions, defined as the decrease in force due to sustained stimulation with fixed parameters, was found to be length dependent and to have a double exponential decay. The first is the acute force loss and is the more significant for functional purposes; the second is the more moderate and asymptotic region, in which partial force receovery in the form of bursts is observed.     

Fatigue response of rat medial longissimus muscles induced with electrical stimulation at various work/rest ratios

Muscle fatigue may be a precursor to workplace musculoskeletal disorders, with the low back resulting in the most frequently injured body part. Work/rest ratios have an effect on fatigue due to the amount of rest allowance provided following muscle contraction. This study explored various work/rest ratios by electrically stimulating rat medial longissimus muscles. A 3 V stimulus with 0.2 ms pulse duration was applied at a frequency of 30 Hz. There were four stimulation groups consisting of the following duty cycles (DC) and cycle times (CT): DC25%:CT20s, DC25%:CT280s, DC75%:CT20s, and DC75%:CT180s. Muscle fatigue was measured as a decrease in M-wave amplitude and area, and an increase in M-wave duration. The results indicated that fatigue occurred immediately in each of the groups. The higher duty cycle and shorter cycle time group resulted in significantly greater fatigue than the lower duty cycle and longer cycle time group, as measured by increased M-wave amplitude and area. A longer M-wave duration was observed in the high duty cycle long cycle time group. This suggests that the combination of low duty cycle and long cycle times leads to less fatigue. In high duty cycle scenarios, short cycle times result in less fatigue.     

Is the change in intracellular pH during fatigue large enough to be the main cause of fatigue?

The intracellular pH of frog sartorius muscles exposed to an extracellular pH 8.0 (25 mM HCO3-, 1% CO2) was 6.9-7.1. Following a fatiguing stimulation period (one tetanic contraction per second for 3 min), the intracellular pH was 6.5-6.7. When similar experiments were repeated with frog sartorius muscles exposed to pH 6.4 (2mM HCO3-, 1% CO2), the intracellular pH was 6.8-6.9 at rest and 6.3-6.4 following fatigue. So, in both experiments the intracellular pH decreased by 0.4-0.5 pH unit during fatigue. When the CO2 concentration of the bathing solution was increased from 1 to 30%, the intracellular pH of resting muscles decreased from 7.0 to 6.2-6.3. Although the effect of CO2 on the intracellular pH was greater than the fatigue effect, the decrease in tetanic force with CO2 was less than 40%, while during fatigue the tetanic force decreased by at least 70%. Therefore in frog sartorius muscle the decrease in tetanic force during fatigue exceeds the decrease that is expected from just a change in intracellular pH.     

Twitch contractile properties of plantar flexor muscles in power and endurance trained athletes

This study compared twitch contractile properties of plantar flexor muscles among three groups of 12 subjects each: endurance and power trained athletes and untrained subjects. The posterior tibial nerve was stimulated by supramaximal square wave pulses of 1-ms duration. Power trained athletes had higher twitch maximal force, maximal rates of force development and relaxation and also maximal voluntary contraction (MVC) force. The trained subjects had a smaller twitch maximal force: MVC force ratio and shorter twitch contraction and half-relaxation times than the untrained subjects with no significant differences between the two groups. Thus, the short time for evoked twitches in the athletes compared to the untrained subjects would seem unrelated to the type of training. It is concluded that power training induces a more evident increase of muscle force-generating capacity and speed of contraction and relaxation than endurance training. Accepted: 24 April 1999     

Effect of low frequency fatigue on human muscle strength and fatigability during subsequent stimulated activity

Fatiguing contractions of the adductor pollicis muscle were produced by intermittent supramaximal stimulation of the ulnar nerve in a set frequency pattern, in six normal subjects. At the end of an initial fatiguing contraction series, low frequency fatigue (LFF) had been induced and persisted at 15 min of recovery. Stimulated fatiguing activity was then repeated in an identical fashion to the initial series. At high frequencies, declines in force were similar for both series. At low frequencies, declines in force were greater during the second series despite similar changes in compound muscle action potential amplitude. This confirmation that LFF persists during subsequent stimulated activity, and reduces low but not high frequency fatigue resistance, suggests that the impaired endurance of fatigued muscle during voluntary activity primarily results from peripheral changes at low frequency. These findings also have implications for therapeutic electrical stimulation of muscle.     

Reports of the length dependence of fatigue are greatly exaggerated.

Relative force depression associated with muscle fatigue is reported to be greater when assessed at short vs. long muscle lengths. This appears to be due to a rightward shift in the force-length relationship. This rightward shift may be caused by stretch of in-series structures, making sarcomere lengths shorter at any given muscle length. Submaximal force-length relationships (twitch, double pulse, 50 Hz) were evaluated before and after repetitive contractions (50 Hz, 300 ms, 1/s) in an in situ preparation of the rat medial gastrocnemius muscle. In some experiments, fascicle lengths were measured with sonomicrometry. Before repetitive stimulation, fascicle lengths were 11.3 +/- 0.8, 12.8 +/- 0.9, and 14.4 +/- 1.2 mm at lengths corresponding to -3.6, 0, and 3.6 mm where 0 is a reference length that corresponds with maximal active force for double-pulse stimulation. After repetitive stimulation, there was no change in fascicle lengths; these lengths were 11.4 +/- 0.8, 12.6 +/- 0.9, and 14.2 +/- 1.2 mm. The length dependence of fatigue was, therefore, not due to a stretch of in-series structures. Interestingly, the rightward shift that was evident when active force was calculated in the traditional way (subtraction of the passive force measured before contraction) was not seen when active force was calculated by subtracting the passive force that was associated with the fascicle length reached at the peak of the contraction. This calculation is based on the assumption that passive force decreases as the fascicles shorten during a fixed-end contraction. This alternative calculation revealed similar postfatigue absolute active force depression at all lengths. In relative terms, a length dependence of fatigue was still evident, but this was greatly diminished compared with that observed when active force was calculated with the traditional method.     

Contractile and endurance properties of geniohyoid and diaphragm muscles

Despite the wealth of information about the neural control of pharyngeal dilator muscles, little is known about their intrinsic physiological properties. In the present study the in situ isometric contractility and endurance of a pharyngeal dilator, the geniohyoid muscle, were compared with properties of the diaphragm in 12 anesthetized artificially ventilated cats. The contraction time (means +/- SE) of the geniohyoid (27 +/- 2 ms) was shorter than that of the diaphragm (36 +/- 3 ms; P less than 0.0005), as was the half-relaxation time (29 +/- 2 vs. 45 +/- 4 ms; P less than 0.002). The faster contraction and relaxation of the geniohyoid compared with the diaphragm were appropriately reflected in the shape of the force-frequency curves for the two muscles, with that of the geniohyoid located to the right of the diaphragm force-frequency curve. The endurance properties of the two muscles were assessed using repetitive stimulation at 40 Hz in trains lasting 0.33 s, with one train repeated every second. The ratio of force at the end of 2 min of repetitive stimulation to initial force was 0.67 +/- 0.06 for the geniohyoid and 0.15 +/- 0.03 for the diaphragm (P less than 0.00001). After the repetitive stimulation, the muscle force generated in response to a range of stimulus frequencies was reduced to a greater extent for the diaphragm than for the geniohyoid muscle. These results indicate that the geniohyoid muscle has a faster physiological profile than does the diaphragm yet is relatively resistant to fatigue when driven at high rates.     

Effects of DAP on diaphragm force and fatigue, including fatigue due to neurotransmission failure

Van Lunteren, Erik, and Michelle Moyer. Effects of DAPon diaphragm force and fatigue, including fatigue due toneurotransmission failure. J. Appl.Physiol. 81(5): 2214-2220, 1996.Among theaminopyridines, 3,4-diaminopyridine (DAP) is a more effectiveK⁺ channel blocker than is4-aminopyridine (4-AP), and, furthermore, DAP enhances neuromusculartransmission. Because 4-AP improves muscle contractility, wehypothesized that DAP would also increase force and, in addition,ameliorate fatigue and improve the neurotransmission failure componentof fatigue. Rat diaphragm strips were studied in vitro (37°C). Infield-stimulated muscle, 0.3 mM DAP significantly increased diaphragmtwitch force, prolonged contraction time, and shifted theforce-frequency relationship to the left without altering peak tetanicforce, resulting in increased force at stimulation frequencies 50 Hz.During 20-Hz intermittent stimulation, DAP increased diaphragm peakforce compared with control during a 150-s fatigue run and,furthermore, significantly improved maintenance of intratrain force.The relative contribution of neurotransmission failure to fatigue wasestimated by comparing the force generated by phrenic nerve-stimulatedmuscles with that generated by curare-treated field-stimulated muscles.DAP significantly increased force in nerve-stimulated muscles and, inaddition, reduced the neurotransmission failure contribution todiaphragm fatigue. Thus DAP increases muscle force atlow-to-intermediate stimulation frequencies, improves overall force andintratrain fatigue during 20-Hz intermittent stimulation, and reducesneurotransmission failure.

     

Effect of lipid infusion on metabolism and force of rat skeletal muscles during intense contractions.

The hypothesis that during intense muscle contraction induced by electrical stimulation, long chain fatty acids (LCFA) might reduce mitochondrial ATP/ADP ratio, raising the contribution of glycolysis for ATP production was examined. The effect of a lipid infusion (Lipovenus emulsion) on UCP-3 mRNA level, lactate, glucose-6-phosphate (G-6P) and glycogen content was investigated in rat. Blood samples for determination of free fatty acids and lactate were collected at 0, 30 and 60 min during rest and at 0, 10 and 20 min during muscle contraction. The content of lactate, glycogen and G-6P was also determined in soleus (SO), red gastrocnemius (RG) and white gastrocnemius (WG) muscles collected immediately after muscle contraction period. In addition, the force level was determined during muscle contractions. The effect of Lipovenus emulsion on respiration of mitochondria isolated from rat skeletal muscle, and content of UCP-3 and lactate in cultured skeletal muscle cells was also determined. The in vivo experiments showed that Lipovenus induced a significant increase of UCP-3 mRNA levels. After Lipovenus infusion, lactate level was increased in RG muscle only, whereas the contents of glycogen and G-6P were decreased in both RG and WG muscles (P < 0.05). Lipovenus infusion failed to exert any effect on muscle force performance (P > 0.05). The in vitro experiments showed that Lipovenus infusion induced a significant increase in mitochondrial respiration, but had no effect on UCP-3 content. Lactate concentration was significantly increased in the culture medium of stimulated cells in the control and Lipovenus groups compared with the respective not-stimulated cells (P< 0.05). We concluded that as mitochondrial function becomes limited by the FFA-uncoupling effect, the ATP demand is mainly supplied by anaerobic glucose metabolism preventing an expected decrease in muscle contraction performance.