Similar effects of cooling and fatigue on eccentric and concentric force-velocity relationships in human muscle.

The purpose of this study was to investigate the effects of muscle temperature and fatigue during stretch (eccentric) and shortening (concentric) contractions of the maximally electrically activated human adductor pollicis muscle. After immersion of the lower arm in water baths of four different temperatures, the calculated muscle temperatures were 36.8, 31.6, 26.6, and 22.3 degrees C. Normalized (isometric force = 100%) eccentric force increased with stretch velocity to maximal values of 136.4 +/- 1.6 and 162.1 +/- 2.0% at 36.8 and 22.3 degrees C, respectively. After repetitive ischemic concentric contractions, fatigue was less at the lower temperatures, and at all temperatures the loss of eccentric force was smaller than the loss of isometric and concentric force. Consequently, normalized eccentric forces increased during fatigue to 159.7 +/- 4.6 and 185.7 +/- 7.3% at 36.8 and 22.3 degrees C, respectively. Maximal normalized eccentric force increased exponentially (r2 = 0.95) when Vmax was reduced by cooling and/or fatiguing contractions. This may indicate that a reduction in cross-bridge cycling rate could underlie the significant increases in normalized eccentric force found with cooling and fatigue.     

Concentric and eccentric muscle strength before, during and after fatigue in 13 year-old boys

The purpose of this study was to investigate the force-producing characteristics of boys aged 13 years in relation to fatigue of elbow flexor muscles. Maximal voluntary force in elbow flexion was measured before and after a muscle endurance test (MET) by using an isokinetic dynamometer isometrically, concentrically and eccentrically at three velocities, i.e. 0.21, 0.52, and 1.05 rad · s^–1. The MET consisted of maximal concentric and eccentric muscle actions performed alternately at 0.52 rad · s^–1 for 50 consecutive trials. Muscle cross-sectional area (CSA) of elbow flexor muscles (biceps brachii and brachialis) was measured by a B-mode ultrasound apparatus. Although eccentric force showed significantly higher values than concentric force during MET, there was no significant difference in the rate of decline in force between the two actions. There was no significant difference in the rate of decline in force after MET for each velocity and muscle action. Isometric, concentric and eccentric force before MET was significantly related to muscle CSA whereas, after MET, concentric force significantly correlated with muscle CSA but there was no significant correlation between muscle CSA and isometric or eccentric force. From our study, it is therefore suggested that in development to maturity, isometric, concentric and eccentric force decrease at the same rate with advancing muscle fatigue; however, there might be differences among muscle actions in facors affecting force development.     

Maximal lengthening contractions increase p70 S6 kinase phosphorylation in human skeletal muscle in the absence of nutritional supply

The aim of this study was to compare the training stimuli of eccentric (lengthening) and concentric (shortening) contractions regarding the effect on signaling enzymes involved in protein synthesis. Ten male subjects performed 4 x 6 maximal eccentric contractions on one leg followed by 4 x 6 maximal concentric contractions on the other. Six additional subjects performed the same protocol, but with maximal concentric and submaximal eccentric exercise of equal force to that of the maximal concentric contractions. Muscle biopsy samples were taken from the vastus lateralis before, immediately after, and 1 and 2 h after exercise in both legs. The average peak force produced during the maximal eccentric exercise was 31% higher than during the maximal concentric exercise, 2,490 (+/-100) vs. 1,894 (+/-108) N (P < 0.05). The maximal eccentric contractions led to two- to eightfold increases in the phosphorylation of p70 S6 kinase (p70(S6k)) and the ribosomal protein S6 that persisted for 2 h into recovery but no significant changes in phosphorylation of Akt or mammalian target of rapamycin (mTOR). Maximal concentric and submaximal eccentric contractions did not induce any significant changes in Akt, mTOR, p70(S6k), or S6 phosphorylation up to 2 h after the exercise. The results indicate that one session of maximal eccentric contractions activates p70(S6k) in human muscle via an Akt-independent pathway and suggest that maximal eccentric contractions are more effective than maximal concentric contractions in stimulating protein synthesis in the absence of a nutritional intake, an effect that may be mediated through a combination of greater tension and stretching of the muscle.     

Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms.

The focus of the present study is the long term element of muscle fatigue provoked by prolonged intermittent contractions at submaximal force levels and analysed by force, surface electromyography (EMG) and mechanomyogram (MMG). It was hypothesized that fatigue related changes in mechanical performance of the biceps muscle are more strongly reflected in low than in high force test contractions, more prominent in the MMG than in the EMG signal and less pronounced following contractions controlled by visual compared to proprioceptive feedback. Further, it was investigated if fatigue induced by 30 min intermittent contractions at 30% as well as 10% of maximal voluntary contraction (MVC) lasted more than 30 min recovery. In six male subjects the EMG and MMG were recorded from the biceps brachii muscle during three sessions with fatiguing exercise at 10% with visual feedback and at 30% MVC with visual and proprioceptive feedback. EMG, MMG, and force were evaluated during isometric test contractions at 5% and 80% MVC before prolonged contraction and after 10 and 30 min of recovery. MVC decreased significantly after the fatiguing exercise in all three sessions and was still decreased even after 30 min of recovery. In the time domain significant increases after the fatiguing exercise were found only in the 5% MVC tests and most pronounced for the MMG. No consistent changes were found for neither EMG nor MMG in the frequency domain and feedback mode did not modify the results. It is concluded that long term fatigue after intermittent contractions at low force levels can be detected even after 30 min of recovery in a low force test contraction. Since the response was most pronounced in the MMG this may be a valuable variable for detection of impairments in the excitation-contraction coupling.     

Force and EMG power spectrum during and after eccentric and concentric fatigue.

Eccentric and concentric force and median frequency of the EMG power spectrum were measured during and immediately after maximal eccentric (EE) and concentric (CE) exercise and during the recovery period of 1 week. Eight male subjects performed EE and CE consisting of 100 maximal eccentric and concentric actions with elbow flexors during two separate exercise sessions. When comparing maximal eccentric and concentric actions before the exercises, the average force was higher (P<0.001) in eccentric than in concentric but the average rectified EMG (aEMG) values were the same with the two types of action. The average eccentric force decreased 53.3% after EE and 30.6% after CE, while the average concentric force decreased 49.9% after CE and 38.4% after EE. The recovery was slower after EE. The median frequency (MF) of biceps brachii (BB) in eccentric action decreased during both EE (P<0.01) and CE (P<0.05). It recovered within 2 days of the exercises but was lower again (P<0.01) 7 days after EE. In concentric action MF of BB decreased during CE (P<0.01), while no changes were observed in EE. Blood lactate concentration increased (P<0.001) in both exercises and serum creatine kinase (CK) activity increased in EE only, being significantly higher (P<0.001) 7 days after than before the eccentric exercise. In the absolute scale, the eccentric force in EE decreased more than the concentric force in CE (P<0.01). Fatigue response was action type specific as seen in the greater reduction in the force of the exercise type. MF decreased immediately after both exercises, which may be at least partly related to elevated blood lactate concentration. Eccentric actions led to possible muscle damage as indicated by elevated serum CK and muscle soreness, and therefore to longer recovery as compared to concentric actions. Decreased MF after EE may be indicative of selective damage of the fast twitch fibers in this type of exercise.     

Eccentric exercise increases EMG amplitude and force fluctuations during submaximal contractions of elbow flexor muscles.

The purpose of this study was to determine the effect of eccentric exercise on the ability to exert steady submaximal forces with muscles that cross the elbow joint. Eight subjects performed two tasks requiring isometric contraction of the right elbow flexors: a maximum voluntary contraction (MVC) and a constant-force task at four submaximal target forces (5, 20, 35, 50% MVC) while electromyography (EMG) was recorded from elbow flexor and extensor muscles. These tasks were performed before, after, and 24 h after a period of eccentric (fatigue and muscle damage) or concentric exercise (fatigue only). MVC force declined after eccentric exercise (45% decline) and remained depressed 24 h later (24%), whereas the reduced force after concentric exercise (22%) fully recovered the following day. EMG amplitude during the submaximal contractions increased in all elbow flexor muscles after eccentric exercise, with the greatest change in the biceps brachii at low forces (3-4 times larger at 5 and 20% MVC) and in the brachialis muscle at moderate forces (2 times larger at 35 and 50% MVC). Eccentric exercise resulted in a twofold increase in coactivation of the triceps brachii muscle during all submaximal contractions. Force fluctuations were larger after eccentric exercise, particularly at low forces (3-4 times larger at 5% MVC, 2 times larger at 50% MVC), with a twofold increase in physiological tremor at 8-12 Hz. These data indicate that eccentric exercise results in impaired motor control and altered neural drive to elbow flexor muscles, particularly at low forces, suggesting altered motor unit activation after eccentric exercise.     

Effects of eccentrically and concentrically induced unilateral fatigue on the involved and uninvolved limbs.

This study extended findings of others related to the fatigue resistance of maximum voluntary knee extension contractions performed eccentrically on an isokinetic dynamometer. Twelve subjects performed either 75 unilateral isokinetic concentric MVCs or 75 unilateral isokinetic eccentric MVCs at 30 degrees s(-1). A uniquely-configured dynamometer provided the opportunity to describe the effect of the concentric or eccentric fatigue protocol on the concentric or eccentric MVC force of the contralateral limb, which was not involved in the fatigue protocol, immediately following the conclusion of the fatigue protocol. Eccentric MVC of the eccentrically fatigued group decreased significantly (13%, p = 0.001) although the decrease was significantly smaller than that of the concentric MVC of the concentrically fatigued group (39%). Concentric MVC of the contralateral limb was unaffected following the concentric fatigue protocol but the eccentric MVC of the contralateral limb increased 11% (p = 0.028) following the eccentric fatigue protocol. These results suggest that eccentric MVCs are not resistant to fatigue but do follow a different time course than fatigue induced with concentric contractions. The extent to which eccentrically performed MVCs fatigue may reflect the influence of protocol parameters such as the isokinetic speed, the number of repetitions, the criteria by which the protocol is terminated and the subject selection. However, the explanations for why eccentric MVCs fatigue to such a smaller extent necessitate further systematic investigation including electrophysiologic methods, as do the results relative to the contralateral leg. Both results can be considered within the framework of current thought about the disparate nature of nervous system control of eccentric contractions.     

Effects of an eccentric exercise session short-term recovery of muscle contractility

Short term effects of 5 sets of 10 maximal eccentric contractions of the elbow flexors, performed using an isokinetic ergometer, were studied. Maximal eccentric, isometric, concentric torque, myoelectrical activity of biceps and triceps brachii, voluntary activation, M-wave amplitude, as well as twitch and maximal contraction and relaxation velocities were measured before (Control), 2 minutes after (Post), 24 hours (Post24 h) and 48 hours (Post48 h) after the exercise session. Torque significantly decreased over the recovery period, whatever the contraction type, excepted concentric torque assessed at 240 degrees.s-1 which recovered its Control value at Post48 h. Activation level significantly decreased at Post (p < 0.05) and returned to its Control value at Post24 h. Twitch, as well as maximal contraction and relaxation velocities had significantly declined among the experimental procedure (p < 0.01). M-wave amplitude was not modified after the exercise. These results indicate that, over a 48 hour rest period, torque decrement following a maximal eccentric exercise session should mainly be due to a failure of the peripheral part of the neuromuscular system, and force recovery should closely be linked to the developed force value.     

Fatigue and recovery contractile properties of young and elderly men

The 24 h recovery pattern of contractile properties of the triceps surae muscle, following a period of muscle fatigue, was compared in physically active young (25 years, n = 10) and elderly (66 years, n = 7) men. The fatigue test protocol consisted of 10 min of intermittent submaximal 20 Hz tetani. The maximal twitch (Pt) and tetanic force at 3 frequencies (10, 20 and 50 Hz) were determined at baseline and at 15 min, 1, 4 and 24 h after fatiguing the muscle. Maximal voluntary contraction (MVC) and vertical jump (MVJ) were also assessed. The loss of force during the fatigue test was not significantly different between the young (18 +/- 13%) and elderly (22 +/- 15%). Both groups showed similar and significant reductions of Pt (15%), tetanic force (10 to 35%) and rate of force development (dp/dt) (20%) 15 min and 1 h into recovery. The loss of force was greater at the lower stimulation frequencies of 10 and 20 Hz. Time-to-peak tension was unchanged from baseline during recovery in either group. The average rate of relaxation of twitch force (-dPt/dt) was decreased (p less than 0.05) and half-relaxation time significantly increased at 15 min and 1 h in the elderly but not the young. The findings indicate that after fatiguing contractions, elderly muscle demonstrates a slower return to resting levels of the rate and time course of twitch relaxation compared to the young.     

Enhanced Glycogen Storage of a Subcellular Hot Spot in Human Skeletal Muscle during Early Recovery from Eccentric Contractions

Unaccustomed eccentric exercise is accompanied by muscle damage and impaired glucose uptake and glycogen synthesis during subsequent recovery. Recently, it was shown that the role and regulation of glycogen in skeletal muscle are dependent on its subcellular localization, and that glycogen synthesis, as described by the product of glycogen particle size and number, is dependent on the time course of recovery after exercise and carbohydrate availability. In the present study, we investigated the subcellular distribution of glycogen in fibers with high (type I) and low (type II) mitochondrial content during post-exercise recovery from eccentric contractions. Analysis was completed on five male subjects performing an exercise bout consisting of 15 x 10 maximal eccentric contractions. Carbohydrate-rich drinks were subsequently ingested throughout a 48 h recovery period and muscle biopsies for analysis included time points 3, 24 and 48 h post exercise from the exercising leg, whereas biopsies corresponding to prior to and at 48 h after the exercise bout were collected from the non-exercising, control leg. Quantitative imaging by transmission electron microscopy revealed an early (post 3 and 24 h) enhanced storage of intramyofibrillar glycogen (defined as glycogen particles located within the myofibrils) of type I fibers, which was associated with an increase in the number of particles. In contrast, late in recovery (post 48 h), intermyofibrillar, intramyofibrillar and subsarcolemmal glycogen in both type I and II fibers were lower in the exercise leg compared with the control leg, and this was associated with a smaller size of the glycogen particles. We conclude that in the carbohydrate-supplemented state, the effect of eccentric contractions on glycogen metabolism depends on the subcellular localization, muscle fiber’s oxidative capacity, and the time course of recovery. The early enhanced storage of intramyofibrillar glycogen after the eccentric contractions may entail important implications for muscle function and fatigue resistance.     

Relationship of contraction capacity to metabolic changes during recovery from a fatiguing contraction

The relationship between changes in muscle metabolites and the contraction capacity was investigated in humans. Subjects (n = 13) contracted (knee extension) at a target force of 66% of the maximal voluntary contraction force (MVC) to fatigue, and the recovery in MVC and endurance (time to fatigue) were measured. Force recovered rapidly [half-time (t 1/2) less than 15 s] and after 2 min of recovery was not significantly different (P greater than 0.05) from the precontraction value. Endurance recovered more slowly (t 1/2 approximately 1.2 min) and was still significantly depressed after 2 and 4 min of recovery (P less than 0.05). In separate experiments (n = 10) muscle biopsy specimens were taken from the quadriceps femoris muscle before and after two successive contractions to fatigue at 66% of MVC with a recovery period of 2 or 4 min in between. The muscle content of high-energy phosphates and lactate was similar at fatigue after both contractions, whereas glucose 6-phosphate was lower after the second contraction (P less than 0.05). During recovery, muscle lactate decreased and was 74 and 43% of the value at fatigue after an elapsed period of 2 and 4 min, respectively. The decline in H+ due to lactate disappearance is balanced, however, by a release of H+ due to resynthesis of phosphocreatine, and after 2 min of recovery calculated muscle pH was found to remain at a low level similar to that at fatigue.(ABSTRACT TRUNCATED AT 250 WORDS)     

A comparison between mechanomyographic condenser microphone and accelerometer measurements during submaximal isometric, concentric and eccentric contractions.

The aim of this study was to compare mechanomyogram (MMG) recorded by a condenser microphone (MIC) and an accelerometer (ACC) during submaximal isometric, concentric and eccentric contractions in 14 males. The maximal voluntary force (MVC) of the biceps brachii was measured. The subjects were asked to do short duration isometric, concentric and eccentric contraction at 10%, 30%, 50%, 70% MVC twice. For the concentric and eccentric contraction, the subject bent his arm for 3s (concentric) then held it for 3s and extended (eccentric) during 3s. The normalized root mean square (RMS) and mean power frequency (MPF) increased linearly with increased force for both transducers. There was a correlation between MIC MPF and ACC MPF at 10%, 30%, 50% MVC, and between MIC RMS and ACC RMS at 30% MVC during isometric contractions. There was significantly higher MPF for the ACC than for the MIC in concentric and eccentric modes, while the RMS did not differ among transducers in the three contraction modes. The RMS and MPF values coefficient of variations were significantly larger during anisometric contractions compared with isometric contractions and were lower for the accelerometer than for the microphone. The present results obtained during isometric, concentric and eccentric contractions of increased intensity showed that the information contained in microphone- and accelerometer-based MMG signals is different despite similar trends. It can be concluded that at low-moderate movement velocity, concentric contractions can be investigated by means of accelerometer and microphone.     

Recovery of the human biceps electromyogram after heavy eccentric, concentric or isometric exercise.

Five men performed submaximal isometric, concentric or eccentric contractions until exhaustion with the left arm elbow flexors at respectively 50%, 40% and 40% of the prefatigued maximal voluntary contraction force (MVC). Subsequently, and at regular intervals, the surface electromyogram (EMG) during 30-s isometric test contractions at 40% of the prefatigued MVC and the muscle performance parameters (MVC and the endurance time of an isometric endurance test at 40% prefatigued MVC) were recorded. Large differences in the surface EMG response were found after isometric or concentric exercise on the one hand and eccentric exercise on the other. Eccentric exercise evoked in two of the three EMG parameters [the EMG amplitude (root mean square) and the rate of shift of the EMG mean power frequency (MPF)] the greatest (P less than 0.001) and longest lasting (up to 7 days) response. The EMG response after isometric or concentric exercise was smaller and of shorter duration (1-2 days). The third EMG parameter, the initial MPF, had already returned to its prefatigued value at the time of the first measurement, 0.75 h after exercise. The responses of EMG amplitude and of rate of MPF shift were similar to the responses observed in the muscle performance parameters (MVC and the endurance time). Complaints of muscle soreness were most frequent and severe after the eccentric contractions. Thus, eccentric exercise evoked the greatest and longest lasting response both in the surface EMG signal and in the muscle performance parameters.     

Maximal and submaximal isometric torque is elevated immediately following highly controlled active stretches of the hamstrings

Hamstring strain rehabilitation programs with an eccentric bias are effective but have a low adherence rate. Post-stretch isometric (PS-ISO) contractions which incorporate a highly controlled eccentric contraction followed by an isometric contraction resulting in elevated torque during following stretch, compared with isometric contractions at the same joint angle. This study measured torque, activation and musculotendinous unit behaviour of the hamstrings during PS-ISO contractions of maximal and submaximal levels using two stretch amplitudes. Ten male participants (24.6 years ± 2.22 years) completed maximal and submaximal baseline isometric contractions at 90°, 120° and 150° knee flexion and PS-ISO contractions of maximal and submaximal intensity initiated at 90° and 120° incorporating active stretch of 30° and 60° at 60°·s⁻¹. Torque and muscle activation of the knee flexors were simultaneously recorded. Musculotendinous unit behaviour of the biceps femoris long head was recorded via ultrasound during all PS-ISO contractions. Compared with baseline, torque was 8% and 39% greater in the maximal and submaximal PS-ISO conditions respectively with no change in muscle activation. The biceps femoris long head muscle lengthened during all PS-ISO contractions. PS-ISO contractions may be beneficial where the effects of highly controlled eccentric contractions and elevated isometric torque are desired, such as hamstring rehabilitation.     

Gender alters impact of hypobaric hypoxia on adductor pollicis muscle performance.

Recently, we reported that, at similar voluntary force development during static submaximal intermittent contractions of the adductor pollicis muscle, fatigue developed more slowly in women than in men under conditions of normobaric normoxia (NN) (Acta Physiol Scand 167: 233-239, 1999). We postulated that the slower fatigue of women was due, in part, to a greater capacity for muscle oxidative phosphorylation. The present study examined whether a gender difference in adductor pollicis muscle performance also exists during acute exposure to hypobaric hypoxia (HH; 4,300-m altitude). Healthy young men (n = 12) and women (n = 21) performed repeated static contractions at 50% of maximal voluntary contraction (MVC) force of rested muscle for 5 s followed by 5 s of rest until exhaustion. MVC force was measured before and at the end of each minute of exercise and at exhaustion. Exhaustion was defined as an MVC force decline to 50% of that of rested muscle. For each gender, MVC force of rested muscle in HH was not significantly different from that in NN. MVC force tended to decline at a faster rate in HH than in NN for men but not for women. In both environments, MVC force declined faster (P < 0.01) for men than for women. For men, endurance time to exhaustion was shorter (P < 0.01) in HH than in NN [6.08 +/- 0.7 vs. 8.00 +/- 0.7 (SE) min]. However, for women, endurance time to exhaustion was similar (not significant) in HH (12.86 +/- 1.2 min) and NN (13.95 +/- 1.0 min). In both environments, endurance time to exhaustion was longer for women than for men (P < 0.01). Gender differences in the impact of HH on adductor pollicis muscle endurance persisted in a smaller number of men and women matched (n = 4 pairs) for MVC force of rested muscle and thus on submaximal absolute force and, by inference, ATP demand in both environments. In contrast to gender differences in the impact of HH on small-muscle (adductor pollicis) exercise performance, peak O(2) uptake during large-muscle exercise was lower in HH than in NN by a similar (P > 0.05) percentage for men and women (-27.6 +/- 2 and -25.1 +/- 2%, respectively). Our findings are consistent with the postulate of a higher adductor pollicis muscle oxidative capacity in women than in men and imply that isolated performance of muscle with a higher oxidative capacity may be less impaired when the muscle is exposed to HH.     

Neuromuscular fatigue during maximal concurrent hand grip and elbow flexion or extension.

The objective was to investigate muscle fatigue measuring changes in force output and force tremor and electromyographic activity (EMG) during two sustained maximal isometric contractions for 60s: (1) concurrent hand grip and elbow flexion (HG and EF); or (2) hand grip and elbow extension (HG and EE). Each force tremor amplitude was decomposed into four frequency bands (1-3, 4-10, 11-20, and 21-50Hz). Surface EMGs were recorded from the flexor digitorum superficialis (FDS), extensor digitorum (ED), biceps brachii (BB) and lateral head of triceps brachii (TB). The HG and EF forces for the HG and EF and the HG force for the HG and EE declined rapidly, whereas the EE force remained almost constant near to the initial value for the first 40s and then declined. The decrease in EMG amplitude was observed not for the FDS muscle but for the ED muscle. The HG tremor amplitude for each frequency band showed similar decreasing rate, whereas the decreases in EF and EE tremor amplitudes for the lower band (below 10Hz) were slower than those for the higher band (above 11Hz). The neuromuscular mechanisms underlying muscle fatigue during sustained maximal concurrent contractions of hand grip and elbow flexion or extension are discussed.     

Acoustic myography reflects force changes during dynamic concentric and eccentric contractions of the human biceps brachii muscle.

The relationship between acoustic myography (AMG), electromyography (EMG) and force during submaximal dynamic contractions was examined in the biceps brachii muscles of eight healthy males (aged 17-26 years). Different weights were lifted and lowered at a constant speed, using a wall pulley system, to perform concentric and eccentric contractions, respectively. Integrated AMG (iAMG) and integrated EMG (iEMG) activity both increased linearly with force during concentric (iAMG r = 0.94; iEMG r = 0.99) and eccentric (iAMG r = 0.90; iEMG r = 0.94) contractions. The slopes of the concentric regression lines were significantly different from the eccentric slopes (P less than 0.01) for both iAMG and iEMG with concentric contractions showing greater levels of activity. The results indicated that AMG can be used to detect changes in force during dynamic contractions which has important implications for the use of AMG in rehabilitation. The differences in iAMG activity between concentric and eccentric contractions are discussed in relationship to the origin of the AMG signal.     

Length-force characteristics of aponeurosis in passive muscle and during isometric and slow dynamic contractions of rat gastrocnemius muscle

Length-force characteristics of aponeurosis of rat gastrocnemius medialis muscle and achilles tendon were studied for passive and active muscle. Active muscle performed isometric as well as slow concentric and eccentric contractions at low velocity. For isometric conditions, different aponeurosis and tendon length-force characteristics were found between passive and active muscle: At comparable low levels of force longer aponeuroses were encountered in passive than in active muscle. Similar results were found for achilles tendon, but the magnitude of the length change involved was smaller than for aponeurosis. For active muscle, no differences of aponeurosis length- force characteristics could be distinguished between the isometric contractions and a slow concentric contraction. Indications that such differences of aponeurosis length-force characteristics may exist between slow concentric and eccentric contractions were found. It is concluded that, for gastrocnemius medialis muscle, aponeurosis and tendon length - force characteristics may be quite variable depending on recent history of muscle length and activity.     

Reduced susceptibility to eccentric exercise-induced muscle damage in resistance-trained men is not linked to resistance training-related neural adaptations

The purpose of this study was to examine the acute effects of maximal concentric vs. eccentric exercise on the isometric strength of the elbow flexor, as well as the biceps brachii muscle electromyographic (EMG) responses in resistance-trained (RT) vs. untrained (UT) men. Thirteen RT men (age: 24 ± 4 years; height: 180.2 ± 7.7 cm; body weight: 92.2 ± 16.9 kg) and twelve UT men (age: 23 ± 4 years; height: 179.2 ± 5.0 cm; body weight: 81.5 ± 8.6 kg) performed six sets of ten maximal concentric isokinetic (CON) or eccentric isokinetic (ECC) elbow flexion exercise in two separate visits. Before and after the exercise interventions, maximal voluntary contractions (MVCs) were performed for testing isometric strength. In addition, bipolar surface EMG signals were detected from the biceps brachii muscle during the strength testing. Both CON and ECC caused isometric strength to decrease, regardless of the training status. However, ECC caused greater isometric strength decline than CON did for the UT group (p = 0.006), but not for the RT group. Both EMG amplitude and mean frequency significantly decreased and increased, respectively, regardless of the training status and exercise intervention. Resistance-trained men are less susceptible to eccentric exercise-induced muscle damage, but this advantage is not likely linked to the chronic resistance training-induced neural adaptations.     

V(O2) max is unaffected by altering the temporal pattern of stimulation frequency in rat hindlimb in situ.

It might be anticipated that fatiguing contractions would impair the aerobic metabolic response in skeletal muscle if significant fatigue developed before full activation of aerobic metabolism. On the basis of this premise, we examined two groups of rats to test the hypothesis that a gradual increase in stimulation frequency would yield a higher maximal O2 uptake (Vo2 max) than beginning immediately with an intense stimulation frequency because of a slower progression of fatigue under the former conditions. In one group of animals, the distal hindlimb muscles were electrically stimulated at a frequency of 60 tetani/min for 4 min (F60; n = 6 rats); in the other group, the muscles were incrementally stimulated for 1 min at each of 7.5, 15, 30, and 60 tetani/min and for 2 min at 90 tetani/min (FInc; n = 5 rats). Despite large differences in rate of fatigue [time to 60% of initial force was 47 +/- 3 (SE) vs. 188 +/- 1 s in F60 and FInc, respectively] and the time at which Vo2 max occurred (120 +/- 15 vs. 264 +/- 6 s), Vo2 max was not different (419 +/- 24 vs. 381 +/- 44 micromol x min-1. 100 g-1). Furthermore, time x tension integral at Vo2 max (3.82 +/- 0.41 vs. 4.07 +/- 0.31 N. s) and peak lactate efflux (910 +/- 45 vs. 800 +/- 98 micromol x min-1. 100 g-1) were not different between groups. Thus our results show that the more rapid progression of fatigue in F60 did not compromise the aerobic metabolic response in electrically stimulated rat hindlimb muscles. However, in both groups, O2 uptake and lactate efflux declined after Vo2 max was attained in similar proportion to a further fall in force, suggesting that ongoing fatigue with intense contractions reduced ATP demand below that requiring maximal aerobic and glycolytic metabolic responses once Vo2 max was reached.