Effect of concentric and eccentric muscle actions on muscle sympathetic nerve activity

The purpose ofthis study was to determine the effects of concentric (Con) andeccentric (Ecc) muscle actions on leg muscle sympathetic nerve activity(MSNA). Two protocols were utilized. In protocol1, eight subjects performed Con and Ecc arm curls for 2 min, with a resistance representing 50% of one-repetition maximum forCon curls. Heart rate (HR) and mean arterial pressure (MAP) weregreater (P < 0.05) during Con thanduring Ecc curls. Similarly, the MSNA was greater(P < 0.05) during Con than during Ecc curls. In protocol 2, eightdifferent subjects performed Con and Ecc arm curls to fatigue, followedby postexercise muscle ischemia, by using the same resistanceas in protocol 1. Endurance time wassignificantly greater for Ecc than for Con curls. The increase in HR,MAP, and MSNA was greater (P < 0.05)during Con than during Ecc curls. However, when the data werenormalized as a function of endurance time, the differences in HR, MAP,and MSNA between Con and Ecc curls were no longer present. HR, MAP, andMSNA responses during postexercise muscle ischemia were similar for Con and Ecc curls. Con curls elicited greater increase(P < 0.05) in blood lactateconcentration than did Ecc curls. In summary, Con actions contributesignificantly more to the increase in cardiovascular and MSNA responsesduring brief, submaximal exercise than do Ecc actions. However, whenperformed to a similar level of effort (i.e., fatigue), Con and Eccmuscle actions elicit similar cardiovascular and MSNA responses. Theseresults indicate that the increase in MSNA during a typical bout ofsubmaximal dynamic exercise is primarily mediated by the musclemetaboreflex, which is stimulated by metabolites produced predominantlyduring Con muscle action.     

Specific effects of eccentric and concentric training on muscle strength and morphology in humans

The purpose of this study was to compare pure eccentric and concentric isokinetic training with respect to their possible specificity in the adaptation of strength and morphology of the knee extensor muscles. Ten moderately trained male physical education students were divided into groups undertaking eccentric (ETG) and concentric (CTG) training. They performed 10 weeks of maximal isokinetic (90 degrees x s(-1)) training of the left leg, 4x10 repetitions - three times a week, followed by a second 10-week period of similar training of the right-leg. Mean eccentric and concentric peak torques increased by 18% and 2% for ETG and by 10% and 14% for CTG, respectively. The highest increase in peak torque occurred in the eccentric 90 degrees x s(-1) test for ETG (35%) whereas in CTG strength gains ranged 8%-15% at velocities equal or lower than the training velocity. Significant increases in strength were observed in the untrained contra-lateral leg only at the velocity and mode used in ipsilateral training. Cross-sectional area of the quadriceps muscle increased 3%-4% with training in both groups, reaching statistical significance only in ETG. No major changes in muscle fibre composition or areas were detected in biopsies from the vastus lateralis muscle for either leg or training group. In conclusion, effects of eccentric training on muscle strength appeared to be more mode and speed specific than corresponding concentric training. Only minor adaptations in gross muscle morphology indicated that other factors, such as changes in neural activation patterns, were causing the specific training-induced gains in muscle strength.     

Experimental knee pain impairs submaximal force steadiness in isometric,eccentric, and concentric muscle actions

IntroductionPopulations with knee joint damage, including arthritis, have noted impairments in the regulation of submaximal muscle force. It is difficult to determine the exact cause of such impairments given the joint pathology and associated neuromuscular adaptations. Experimental pain models that have been used to isolate the effects of pain on muscle force regulation have shown impaired force steadiness during acute pain. However, few studies have examined force regulation during dynamic contractions, and these findings have been inconsistent. The goal of the current study was to examine the effect of experimental knee joint pain on submaximal quadriceps force regulation during isometric and dynamic contractions.MethodsThe study involved fifteen healthy participants. Participants were seated in an isokinetic dynamometer. Knee extensor force matching tasks were completed in isometric, eccentric, and concentric muscle contraction conditions. The target force was set to 10 % of maximum for each contraction type. Hypertonic saline was then injected into the infrapatella fat pad to generate acute joint pain. The force matching tasks were repeated during pain and once more 5 min after pain had subsided.ResultsHypertonic saline resulted in knee pain with an average peak pain rating of 5.5 ± 2.1 (0–10 scale) that lasted for 18 ± 4 mins. Force steadiness significantly reduced during pain across all three muscle contraction conditions. There was a trend to increased force matching error during pain but this was not significant.ConclusionExperimental knee pain leads to impaired quadriceps force steadiness during isometric, eccentric, and concentric contractions, providing further evidence that joint pain directly affects motor performance. Given the established relationship between submaximal muscle force steadiness and function, such an effect may be detrimental to the performance of tasks in daily life. In order to restore motor performance in people with painful arthritic conditions of the knee, it may be important to first manage their pain more effectively.     

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.     

The effects of diverting activities on recovery from fatiguing concentric isokinetic muscle actions

The purpose of this study was to examine the effects of diverting activities on recovery from fatiguing concentric isokinetic muscle actions. On 3 separate occasions, 11 men (mean ± SD age = 22.2 ± 1.5 years) and 8 women (mean ± SD age = 22.2 ± 2.1 years) performed 2 bouts of 50 consecutive maximal concentric isokinetic muscle actions of the dominant leg extensors. Between these bouts, the subjects either performed math problems (mental diverting activity), contralateral dynamic constant external resistance (DCER) leg extensions (physical diverting activity), or rested quietly (control). For each trial, the peak torque data from the first and second bouts of 50 muscle actions served as the pretest (Pre) and posttest (Post) data, respectively. The results indicated that when the subjects rested quietly or performed contralateral DCER leg extensions between the fatiguing bouts, the initial peak torque values observed for Post were significantly less than those for Pre. When the subjects performed math problems, however, no decline in the initial peak torque values was observed, thus indicating better recovery. In addition, a decline in the average torque values was observed from Pre to Post for the control trial but not for the math problem or contralateral exercise trials. No differences were observed among the trials for final peak torque, percent decline, or the linear slope of the decline in peak torque. These findings demonstrated that performing either mental or physical diverting activities after fatiguing isokinetic muscle actions enhanced recovery.     

Doublet potentiation during eccentric and concentric contractions of cat soleus muscle

Sandercock, Thomas G., and C. J. Heckman. Doubletpotentiation during eccentric and concentric contractions of cat soleusmuscle. J. Appl. Physiol. 82(4):1219-1228, 1997.The addition of an extra stimulus pulse, ordoublet, at the beginning of a low-frequency train has been shown tosubstantially increase isometric force. This study examined the effectsof muscle movement on this doublet potentiation. The soleus muscles ofanesthetized cats were stimulated at 10 Hz for 1 s, with and without anadded doublet (0.01-s interval). Isovelocity releases reduced but didnot eliminate peak and early doublet potentiation (average 0.0-0.5s after the doublet). Large releases, >0.4 s after the doublet,completely abolished sustained doublet potentiation (average0.5-1.0 s after the doublet). In contrast, early isovelocitystretches boosted peak doublet potentiation. Yet, large stretches laterin the stimulus almost completely eliminated sustained doubletpotentiation. This suggests that a different mechanism is responsiblefor early and sustained doublet potentiations. Because peak and averageinitial doublet potentiation were not strongly affected by movement,doublets still offer a viable control strategy to increase force during movement while minimizing the number of stimulus pulses.

     

Influence of preactivity and eccentric muscle activity on concentric performance during vertical jumping

The purpose of this investigation was to observe the influence of increasing amounts of preactivity and eccentric muscle activity imposed by three different jump types on concentric vertical jumping performance. Sixteen athletes involved in jumping-related sports at Appalachian State University, which is a Division IA school, performed a static jump (SJ), counter-movement jump (CMJ), and drop jump (DJ). Force, power, velocity, and jump height were measured during each jump type. In addition, muscle activity was measured from two agonist muscles (vastus lateralis, vastus medialis) and one antagonist muscle (biceps femoris). Preactivity and eccentric phase muscle activity of the agonist muscles (average integrated electromyography) was significantly (p < or = 0.05) higher during the DJ (preactivity, 0.2 +/- 0.11 mV; eccentric phase, 1.00 +/- 0.36 mV) in comparison with the CMJ (preactivity, 0.11 +/- 0.10 mV; eccentric phase, 0.45 +/- 0.17 mV). Peak concentric force was highest during the DJ and was significantly different among all three jump types (SJ, CMJ, DJ). Maximal jump height was significantly higher during the DJ (0.41 +/- 0.05 m) and CMJ (0.40 +/- 0.06 m) compared with the SJ (0.37 +/- 0.07 m). However, no significant difference in jump height existed between the CMJ and DJ. A positive energy balance, as assessed by force-displacement curves during the eccentric and concentric phases, was observed during the CMJ, and a negative energy balance was observed during the DJ. The data from this investigation indicate that a significant increase in concentric vertical jump performance is associated with increased levels of preactivity and eccentric phase muscle activity (SJ to CMJ). However, higher eccentric loading (CMJ to DJ) leads to a negative energy balance during the eccentric phase, which may relate to a non-significant increase in vertical jump height, even with coincidental increases in peak concentric force. Practitioners may want to focus on improving eccentric phase muscle activity through the use of plyometrics to improve overall jumping performance in athletes.     

Electromechanical delay in human skeletal muscle under concentric and eccentric contractions

In contraction of skeletal muscle a delay exists between the onset of electrical activity and measurable tension. This delay in electromechanical coupling has been stated to be between 30 and 100 ms. Thus, in rapid movements it may be possible for electromyographic (EMG) activity to have terminated before force can be detected. This study was designed to determine the dependence of the EMG-tension delay upon selected initial conditions at the time of muscle activation. The right forearms of 14 subjects were passively oscillated by a motor-driven dynamometer through flexion-extension cycles of 135 deg at an angular velocity of approximately equal to 0.5 rad/s. Upon presentation of a visual stimulus the subjects maximally contracted the relaxed elbow flexors during flexion, extension, and under isometric conditions. The muscle length at the time of the stimulus was the same in all three conditions. An on-line computer monitoring surface EMG (Biceps and Brachioradialis) and force calculated the electromechanical delay. The mean value for the delay under eccentric condition, 49.5 ms, was significantly different (p less than 0.05) from the delays during isometric (53.9 ms) and concentric activity (55.5 ms). It is suggested that the time required to stretch the series elastic component (SEC) represents the major portion of the measured delay and that during eccentric muscle activity the SEC is in a more favorable condition for rapid force development.     

Different neuromuscular recruitment patterns during eccentric, concentric and isometric contractions

Aim. The purpose of this study was to determine the neuromuscular fatigue profiles during 100 s isometric (ISO), concentric (CON), and eccentric (ECC) activity.

Methods. Twelve subjects (age 25.1±3.7 years, mass 70.1±8.2 kg, mean±SD) performed ISO, CON and ECC maximal voluntary contractions and 100 s endurance trials on an isokinetic dynamometer. Raw EMG data were recorded throughout each trial from the rectus femoris of the right limb. Corresponding data for integrated electromyography (IEMG), percentile frequency shifts (MPFS) and peak torque output were divided into five 5 s epochs and subsequently normalised with the first epoch being the reference point, in order to assess changes over time.

Results. There were no significant differences between ECC, CON and ISO peak torque output (211±63 vs 169±41 vs 177±61 Nm; ECC, CON, ISO) and IEMG activity (280±143 vs 305±146 vs 287±143 mV; ECC, CON, ISO) during maximal contractions. Serial reductions in torque output were greatest in ISO in which torque output during the final epoch was 31±13% of initial values, similar to the final torque values in CON (58±15%), but significantly less than ECC (108.6±38.6%; P<0.001) values. In CON and ECC, IEMG was maintained (95±27% and 93±21%; CON and ECC), whereas IEMG for ISO decreased to 38±13% of initial values. The greatest reduction in MPFS occurred in CON (69±10%) compared to ISO (78±9%; P<0.05) and ECC (93±6%; P<0.001).

Conclusion. These data demonstrate distinct neuromuscular fatigue profiles for the different types of muscle contraction. Whereas eccentric activity was largely fatigue resistant, isometric and concentric contractions displayed different neuromuscular fatigue profiles.     

Motor unit activation patterns during isometric, concentric and eccentric actions at different force levels.

Motor unit activation patterns were studied during four different force levels of concentric and eccentric actions. Eight male subjects performed concentric and eccentric forearm flexions with the movement range from 100 degrees to 60 degrees in concentric and from 100 degrees to 140 degrees elbow angle in eccentric actions. The movements were started either from zero preactivation or with isometric preactivation of the force levels of 20, 40, 60 and 80% MVC. The subjects were then instructed to maintain the corresponding relative force levels during the dynamic actions. Intramuscular and surface EMG was recorded from biceps brachii muscle. Altogether 28 motoneuron pools were analyzed using the intramuscular spike-amplitude frequency (ISAF) analysis technique of Moritani et al. The mean spike amplitude was lower and the mean spike frequency higher in the isometric preactivation phase than in the consequent concentric and eccentric actions. When the movements started with isometric preactivation the mean spike amplitude increased significantly (P<0.001) up to 80% in isometric and concentric actions but in eccentric actions the increase continued only up to 60% (P<0.01). The mean spike frequency in isometric preactivation and in concentric action with preactivation was lower only at the 20% force level (P<0.01) as compared to the other force levels while in eccentric action with preactivation the increase between the force levels was significant (P<0.01) up to 60%. When the movement was started without preactivation the mean spike amplitude at 20% and at 40% force level was higher (P<0.01) in eccentric action than in concentric actions. It was concluded that the recruitment threshold may be lower in dynamic as compared to isometric actions. The recruitment of fast motor units may continue to higher force levels in isometric and in concentric as in eccentric actions which, on the other hand, seems to achieve the higher forces by increasing the firing rate of the active units. At the lower force levels mean spike amplitude was higher in eccentric than in concentric actions which might indicate selective activation of fast motor units. This was, however, the case only when the movements were started without isometric preactivation.     

Mechanomyographic and electromyographic responses of the vastus medialis muscle during isometric and concentric muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) vs. torque relationships during submaximal to maximal isometric and isokinetic muscle actions. Seven men (mean +/- SD age, 22.4 +/- 1.3 years) volunteered to perform isometric and concentric isokinetic leg extension muscle actions at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC) and peak torque (PT) on a Cybex II dynamometer. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes on the vastus medialis. Polynomial regression and separate 1-way repeated-measures analysis of variance were used to analyze the EMG amplitude, MMG amplitude, EMG MPF, and MMG MPF data for the isometric and isokinetic muscle actions. For the isometric muscle actions, EMG amplitude (R(2) = 0.999) and MMG MPF (R(2) = 0.946) increased to MVC, mean MMG amplitude increased to 60% MVC and then plateaued, and mean EMG MPF did not change (p > 0.05) across torque levels. For the isokinetic muscle actions, EMG amplitude (R(2) = 0.988) and MMG amplitude (R(2) = 0.933) increased to PT, but there were no significant mean changes with torque for EMG MPF or MMG MPF. The different torque-related responses for EMG and MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for isometric vs. dynamic muscle actions. These results support the findings of others and suggest that isometric torque production was modulated by a combination of recruitment and firing rate, whereas dynamic torque production was modulated primarily through recruitment.     

Effects of concentric and eccentric training on muscle strength, cross-sectional area, and neural activation

Higbie, Elizabeth J., Kirk J. Cureton, Gordon L. Warren III,and Barry M. Prior. Effects of concentric and eccentric trainingon muscle strength, cross-sectional area, and neural activation.J. Appl. Physiol. 81(5):2173-2181, 1996.We compared the effects of concentric (Con) andeccentric (Ecc) isokinetic training on quadriceps muscle strength,cross-sectional area, and neural activation. Women (age 20.0 ± 0.5 yr) randomly assigned to Con training (CTG;n = 16), Ecc training (ETG;n = 19), and control (CG;n = 19) groups were tested before andafter 10 wk of unilateral Con or Ecc knee-extension training. Averagetorque measured during Con and Ecc maximal voluntary knee extensions increased 18.4 and 12.8% for CTG, 6.8 and 36.2% for ETG, and 4.7 and1.7% for CG, respectively. Increases by CTG and ETG were greater than for CG (P < 0.05). ForCTG, the increase was greater when measured with Con than with Ecctesting. For ETG, the increase was greater when measured with Ecc thanwith Con testing. The increase by ETG with Ecc testing was greater thanthe increase by CTG with Con testing. Corresponding changes in theintegrated voltage from an electromyogram measured during strengthtesting were 21.7 and 20.0% for CTG, 7.1 and 16.7% for ETG, and8.0 and 9.1% for CG. Quadriceps cross-sectional areameasured by magnetic resonance imaging (sum of 7 slices) increased morein ETG (6.6%) than in CTG (5.0%) (P < 0.05). We conclude that Ecc is more effective than Con isokinetictraining for developing strength in Ecc isokinetic muscle actions andthat Con is more effective than Ecc isokinetic training for developingstrength in Con isokinetic muscle actions. Gains in strength consequentto Con and Ecc training are highly dependent on the muscle action usedfor training and testing. Muscle hypertrophy and neural adaptationscontribute to strength increases consequent to both Con and Ecctraining.

     

Fatigue and functional performance of human biceps muscle following concentric or eccentric contractions.

A long-lasting fatigue was measured in human biceps muscle, following 40 maximal isokinetic concentric or eccentric contractions of the forearm, as the response to single-shock stimuli every minute for 4 h. This protocol allowed new observations on the early time course of long-lasting fatigue. Concentric contractions induced a novel progressive decline to 30.2% (SE 7.8, n = 7) of control at 23 min with complete recovery by 120 min. Eccentric contractions lead initially to a smaller force reduction of similar time course followed by a slower decline to 40.0% (SE 5.1, n = 7) control at 120 min with recovery less than half complete at 4 h. A 50-Hz test stimuli overcame both fatigues, identifying low-frequency fatigue. EMG recordings from the biceps muscle showed moderate (<20%) changes during the fatigue. A visual-tracking task showed no decrement in performance at the time of maximal fatigue of the single-shock response. Because the eccentric contractions have a similar activation, a larger force, but much smaller metabolic usage than concentric contractions, it is concluded that the initial decline is related to the effects of metabolites, whereas the slower phase after eccentric contractions is associated with higher mechanical stress.     

The effects of isokinetic contraction velocity on concentric and eccentric strength of the biceps brachii

The purpose of this investigation was to determine the influence of contraction velocity on the eccentric (ECC) and concentric (CON) torque production of the biceps brachii. After performing warm-up procedures, each male subject (n = 11) completed 3 sets of 5 maximal bilateral CON and ECC isokinetic contractions of the biceps at speeds of 90, 180, and 300 degrees x s(-1) on a Biodex System 3 dynamometer. The men received a 3-minute rest between sets and the order of exercises was randomized. Peak torque (Nm) values were obtained for CON and ECC contractions at each speed. Peak torque scores (ECC vs. CON) were compared using a t-test at each speed. A repeated measures analysis of variance was used to determine differences between speeds. ECC peak torque scores were greater than CON peak torque scores at each given speed: 90 degrees x s(-1), p = 0.0001; 180 degrees x s(-1), p = 0.0001; and 300 degrees x s(-1), p = 0.0001. No differences were found between the ECC peak torque scores (p = 0.62) at any of the speeds. Differences were found among the CON scores (p = 0.004). Post hoc analysis revealed differences between 90 degrees x s(-1) (114.61 +/- 23) and 300 degrees x s(-1) (94.17 +/- 18). These data suggest that ECC contractions of the biceps brachii were somewhat resistant to a force decrement as the result of an increase in velocity, whereas CON muscular actions of the biceps brachii were unable to maintain force as velocity increased.     

Theoretical and experimental behaviour of the muscle viscosity coefficient during maximal concentric actions

The aim of this study was to calculate the theoretical variation of the nonlinear damping factor (B) as a function of the muscle shortening velocity, and then to compare the theoretical values with the experimental data obtained on both the elbow flexor and the ankle extensor muscles. The theoretical variation of the B factor was determined from a muscle model consisting of a contractile component in parallel with a viscous damper both in series with an elastic component, and by using, the charateristic equation of the force velocity curve. In this muscle model, the viscous element modelled the inability of the muscle to generate as big a contracting force (while shortening) as possible under isometric conditions. Eight volunteer subjects performed maximal concentric elbow flexions and ankle extensions on an isokinetic ergometer at angular velocities of 60, 120, 180, 240, 300 and 360°·s^–1, and held two maximal isometric actions at an elbow angle of 90° (0° corresponds to the full extension) and at an ankle angle of 0° (0° corresponds to the foot flexion of 90° relative to the leg axis). From these measurements, the force and the shortening velocity values of each muscle were determined by using a musculo-skeletal model of the joint. The results showed that the theoretical behaviour of the B factor would seem to be dependent on the shortening velocity and on the parameter which varies according to the muscle fibre type composition and affects the curvature of the force-velocity curve (a_f). For each muscle group, the experimental data of B fitted with the theoretical equation, and the best fit was obtained for an of of 0.28 for the ankle extensor and of 0.32 for the elbow flexor muscles. These results indicated that from the muscle model used in the present study it is possible to describe the mechanical behaviour of the muscle during maximal concentric action.     

Effect of angular velocity on soleus and medial gastrocnemius H-reflex during maximal concentric and eccentric muscle contraction

At rest, the H-reflex is lower during lengthening than shortening actions. During passive lengthening, both soleus (SOL) and medial gastrocnemius (MG) H-reflex amplitudes decrease with increasing angular velocity. This study was designed to investigate whether H-reflex amplitude is affected by angular velocity during concentric and eccentric maximal voluntary contraction (MVC). Experiments were performed on nine healthy men. At a constant angular velocity of 60°/s and 20°/s, maximal H-reflex and M-wave potentials were evoked at rest (i.e., H_max and M_max, respectively) and during concentric and eccentric MVC (i.e., H_sup and M_sup, respectively). Regardless of the muscle, H_max/M_max was lower during lengthening than shortening actions and the H_sup/M_sup ratio was higher than H_max/M_max during lengthening actions. Whereas no action type and angular velocity effects on the MG H_sup/M_sup were found, the SOL H_sup/M_sup was lower during eccentric than concentric MVC and this depression was increased with higher angular velocity. Our findings indicate that the depression of the H-reflex amplitude during eccentric compared to concentric MVC depends mainly on the amount of inhibition induced by lengthening action. In conclusion, H-reflex should be evoked during both passive and active dynamic trials to evaluate the plasticity of the spinal loop.     

Efficiency of human skeletal muscle in vivo: comparison of isometric, concentric, and eccentric muscle action

Ryschon, T. W., Fowler, R. E. Wysong, A.-R. Anthony, and R. S. Balaban. Efficiency of human skeletal muscle in vivo: comparison of isometric, concentric, and eccentric muscle action. J. Appl. Physiol. 83(3): 867-874, 1997.The purpose of this study was to estimate the efficiency of ATPutilization for concentric, eccentric, and isometric muscle action inthe human tibialis anterior and extensor digitorum longus in vivo. Adynamometer was used to quantitate muscle work, or tension, whilesimultaneous ³¹P-nuclear magneticresonance data were collected to monitor ATP, phosphocreatine,inorganic phosphate, and pH. The relative efficiency of the actions wasestimated in two ways: steady-state effects on high-energyphosphates and a direct comparison of ATP synthesis rates with work. Inthe steady state, the cytosolic free energy dropped to the lowest valuewith concentric activity, followed by eccentric and isometric actionfor comparative muscle tensions. Estimates of ATP synthesis ratesrevealed a mechanochemical efficiency [i.e., ATP productionrate/work (both in J/s)] of 15.0 ± 1.3% in concentric and34.7 ± 6.1% in eccentric activity. The estimated maximum ATPproduction rate was highest in concentric action, suggesting anactivation of energy metabolism under these conditions. By using directmeasures of metabolic strain and ATP turnover, these data demonstrate adecreasing metabolic efficiency in human muscle action from isometric,to eccentric, to concentric action.

     

Mechanomyographic and electromyographic responses to repeated concentric muscle actions of the quadriceps femoris.

In comparison to isometric muscle action models, little is known about the electromyographic (EMG) and mechanomyographic (MMG) amplitude and mean power frequency (MPF) responses to fatiguing dynamic muscle actions. Simultaneous examination of the EMG and MMG amplitude and MPF may provide additional insight with regard to the motor control strategies utilized by the superficial muscles of the quadriceps femoris during a concentric fatiguing task. Thus, the purpose of this study was to examine the EMG and MMG amplitude and MPF responses of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) during repeated, concentric muscle actions of the dominant leg. Seventeen adults (21.8+/-1.7 yr) performed 50 consecutive, maximal concentric muscle actions of the dominant leg extensors on a Biodex System 3 Dynamometer at velocities of 60 degrees s(-1) and 300 degrees s(-1). Bipolar surface electrode arrangements were placed over the mid portion of the VL, RF, and VM muscles with a MMG contact sensor placed adjacent to the superior EMG electrode on each muscle. Torque, MMG and EMG amplitude and MPF values were calculated for each of the 50 repetitions. All values were normalized to the value recorded during the first repetition and then averaged across all subjects. The cubic decreases in torque at 60 degrees s(-1) (R2 = 0.972) and 300 degrees s(-1) (R2 = 0.931) was associated with a decline in torque of 59+/-24% and 53+/-11%, respectively. The muscle and velocity specific responses for the MMG amplitude and MPF demonstrated that each of the superficial muscles of the quadriceps femoris uniquely contributed to the control of force output across the 50 repetitions. These results suggested that the MMG responses for the VL, RF, VM during a fatiguing task may be influenced by a number of factors such as fiber type differences, alterations in activation strategy including motor unit recruitment and firing rate and possibly muscle wisdom.