Maximal Voluntary Eccentric Contractions Research Articles

Neuromuscular characteristics of eccentric, concentric and isometric contractions of the knee extensors.

We compared voluntary drive and corticospinal responses during eccentric (ECC), isometric (ISOM) and concentric (CON) muscle contractions to shed light on neurophysiological mechanisms underpinning the lower voluntary drive in a greater force production in ECC than other contractions. Sixteen participants (20-33 years) performed ISOM and isokinetic (30°/s) CON and ECC knee extensor contractions (110°-40° knee flexion) in which electromyographic activity (EMG) was recorded from vastus lateralis. Voluntary activation (VA) was measured during ISOM, CON and ECC maximal voluntary contractions (MVCs). Transcranial magnetic stimulation elicited motor-evoked potentials (MEPs) and corticospinal silent periods (CSP) during MVCs and submaximal (30%) contractions, and short-interval intracortical inhibition (SICI) in submaximal contractions. MVC torque was greater (P < 0.01) during ECC (302.6 ± 90.0 Nm) than ISOM (269.8 ± 81.5 Nm) and CON (235.4 ± 78.6 Nm), but VA was lower (P < 0.01) for ECC (68.4 ± 14.9%) than ISOM (78.3 ± 13.1%) and CON (80.7 ± 15.4%). In addition, EMG/torque was lower (P < 0.02) for ECC (1.9 ± 1.1μV.Nm-1) than ISOM (2.2 ± 1.2μV.Nm-1) and CON (2.7 ± 1.6μV.Nm-1), CSP was shorter (p < 0.04) for ECC (0.097 ± 0.03s) than ISOM (0.109 ± 0.02s) and CON (0.109 ± 0.03s), and MEP amplitude was lower (P < 0.01) for ECC (3.46 ± 1.67mV) than ISOM (4.21 ± 2.33mV) and CON (4.01 ± 2.06mV). Similar results were found for EMG/torque and CSP during 30% contractions, but MEP and SICI showed no differences among contractions (p > 0.05). The lower voluntary drive indicated by reduced VA during ECC may be partly explained by lower corticospinal excitability, while the shorter CSP may reflect extra muscle spindle excitation of the motoneurons from vastus lateralis muscle lengthening.

Plantar Flexor Weakness and Pain Sensitivity Cannot Be Assumed in Midportion Achilles Tendinopathy.

The purpose of this study was to determine the following in persons with midportion Achilles tendinopathy (AT): 1) maximal strength and power; 2) neural drive during maximal contractions and contractile function during electrically evoked resting contractions; and 3) whether pain, neural drive, and contractile mechanisms contribute to differences in maximal strength. Twenty-eight volunteers (14 AT, 14 controls) completed isometric, concentric, and eccentric maximal voluntary contractions (MVCs) of the plantar flexors in a Biodex™ dynamometer. Supramaximal electrical stimulation of the tibial nerve was performed to quantify neural drive and contractile properties of the plantar flexors. Pain sensitivity was quantified as the pressure-pain thresholds of the Achilles tendon, medial gastrocnemius, and upper trapezius. There were no differences in plantar flexion strength or power between AT and controls (isometric MVC: P = 0.95; dynamic MVC: P = 0.99; power: P = 0.98), nor were there differences in neural drive and contractile function (P = 0.55 and P = 0.06, respectively). However, the mechanisms predicting maximal strength differed between groups: neural drive predicted maximal strength in controls (P = 0.02) and contractile function predicted maximal strength in AT (P = 0.001). Although pain did not mediate these relationships (i.e., between maximal strength and its contributing mechanisms), pressure-pain thresholds at the upper trapezius were higher in AT (P = 0.02), despite being similar at the calf (P = 0.24) and Achilles tendon (P = 0.40). There were no deficits in plantar flexion strength or power in persons with AT, whether evaluated isometrically, concentrically, or eccentrically. However, the mechanisms predicting maximal plantar flexor strength differed between groups, and systemic pain sensitivity was diminished in AT.

Greater effects by performing a small number of eccentric contractions daily than a larger number of them once a week.

Our previous study found that one maximal voluntary eccentric contraction (MVC-ECC) performed daily for 5 days a week for 4 weeks increased MVC-ECC, isometric (MVC-ISO), and concentric contraction (MVC-CON) torque of the elbow flexors more than 10%. The present study investigated the effects of six maximal voluntary eccentric contractions on the MVC torques and biceps brachii and brachialis muscle thickness (MT). Thirty-six healthy young adults were placed to one of the three groups (N=12 per group); the 6 × 1 group that performed one set of six contractions once a week, the 6 × 5 group that performed one set of six contractions a day for 5 days a week, and the 30 × 1 group that performed five sets of six contractions a day in a week. The training duration was 4 weeks for all groups, and changes in MVC-ECC, MVC-CON and MVC-ISO torque, and MT before and after the 4-week training were compared among the groups. The 6 × 1 group did not show significant changes in muscle strength and MT. Significant (p < 0.05) increases in MVC-ECC (13.5 ± 11.5%), MVC-ISO (9.3 ± 5.5%), MVC-CON torque (11.1 ± 7.4%) were evident for the 6 × 5 group only, and increases in MT were found for the 6 × 5 (10.4 ± 4.4%) and 30 × 1 (8.0 ± 5.8%) groups without a significant difference. These results suggest that performing a small number of eccentric contractions 5 days a week is more effective for increasing muscle strength than performing a larger volume of eccentric contractions once a week. However, it appears that training volume is a factor for muscle hypertrophy in a short-term training.

Influence of fascicle strain and corticospinal excitability during eccentric contractions on force loss.

What is the central question of this study? Do neural and/or mechanical factors determine the extent of muscle damage induced by eccentric contractions? What is the main finding and its importance? The extent of muscle damage induced by eccentric contractions is related to both mechanical strain and corticospinal excitability measured at long muscle lengths during eccentric contractions. In this study, we investigated whether the mechanical and neural characteristics of maximal voluntary eccentric contractions would determine the extent of change in postexercise maximal voluntary isometric contraction (MVC) torque and muscle soreness. Eleven men performed 10 sets of 15 isokinetic (45degs-1 ) maximal voluntary eccentric knee extensions. Knee-extension torque and vastus lateralis fascicle length were assessed at sets 1, 5 and 9. Vastus lateralis motor evoked potential, maximal Mwave (MEP/M) and the cortical silent period (CSP) were measured at 75 and 100deg of knee flexion (0deg=full extension) during contractions and were normalized to MEP/M (MEP/Mecc/iso ) and CSP (CSPecc/iso ) recorded during isometric MVC at each angle. The MVC torque and muscle soreness of the knee extensors were assessed before, 24, 48 and 96 h after the eccentric contractions. The extent of relative decrease in MVC torque at 24h postexercise (r2 =0.38) and peak muscle soreness (r2 =0.69) were correlated (P<0.05) with MEP/Mecc/iso measured at 100deg, but not at 75deg. The average torque on the descending limb of the torque-angle relationship (r2 =0.16), fascicle elongation (r2 =0.18) and CSPecc/iso at both 75 (r2 =0.00) and 100deg (r2 =0.02) were not significantly correlated with the relative decrease in MVC torque. The relative decrease in MVC torque was best predicted by a combination of mean torque on the descending limb, fascicle elongation and MEP/Mecc/iso (R2 =0.93). It is concluded that the extent of muscle damage based on the reduction in MVC torque is determined by mechanical strain and corticospinal excitability at long muscle lengths during maximal voluntary eccentric contractions.

Prolonged low-frequency force depression is underestimated when assessed with doublets compared with tetani in the dorsiflexors.

Prolonged low-frequency force depression (PLFFD) after damaging eccentric exercise may last for several days. Historically, PLFFD has been calculated from the tetanic force responses to trains of supramaximal stimuli. More recently, for methodological reasons, stimulation has been reduced to two pulses. However, it is unknown whether doublet responses provide a valid measure of PLFFD in the days after eccentric exercise. In 12 participants, doublets and tetani were elicited at 10 and 100 Hz before and after (2, 3, 5 min, 48 and 96 h) 200 eccentric maximal voluntary contractions of the dorsiflexors. Doublet and tetanic torque responses at 10 Hz were similarly depressed throughout recovery (P > 0.05; e.g., 2 min: 58.9 ± 12.8% vs. 57.1 ± 14.5% baseline; 96 h: 85.6 ± 11.04% vs. 85.1 ± 10.8% baseline). At 100 Hz, doublet torque was impaired more than tetanic torque at all time points (P < 0.05; e.g., 2 min: 70.5 ± 14.2% vs. 88.1 ± 11.7% baseline; 96 h: 83.0 ± 14.2% vs. 98.7 ± 9.5% baseline). As a result, the postfatigue reduction of the 10 Hz-to-100 Hz ratio (PLFFD) was markedly greater for tetani than for doublets (P < 0.05; e.g., 2 min: 64.3 ± 15.1% vs. 83.0 ± 5.8% baseline). In addition, the doublet ratio recovered by 48 h (99.2 ± 5.0% baseline), whereas the tetanic ratio was still impaired at 96 h (88.2 ± 9.7% baseline). Our results indicate that doublets are not a valid measure of PLFFD in the minutes and days after eccentric exercise. If study design favors the use of paired stimuli, it should be acknowledged that the true magnitude and duration of PLFFD are likely underestimated. NEW & NOTEWORTHY Prolonged low-frequency force depression (PLFFD) will result from damaging exercise and may last for several days. After 200 eccentric maximal dorsiflexor contractions, we compared the gold-standard measure of PLFFD (calculated using trains of supramaximal stimulation) to the value obtained from an alternative technique that is becoming increasingly common (paired supramaximal stimuli). Doublets underestimated the magnitude and duration of PLFFD compared with tetani, so caution must be used when reporting PLFFD derived from paired stimuli.

Intrarater Reliability of Muscle Strength and Hamstring to Quadriceps Strength Imbalance Ratios During Concentric, Isometric, and Eccentric Maximal Voluntary Contractions Using the Isoforce Dynamometer.

To determine intrasession and intersession reliability of strength measurements and hamstrings to quadriceps strength imbalance ratios (H/Q ratios) using the new isoforce dynamometer. Repeated measures. Exercise science laboratory. Thirty healthy subjects (15 females, 15 males, 27.8 years). Coefficient of variation (CV) and intraclass correlation coefficients (ICC) were calculated for (1) strength parameters, that is peak torque, mean work, and mean power for concentric and eccentric maximal voluntary contractions; isometric maximal voluntary torque (IMVT); rate of torque development (RTD), and (2) H/Q ratios, that is conventional concentric, eccentric, and isometric H/Q ratios (Hcon/Qcon at 60 deg/s, 120 deg/s, and 180 deg/s, Hecc/Qecc at -60 deg/s and Hiso/Qiso) and functional eccentric antagonist to concentric agonist H/Q ratios (Hecc/Qcon and Hcon/Qecc). High reliability: CV <10%, ICC >0.90; moderate reliability: CV between 10% and 20%, ICC between 0.80 and 0.90; low reliability: CV >20%, ICC <0.80. (1) Strength parameters: (a) high intrasession reliability for concentric, eccentric, and isometric measurements, (b) moderate-to-high intersession reliability for concentric and eccentric measurements and IMVT, and (c) moderate-to-high intrasession reliability but low intersession reliability for RTD. (2) H/Q ratios: (a) moderate-to-high intrasession reliability for conventional ratios, (b) high intrasession reliability for functional ratios, (c) higher intersession reliability for Hcon/Qcon and Hiso/Qiso (moderate to high) than Hecc/Qecc (low to moderate), and (d) higher intersession reliability for conventional H/Q ratios (low to high) than functional H/Q ratios (low to moderate). The results have confirmed the reliability of strength parameters and the most frequently used H/Q ratios.

Stretching the limits of maximal voluntary eccentric force production in vivo

During eccentric contractions, muscular force production capacity is enhanced compared to isometric contractions. Although this is well accepted in terms of muscle mechanics, maximal voluntary eccentric contractions are associated with neural inhibition that prevents increased force production of in vivo human muscles. However, because it was shown that maximal voluntary eccentric forces can exceed maximum isometric forces by a factor of 1.2–1.4, this review focuses on the question of whether the absent eccentric force enhancement, as observed in many studies, can unambiguously be attributed to an inherent neural inhibition. First, we demonstrate that participant familiarization, preload, and fascicle behavior are crucial factors influencing maximal voluntary eccentric force production. Second, we show that muscle mechanics such as muscle length, lengthening velocity, and stretch amplitude interact when it comes to maximal voluntary eccentric force production. Finally, we discuss the diverging findings on neural inhibition during maximal voluntary eccentric contractions. Because there was no inhibition of the major motor pathways in the presence of enhanced maximal voluntary eccentric forces, further research is needed to test the concept of neural inhibition and to understand why maximal voluntary force production is reduced compared to the force capacity of isolated muscle preparations.

Prolonged Low‐frequency Force Depression is Underestimated When Assessed with Doublets Compared to Trains

Unaccustomed eccentric exercise induces muscle damage, which leads to an impairment of force or torque that can last for several days. A common indirect measure of muscle damage is a greater depression of the force response to low‐frequency (e.g., 10Hz) compared to high‐frequency (e.g., 100Hz) stimulation; i.e., a post‐exercise reduction in the 10:100Hz ratio. This phenomenon is termed, prolonged low‐frequency force depression (PLFFD). Historically, trains of stimuli (e.g., 1s) have been used to evoke tetanic responses for the ratio. More recently, some authors have used only paired stimuli to evoke doublets. However, it is unknown if the PLFFD indicated by doublet responses is equivalent to the value derived from the traditional tetanic responses. Hence, the purpose of this study was to compare the magnitude of PLFFD determined by doublet vs. tetanic responses. We hypothesized that the doublet ratio (DR) would indicate significantly less PLFFD than the train ratio (TR). Eight participants (four females) performed 200 eccentric maximal voluntary contractions (4 sets of 50 repetitions, 1s rest between repetitions and 1 min rest between sets) of the dorsiflexors to induce muscle damage. Eccentric contractions were performed at 60°/s, started at a neutral ankle position, and finished at 30° of plantar flexion. Before the fatiguing protocol, baseline neuromuscular function was measured by a sequence of isometric contractions evoked by supramaximal electrical stimulation of the fibular nerve (paired stimuli at 10 and 100Hz, and 1s trains at 10 and 100Hz, each separated by 1s rest). This neuromuscular function test was repeated three times prior to fatigue and 2 min, 3 min, 5 min, 10 min and 48h after the fatiguing protocol. Using the peak torque of each response, 10:100Hz ratios were calculated at all time points; post‐exercise values were expressed as a percentage of the respective baseline ratio. At baseline, the DR was greater than TR (0.81 ± 0.04 vs. 0.48 ± 0.09; P &lt; 0.01). PLFFD was observed by 2 min for TR (73 ± 12%) and 3 min for DR (83 ± 9%), continued to develop over the acute recovery phase (at 10 min, DR = 73 ± 9%; TR = 51 ± 11%), and recovered by 48h (DR = 98 ± 6%; TR = 91 ± 12%). The TR was reduced more than the DR at all acute recovery time points (P &lt; 0.05), but not 48h. With DR and TR revealing different levels of PLFFD following damaging eccentric contractions, our results indicate that these techniques should not be used interchangeably. Whenever possible, trains of stimuli should be used to indicate the true extent of PLFFD. In cases where the reproducibility of the tetanic force response may be problematic (e.g., femoral nerve stimulation) or a train is deemed prohibitively uncomfortable, it should be acknowledged that paired stimuli will underestimate PLFFD.Support or Funding InformationSupported by NSERC, CFI and BCKDFThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Neural adaptations to submaximal isokinetic eccentric strength training.

This study investigated the neural adaptations following submaximal isokinetic eccentric strength training of the plantar flexors. The modulation of electromyographic (EMG) activity and spinal excitability were compared in the soleus muscle (SOL) during isometric, concentric and eccentric maximal voluntary contractions (MVC) before and after submaximal isokinetic eccentric training. Eighteen healthy subjects were divided into a training group (n=8) and a control group (n=10). The training protocol consisted of sixteen sessions of isokinetic eccentric strength training during 8weeks. Normalized EMG was used to assess the activity of SOL and medial gastrocnemius muscle (MG). For SOL, maximal Hoffmann reflex (H-reflex) and compound motor potential were evoked during isometric, concentric and eccentric actions at rest (Hmax and Mmax, respectively) and during MVC (Hsup and Msup, respectively). The results showed that the torque and normalized EMG of SOL significantly increased after training during eccentric (+20.5 and +28.8%, respectively) and isometric (+18.2 and +23.0%, respectively) MVC (p<0.05). Hmax/Mmax and Hsup/Msup ratios were not significantly modified after training for SOL (p>0.05), and remained significantly depressed during eccentric compared to isometric and concentric actions (p<0.05). In contrast, no significant difference was observed on normalized EMG of MG (p>0.05). These results suggested that the increase in voluntary torque after submaximal isokinetic eccentric training can be at least partly ascribed to enhanced neural drive for SOL that does not affect the H-reflex pathway.

Velocity dependence of eccentric strength in young and old men: the need for speed!

Older adults better maintain eccentric strength relative to isometric strength, as indicated by a higher ratio of eccentric:isometric torque as compared with younger adults. The effect of increasing angular velocities (>200°/s) on the age-related maintenance of eccentric strength has not been tested and thus it is unknown whether the eccentric:isometric ratio is velocity dependent in old age. The purpose of this study was to investigate eccentric strength of the ankle dorsiflexors over a large range of lengthening angular velocities in young and older men. Isometric neuromuscular properties were assessed on a HUMAC NORM dynamometer. Nine young (∼24 years) and 9 older (∼76 years) healthy men performed maximal voluntary eccentric contractions at angular velocities of 15-360°/s. Despite near full voluntary activation (>95%), the older men were ∼30% weaker than the young men for isometric strength (P < 0.05). Across all lengthening velocities, older men had a greater eccentric:isometric ratio than young men (P < 0.05). Additionally, there was a velocity dependence of strength in both young and older men: eccentric strength increased as velocity increased up to 120°/s (P < 0.05) and plateaued thereafter. In young and older men, eccentric strength at 15°/s was ∼20% and ∼40% greater than isometric strength (P < 0.05), while at 360°/s eccentric strength was ∼50% and ∼90% greater, respectively (P < 0.05). These findings indicate that with increasing angular velocity, both young and older men have considerable increases in the eccentric:isometric ratio of torque production.

Muscle activation varies with contraction mode in human spinal cord injury.

To better understand volitional force generation after chronic incomplete spinal cord injury (SCI), we examined muscle activation during single and repeated isometric, concentric, and eccentric knee extensor (KE) maximal voluntary contractions (MVCs). Torque and electromyographic (EMG) activity were recorded during single and repeated isometric and dynamic KE MVCs in 11 SCI subjects. Central activation ratios (CARs) were calculated for all contraction modes in SCI subjects and 11 healthy controls. SCI subjects generated greater torque, KE EMG, and CARs during single eccentric vs. isometric and concentric MVCs (all P < 0.05). Torque and EMG remained similar during repeated eccentric MVCs; however, both increased during repeated isometric (>25%) and concentric (>30%) MVCs. SCI subjects demonstrated greater muscle activation during eccentric MVCs vs. isometric and concentric MVCs. This pattern of activation contrasts with the decreased eccentric activation demonstrated by healthy controls. Such information may aid development of novel rehabilitation interventions.

Changes in the number of circulating CD34+ cells after eccentric exercise of the elbow flexors in relation to muscle damage

BackgroundIt has been reported that strenuous exercise increases the number of bone marrow-derived progenitor cells such as CD34+ cells in the blood, but no previous studies have investigated the changes in circulating CD34+ cells following resistance exercise. This study tested the hypothesis that the number of CD34+ cells in the blood would increase after eccentric exercise of the elbow flexors, but decrease in recovery, and the magnitude of the changes would be dependent on the magnitude of muscle damage. MethodsNine men (28.0 ± 6.6 years) performed exercises consisting of 10 sets of six maximal voluntary eccentric contractions of the elbow flexors with their non-dominant arm. Six of them performed the same exercise with the same arm 4 weeks later. Changes in indirect markers of muscle damage were measured before, within 10 min after, and at 24, 48, 72, and 96 h after eccentric exercise. Differential leukocyte counts (total leukocytes, neutrophils, lymphocytes, monocytes) and CD34+ cells in the blood were measured before, immediately after, and at 2, 24, 48, 72, and 96 h following the exercises. ResultsAfter eccentric exercise, significant (p < 0.05) decreases in maximal voluntary isometric contraction torque and increases in delayed onset muscle soreness and plasma creatine kinase activity were observed. However, no significant changes in leukocytes and CD34+ cells were evident. The changes in muscle damage markers were significantly (p < 0.05) smaller following the second exercise session as compared with the first exercise session, but the changes in leukocytes and CD34+ cells were not significantly different between sessions. ConclusionThese results did not support the hypothesis, and showed that eccentric exercise-induced muscle damage to the elbow flexors did not influence the number of circulating CD34+ cells.

Multi-channel electromyography during maximal isometric and dynamic contractions

Motor unit behavior differs between contraction types at submaximal contraction levels, however is challenging to study during maximal voluntary contractions (MVCs). With multi-channel surface electromyography (sEMG), mean physiological characteristics of the active motor units can be extracted. Two 8-electrode sEMG arrays were attached on biceps brachii muscle (one on each head) to examine behavior of sEMG variables during isometric, eccentric and concentric MVCs of elbow flexors in 36volunteers.On average, isometric (364±88N) and eccentric (353±74N) MVCs were higher than concentric (290±73N) MVC (p<0.001). Mean muscle fiber conduction velocity (CV) was highest during eccentric MVC (4.42±0.49m/s) than concentric (4.25±0.49m/s, p<0.01) and isometric (4.14±0.45m/s, p<0.001) MVCs. Furthermore, eccentric MVC showed lower sEMG amplitude at the largest elbow joint angles (120–170°) and higher CV at the smallest (70–150°) elbow joint angles (p<0.05–0.001) than concentric MVC.The differences in CV and sEMG amplitude between the MVCs suggest that the control strategy of motor units differs between the contraction types during MVCs, and is dependent on the muscle length between the dynamic MVCs.

Tactile stimulation with Kinesiology tape alleviates muscle weakness attributable to attenuation of Ia afferents

ObjectiveProlonged vibration stimulation to normal individuals could lead to muscle weakness attributable to attenuation of afferent feedback. This weakness is neurophysiologically similar to that seen in patients with knee injury. Theoretically, increasing input to gamma motor neurons could reverse this weakness. Sensory input to these neurons from skin could indirectly increase Ia afferent feedback. The present study examined the effect of this tactile stimulation in the form of Kinesiology tape on muscle weakness attributable to attenuation of afferent feedback. DesignRandomized, crossover design. MethodsAll participants were measured their eccentric maximal voluntary contractions under the 2 conditions (taping and non-taping). First, maximal voluntary contraction during eccentric contraction was measured as baseline. For the taping condition, Kinesiology tape was applied around each subject's knee joint during maximal voluntary contraction measurement after vibration. For the non-taping condition, tape was not applied during maximal voluntary contraction measurement after vibration. Mean percentage changes between pre- and post-vibration stimulation were compared between two conditions. ResultsMaximal voluntary contraction and average electromyography of taping condition was significantly larger than that of non-taping condition. ConclusionsOur results suggest that tactile stimulation in the form of Kinesiology tape inhibits the decline of both strength and electromyography. Alpha motor neuron activity attenuated by prolonged vibration would thus be partially rescued by tactile stimulation. These results indirectly suggest that stimulation of skin around the knee could counter quadriceps femoris weakness due to attenuated Ia afferent activity.

Interferential therapy effect on mechanical pain threshold and isometric torque after delayed onset muscle soreness induction in human hamstrings

This study was undertaken to examine the acute effect of interferential current on mechanical pain threshold and isometric peak torque after delayed onset muscle soreness induction in human hamstrings. Forty-one physically active healthy male volunteers aged 18−33 years were randomly assigned to one of two experimental groups: interferential current group (n = 21) or placebo group (n = 20). Both groups performed a bout of 100 isokinetic eccentric maximal voluntary contractions (10 sets of 10 repetitions) at an angular velocity of 1.05 rad · s−1 (60° · s−1) to induce muscle soreness. On the next day, volunteers received either an interferential current or a placebo application. Treatment was applied for 30 minutes (4 kHz frequency; 125 μs pulse duration; 80−150 Hz bursts). Mechanical pain threshold and isometric peak torque were measured at four different time intervals: prior to induction of muscle soreness, immediately following muscle soreness induction, on the next day after muscle soreness induction, and immediately after the interferential current and placebo application. Both groups showed a reduction in isometric torque (P < 0.001) and pain threshold (P < 0.001) after the eccentric exercise. After treatment, only the interferential current group showed a significant increase in pain threshold (P = 0.002) with no changes in isometric torque. The results indicate that interferential current was effective in increasing hamstrings mechanical pain threshold after eccentric exercise, with no effect on isometric peak torque after treatment.

Muscle damage induced by electrical stimulation

Electrical stimulation (ES) induces muscle damage that is characterised by histological alterations of muscle fibres and connective tissue, increases in circulating creatine kinase (CK) activity, decreases in muscle strength and development of delayed onset muscle soreness (DOMS). Muscle damage is induced not only by eccentric contractions with ES but also by isometric contractions evoked by ES. Muscle damage profile following 40 isometric contractions of the knee extensors is similar between pulsed current (75Hz, 400μs) and alternating current (2.5kHz delivered at 75Hz, 400μs) ES for similar force output. When comparing maximal voluntary and ES-evoked (75Hz, 200μs) 50 isometric contractions of the elbow flexors, ES results in greater decreases in maximal voluntary contraction strength, increases in plasma CK activity and DOMS. It appears that the magnitude of muscle damage induced by ES-evoked isometric contractions is comparable to that induced by maximal voluntary eccentric contractions, although the volume of affected muscles in ES is not as large as that of eccentric exercise-induced muscle damage. It seems likely that the muscle damage in ES is associated with high mechanical stress on the activated muscle fibres due to the specificity of motor unit recruitment (i.e., non-selective, synchronous and spatially fixed manner). The magnitude of muscle damage induced by ES is significantly reduced when the second ES bout is performed 2-4weeks later. It is possible to attenuate the magnitude of muscle damage by "pre-conditioning" muscles, so that muscle damage should not limit the use of ES in training and rehabilitation.

ANTIOXIDANT SUPPLEMENTATION HAS NO EFFECT ON MUSCLE DAMAGE FOLLOWING AN ULTRAMARATHON RUN

PURPOSE To determine if prolonged antioxidant (AO) supplementation enhances recovery from endurance exercise-induced muscle damage. METHODS 22 runners consumed either placebos or AO (400 IU vitamin E and 2 × 500 mg vitamin C) daily for 6 w prior to and 1 w after a 50 k ultramarathon. All subjects consumed a prescribed diet low in vitamins E and C for the 7 w of the study; foods limited in AO were provided for subjects on race day. An isokinetic dynanometer was used to determine concentric and eccentric maximal voluntary contractions (MVC) in quadriceps and hamstrings. Measurements were taken at baseline, 1 d pre-race, 2 h post-race, and daily for 6 d post-race. Plasma creatine kinase (CK) and lactate dehydrogenase (LDH) were assessed at additional time points: pre-, mid- and post-race. Energy expenditure (EE) was calculated for each individual using the average heart rate (HR) during the run (recorded by HR monitors) and previously measured VO2. RESULTS All subjects completed the race; run time was 7.1 ± 0.1 h; EE was 5008 ± 80 for women (N=11) and 6932 ± 206 Kcal for men (N=11). Compared to pre-race, CK was elevated at mid-, post, 2 h post-, and peaked at 1 d post- (17-fold increase, p > 0.0001) returning to baseline 6 d post-race. LDH was elevated at mid-, peaked at post- and remained elevated for 2 d post-race (p < 0.03). MVC were depressed at 2 h post- (p < 0.05) compared to 1 d pre-race. Concentric quadriceps MVC remained depressed until 3 d post- (p < 0.009) while concentric hamstring MVC recovered by 2 d post-race (p < 0.02). Eccentric quadriceps MVC recovered by 1 d post-, but eccentric hamstring MVC did not recover until 3 d post-race. CONCLUSION Endurance exercise resulted in a reduction in MVC and increased plasma muscle damage markers indicating that the exercise bout was strenuous enough to cause persistent muscle damage and fatigue. Most importantly, AO supplements had no effect on the severity of the muscle damage markers, MVC impairment or on recovery rates. Supported by NIH Grant ES1153

Characteristics of isometric and dynamic strength loss following eccentric exercise‐induced muscle damage

Angle-specific isometric strength and angular velocity-specific concentric strength of the knee extensors were studied in eight subjects (5 males and 3 females) following a bout of muscular damaging exercise. One hundred maximal voluntary eccentric contractions of the knee extensors were performed in the prone position through a range of motion from 40 degrees to 140 degrees (0 degrees = full extension) at 1.57 rads(-1). Isometric peak torque was measured whilst seated at 10 degrees and 80 degrees knee flexion, corresponding to short and optimal muscle length, respectively. Isokinetic concentric peak torque was measured at 0.52 and 3.14 rad x s(-1). Plasma creatine kinase (CK) activity was also measured from a fingertip blood sample. These measures were taken before, immediately after and on days 1, 2, 4, and 7 following the eccentric exercise. The eccentric exercise protocol resuited in a greater relative loss of strength (P< 0.05) at short muscle length (76.3 +/- 2.5% of pre-exercise values) compared to optimal length (82.1 +/- 2.7%). There were no differences in the relative strength loss between isometric strength at optimal length and isokinetic concentric strength at 0.52 and 3.14 rad x s(-1). CK activity was significantly elevated above baseline at days 4 (P < 0.01) and 7 (P < 0.01). The greater relative strength loss at short muscle length appeared to persist throughout the seven-day testing period and provides indirect evidence of a shift in the angle-torque relationship towards longer muscle lengths. The results lend partial support to the popping sarcomere hypothesis of muscle damage, but could also be explained by an impairment of activation at short muscle lengths.

The human force:velocity relationship; activity in the knee flexor and extensor muscles before and after eccentric practice.

The human voluntary force:velocity relationship frequently fails to demonstrate the expected high eccentric forces. Possible explanations include unique activation strategies which might be affected by neural learning mechanisms. We investigated the effect of practicing eccentric contractions on (1) the force:velocity relationship of the human knee extensor muscles and (2) the extent of agonist and antagonist muscle activity. Eight healthy adults [seven women, group mean age 31 (SEM 5) years +/-] practiced twice a week for 4 weeks using their non-dominant legs. Each session comprised three isokinetic concentric and eccentric maximal voluntary contractions (MVC) at randomised angular velocities of 100, 200 and 300 degrees.s-1. Before and after, the force:velocity relationship was determined bilaterally (angular velocities 0-300 degrees.s-1). There were no significant differences in the forces generated or relative electromyogram (EMG) activity after practice, although there was a trend for dynamic forces to increase. Beforehand, the bilateral eccentric MVC forces were lower than isometric (P < 0.0025); afterwards they were broadly similar. The agonist EMG was similar during isometric and eccentric contractions, but lower during concentric (P < 0.03). Antagonist EMG activity showed considerable individual variation, was similar during all contraction types and tended to be greater during dynamic contractions. These data indicate that neither central learning mechanisms nor total muscle activation strategies underlie the human failure to produce the expected high eccentric voluntary forces in humans.

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.