Electrically evoked eccentric and concentric torque-velocity relationships in human knee extensor muscles

The torque-velocity relationship, obtained during in situ conditions in humans, demonstrates a levelling-off of eccentric torque output at the isometric torque level, at least for knee extensor actions. In contrast, the in vitro force-velocity relationship for animal muscle preparations is characterized by a sharp rise in eccentric force from isometric maximum. A force-regulating 'protective' mechanism has been suggested during maximal voluntary high-tension eccentric muscle actions. To investigate this phenomenon, maximal voluntary and three different levels of submaximal, electrically induced torques were compared during isometric and low velocity (10, 20 and 30 degrees s-1) isokinetic eccentric and concentric knee extensor actions in 10 healthy, moderately trained subjects. Eccentric torque was higher than isometric during electrically evoked, but not during maximal voluntary muscle actions. In contrast, concentric torque was significantly lower than isometric for both maximal voluntary and submaximal, electrically evoked conditions. Comparisons of normalized torques (isometric value under each condition set to 100%) demonstrated that the maximal voluntary eccentric torque had to be increased by 20%, and the isometric by 10% in order for the maximal voluntary torque-velocity curve to coincide with the electrically stimulated submaximal ones. These results support the notion that a tension-regulating mechanism is present primarily during eccentric maximal voluntary knee extensor actions.     

Effects of electrical stimulation on eccentric and concentric torque-velocity relationships during knee extension in man

The purpose of this study was to examine the effects of electrical stimulation on torque output during knee extension. Nine well-trained males (19-43 years) performed maximal voluntary, electrically evoked and superimposed eccentric and concentric knee extensions at velocities of 60, 180 and 360 degrees s-1, plus an isometric test (torque was always recorded at a 60 degree knee angle). Fifty-hertz stimulation was applied percutaneously at the maximum tolerated voltage (140-200 V). By superimposing electrical stimulation, eccentric torque could be increased by an average of 21-24% above the voluntary level (P less than 0.05). No corresponding differences were observed between superimposed and voluntary torques under isometric or concentric conditions. Electrically evoked torque also exceeded voluntary torque under eccentric conditions (11-12%, P less than 0.05), but was less under isometric and concentric conditions (-10 to -52%, P less than 0.05). Within the limitations of the study, it was concluded that eccentric knee extension torque under maximal voluntary conditions does not represent the maximal torque-producing capacity. The action of a neural inhibitory mechanism was proposed as an explanation for this finding. If active, this mechanism may protect against the extreme muscle tension that could otherwise develop under truly maximal eccentric conditions.     

Potentiation of concentric plantar flexion torque following eccentric and isometric muscle actions

In a stretch-shortening cycle (SSC) the concentric muscle action is enhanced by a preceding eccentric muscle action. The hypothesis of the present study is that a preceding isometric action can also have an effect on a following concentric action, but to a lesser degree. A KINetic-COMmunicator II dynamometer was used to test muscle strength of the plantar flexion of the right foot in 20 healthy women. Maximal voluntary torque measurements were made at different angular velocities (120^o s^-1 and 240^o s^-1) and the range of motion of the ankle joint was 78–125^o. The assessment was based on concentric torque output and EMG recording from the gastrocnemius muscle under three different types of testing conditions (concentric actions with and without preceding eccentric or isometric actions, all with maximal efforts). The results showed that preceding muscle actions led to greater concentric torque output (P < 0.01) between 90 and 99^o plantar flexion. However, the increase in the concentric action was significantly (P < 0.01) larger with eccentric than with isometric preceding action, regardless of velocity. The EMG activity of the concentric action showed unchanged or lower values when preceded by a muscle action. In this model our conclusion is that the main reason for larger concentric torque values after a preceding muscle action is that time is sufficient for maximal muscle tension development; in addition, elastic energy is stored, particularly during the preceding eccentric action. Our results show that the effect of preceding muscle actions should be taken into account when measuring isokinetic muscle strength at relatively small angular movements.     

Muscle activation during maximal voluntary eccentric and concentric knee extension

Summary The aim of this investigation was to study the relationships among movement velocity, torque output and electromyographic (EMG) activity of the knee extensor muscles under eccentric and concentric loading. Fourteen male subjects performed maximal voluntary eccentric and concentric constant-velocity knee extensions at 45, 90, 180 and 360° · s^–1. Myoelectric signals were recorded, using surface electrodes, from the vastus medialis, vastus lateralis and rectus femoris muscles. For comparison, torque and full-wave rectified EMG signals were amplitude-averaged through the central half (30°–70°) of the range of motion. For each test velocity, eccentric torque was greater than concentric torque (range of mean differences: 20%–146%,P < 0.05). In contrast, EMG activity for all muscles was lower under eccentric loading than velocity-matched concentric loading (7%–31%,P < 0.05). Neither torque output nor EMG activity for the three muscles changed across eccentric test velocities (P > 0.05). While concentric torque increased with decreasing velocity, EMG activity for all muscles decreased with decreasing velocity (P < 0.05). These data suggest that under certain high-tension loading conditions (especially during eccentric muscle actions), the neural drive to the agonist muscles was reduced, despite maximal voluntary effort. This may protect the musculoskeletal system from an injury that could result if the muscle was to become fully activated under these conditions.     

The force–velocity relationship of the human soleus muscle during submaximal voluntary lengthening actions

In experiments on isolated animal muscle, the force produced during active lengthening contractions can be up to twice the isometric force, whereas in human experiments lengthening force shows only modest, if any, increase in force. The presence of synergist and antagonist muscle activation associated with human experiments in situ may partly account for the difference between animal and human studies. Therefore, this study aimed to quantify the force–velocity relationship of the human soleus muscle and assess the likelihood that co-activation of antagonist muscles was responsible for the inhibition of torque during submaximal voluntary plantar flexor efforts. Seven subjects performed submaximal voluntary lengthening, shortening(at angular, velocities of +5, –5, +15, –15 and +30, and –30° s^–1) and isometric plantar flexor efforts against an ankle torque motor. Angle-specific (90°) measures of plantar flexor torque plus surface and intramuscular electromyography from soleus, medial gastrocnemius and tibialis anterior were made. The level of activation (30% of maximal voluntary isometric effort) was maintained by providing direct visual feedback of the soleus electromyogram to the subject. In an attempt to isolate the contribution of soleus to the resultant plantar flexion torque, activation of the synergist and antagonist muscles were minimised by: (1) flexing the knee of the test limb, thereby minimising the activation of gastrocnemius, and (2) applying an anaesthetic block to the common peroneal nerve to eliminate activation of the primary antagonist muscle, tibialis anterior and the synergist muscles, peroneus longus and peroneus brevis. Plantar flexion torque decreased significantly (P<0.05) after blocking the common peroneal nerve which was likely due to abolishing activation of the peroneal muscles which are synergists for plantar flexion. When normalised to the corresponding isometric value, the force–velocity relationship between pre- and post-block conditions was not different. In both conditions, plantar flexion torques during shortening actions were significantly less than the isometric torque and decreased at faster velocities. During lengthening actions, however, plantar flexion torques were not significantly different from isometric regardless of angular velocity. It was concluded that the apparent inhibition of lengthening torques during voluntary activation is not due to co-activation of antagonist muscles. Results are presented as mean (SEM).     

Tension regulation during lengthening and shortening actions of the human soleus muscle

In the present study we investigated tension regulation in the human soleus (SOL) muscle during controlled lengthening and shortening actions. Eleven subjects performed plantar flexor efforts on an ankle torque motor through 30° of ankle displacement (75°–105° internal ankle angle) at lengthening and shortening velocities of 5, 15 and 30° · s⁻¹. To isolate the SOL from the remainder of the triceps surae, the subject's knee was flexed to 60° during all trials. Voluntary plantar flexor efforts were performed under two test conditions: (1) maximal voluntary activation (MVA) of the SOL, and (2) constant submaximal voluntary activation (SVA) of the SOL. SVA trials were performed with direct visual feedback of the SOL electromyogram (EMG) at a level resulting in a torque output of 30% of isometric maximum. Angle-specific (90° ankle angle) torque and EMG of the SOL, medial gastrocnemius (MG) and tibialis anterior (TA) were recorded. In seven subjects from the initial group, the test protocol was repeated under submaximal percutaneous electrical activation (SEA) of SOL (to 30% isometric maximal effort). Lengthening torques were significantly greater than shortening torques in all test conditions. Lengthening torques in MVA and SVA were independent of velocity and remained at the isometric level, whereas SEA torques were greater than isometric torques and increased at higher lengthening velocities. Shortening torques were lower than the isometric level for all conditions. However, whereas SVA and SEA torques decreased at higher velocities of shortening, MVA torques were independent of velocity. These results indicate velocity- and activation-type-specific tension regulation in the human SOL muscle. Accepted: 11 October 1999     

Muscle strength and myoelectric activity in prepubertal and adult males and females

The purpose of this investigation was to compare children and adults of both genders with respect to torque-velocity, electromyogram (EMG)-velocity and torque-EMG relationships during maximal voluntary knee extensor muscle actions. Four groups of ten subjects each were studied comprising 11-year-old girls and boys and female and male physical education students (22–35 years). Maximal voluntary eccentric (lengthening) and concentric (shortening) actions of the knee extensors were performed at the constant velocities of 45, 90 and 180° · s^–1. Average values for torque and EMG activity, recorded by surface electrodes from the quadriceps muscle, were taken for the mid 40° of the 80° range of motion. The overall shapes of the torque- and EMG-velocity relationships were similar for all four groups, showing effects of velocity under concentric (torque decrease and EMG increase) but not under eccentric conditions. Eccentric torques were always greater than velocity-matched concentric ones, whereas the eccentric EMG values were lower than the concentric ones at corresponding velocities. Torque output per unit EMG activity was clearly higher for eccentric than for concentric conditions and the difference was of similar magnitude for all groups. Thus, the torque-EMG-velocity relationships would appear to have been largely independent of gender and to be fully developed at a prepubertal age.     

Muscle hypertrophy following 5‐week resistance training using a non‐gravity‐dependent exercise system

Aim: The efficacy of a mechanical, gravity‐independent resistance exercise (RE) system to induce strength gains and muscle hypertrophy was validated. Designed for space crew in orbit, this technique offers resistance during coupled concentric and eccentric actions by utilizing the inertia of a rotating flywheel(s), set in motion by the trainee. Methods: Ten middle‐aged (30–53 years) men and women performed four sets of seven maximal, unilateral (left limb) knee extensions two or three times weekly for 5 weeks. Knee extensor force and electromyographic (EMG) activity of the three superficial quadriceps muscles were measured before and after this intervention. In addition, with the use of magnetic resonance imaging (MRI), volume of individual knee extensor and ankle plantar flexor muscles was assessed. Results: Over the 12 training sessions, the average concentric (CON) and eccentric (ECC) force generated during exercise increased by 11% (P < 0.05). Likewise, maximal isometric strength (maximal voluntary contraction, MVC) at 90 and 120° knee angle increased by (P < 0.05) 11 and 12% respectively, after training. Neither individual quadriceps muscle showed a change (P > 0.05) in maximal integrated EMG (iEMG) activity. Quadriceps muscle volume increased by 6.1% (P < 0.05). Although the magnitude of response varied, all individual quadriceps muscles showed increased (P < 0.05) volume after training. As expected, ankle plantar flexor volume of the trained limb was unchanged (P > 0.05). Likewise, MVC, CON and ECC force, iEMG and knee extensor and plantar flexor muscle volume were unaltered (P > 0.05) in the right, non‐trained limb. Conclusion: The results of this study show that the present RE regimen produces marked muscle hypertrophy and important increases in maximal voluntary strength and appears equally effective as RE paradigms using gravity‐dependent weights, in this regard.     

Eccentric and concentric torque-velocity characteristics of the quadriceps femoris in man

Summary The primary purpose of this investigation was to study the eccentric and concentric torque-velocity characteristics of the quadriceps femoris in man using a recently developed combined isometric, concentric and eccentric controlled velocity dynamometer (the SPARK System). A secondary purpose was to compare the method error associated with maximal voluntary concentric and eccentric torque output over a range of testing velocities. 21 males (21–32 years) performed on two separate days maximal voluntary isometric, concentric and eccentric contractions of the quadriceps femoris at 4 isokinetic lever arm velocities of 0° · s^–1 (isometric), 30° · s^–1 120° · s^–1 and 270° · s^–1. Eccentric peak torque and angle-specific torques (measured every 10° from 30° to 70°) did not significantly change from 0° · s^–1 to 270° · s^–1 (p>0.05) (with the exception of angle-specific 40° torque, which significantly increased;p<0.05). The mean method error was significantly higher for the eccentric tests (10.6%±1.6%) than for the concentric tests (8.1%±1.7%) (p<0.05). The mean method error decreased slightly with increasing concentric velocity (p>0.05), and increased slightly with increasing eccentric velocity (p>0.05). A tension restricting neural mechanism, if active during maximal eccentric contractions, could possibly account for the large difference seen between the present eccentric torque-velocity results and the classic results obtained from isolated animal muscle.     

Co-activation and tension-regulating phenomena during isokinetic knee extension in sedentary and highly skilled humans

The aim of this study was to examine isokinetic torque produced by highly skilled (HS) and sedentary (S) human subjects, during knee extension, during maximal voluntary and superimposed electrical activation. To verify the level of activation of agonist (vastus lateralis, VL, and vastus medialis, VM) and antagonist muscles (semi-tendineous, ST), during maximal voluntary activation, their myo-electrical activities were detected and quantified as root mean square (rms) amplitude. Ten HS and ten S subjects performed voluntary and superimposed isometric actions and isokinetic knee extensions at 14 angular velocities (from –120 to 300°·s^–1). The rms amplitude of each muscle was normalized with respect to its rms amplitude when acting as agonist at 15°·s^–1. Whatever the angular velocity considered, peals torque and constant angular torque at 65° HS were significantly higher (P < 0.05) than those of S. Eccentric superimposed torque of S, but not HS, was significantly higher (P < 0.05) than voluntary torque at –120, –90, –60 and –30°·s^–1 angular velocities. For a given velocity, the rms amplitude of VL and VM were significantly lower (P < 0.05), during eccentric than during concentric actions, in S, but not in HS. However, whatever the angular velocity, ST co-activation in HS was significantly lower (P < 0.05) than in S. We concluded that co-activation phenomenon could partly explain differences in isokinetic performances. Differences between voluntary and superimposed eccentric torques as well as lower agonist rms amplitude during eccentric action in S, support the possibility of the presence of a tension-regulating mechanism in sedentary subjects.     

Maximal force during eccentric and isometric actions at different elbow angles

The purpose of this study was to examine a course of force potentiation and/or inhibition during maximal voluntary eccentric action. Maximal voluntary force (MVC) of elbow flexion of ten healthy male volunteers was measured during isometric and isokinetic eccentric action starting from 80° or 110° and ending at 140° elbow angle. Surface EMG was recorded from biceps brachii (BB) and brachioradialis (BR) muscles. Maximal voluntary eccentric force during the first 10° of the movement was higher (P<0.001) than the maximal voluntary isometric preactivation force both in 80° and in 110° starting position at all three velocities (1, 2, and 4 rad s⁻¹). The relative force potentiation was velocity dependent being smallest at the lowest stretching speed (P<0.01). Average EMG (aEMG) of BB and BR decreased as the joint angle increased both in eccentric and in isometric actions but the decrease in aEMG towards extension was somewhat higher in eccentric actions as compared to isometric. It was concluded that the force measured during the first 10° of eccentric contraction always exceeded the maximal voluntary isometric preactivation force regardless of the joint angle or of the movement velocity. When maximal voluntary preactivation preceded the stretch, the relative force potentiation seemed to be greater at higher stretching velocities (velocity dependent) while at lower preactivation levels, the velocity dependence was not observed. Decreased muscle activation and lower maximal voluntary force towards the end of the movement suggested inhibition during maximal voluntary eccentric actions.     

The repeated bout effect of eccentric exercise is not associated with changes in voluntary activation

The aim of this study was to compare the possible changes in muscle activation level between a first and second bout of damaging eccentric exercise performed at 2 weeks interval (i.e. repeated bout effect). To that purpose, ten physically active males took part in this study. The eccentric exercise consisted of 10 sets of 12 maximal voluntary contractions (MVC) produced by the knee extensors during movements performed at a constant speed of 160°s⁻¹. Changes in voluntary and electrically evoked torque in concentric and/or isometric conditions were assessed at the following time points: pre-exercise, and 2 min, 1 and 24 h after each eccentric exercise. At the same time points, voluntary activation was quantified by the superimposed electrical stimulation technique. Muscle soreness and plasma CK activity were measured within 48 h after the eccentric exercise. The results showed that the decrease in eccentric peak torque was linear throughout the exercise protocol. At the end of bouts 1 and 2, torque was significantly reduced by 27.7 ± 9.1 and 23.4 ± 11.2, respectively, with no difference between bouts (P > 0.05). At 24 h post-exercise, a lower reduction (P < 0.05) in MVC (17.8 ± 5.4%) and electrically evoked (16.7 ± 4.6%) isometric torque was observed for bout 2. In contrast, no statistical difference was found in the deficit in voluntary activation between the two bouts. In conclusion, our results indicate that the repeated bout effect of eccentric exercise appears to reduce muscle damage, but does not influence the level of voluntary activation.     

Effects of 17-day spaceflight on knee extensor muscle function and size

It is generally held that space travelers experience muscle dysfunction and atrophy during exposure to microgravity. However, observations are scarce and reports somewhat inconsistent with regard to the time course, specificity and magnitude of such changes. Hence, we examined four male astronauts (group mean ~43 years, 86 kg and 183 cm) before and after a 17-day spaceflight (Space Transport System-78). Knee extensor muscle function was measured during maximal bilateral voluntary isometric and iso-inertial concentric, and eccentric actions. Cross-sectional area (CSA) of the knee extensor and flexor, and gluteal muscle groups was assessed by means of magnetic resonance imaging. The decrease in strength (P<0.05) across different muscle actions after spaceflight amounted to 10%. Eight ambulatory men, examined on two occasions 20 days apart, showed unchanged (P>0.05) muscle strength. CSA of the knee extensor and gluteal muscles, each decreased (P<0.05) by 8%. Knee flexor muscle CSA showed no significant (P>0.05) change. The magnitude of these changes concord with earlier results from ground-based studies of similar duration. The results of this study, however, do contrast with the findings of no decrease in maximal voluntary ankle plantar flexor force previously reported in the same crew.     

Muscle hypertrophy following 5-week resistance training using a non-gravity-dependent exercise system

AIM: The efficacy of a mechanical, gravity-independent resistance exercise (RE) system to induce strength gains and muscle hypertrophy was validated. Designed for space crew in orbit, this technique offers resistance during coupled concentric and eccentric actions by utilizing the inertia of a rotating flywheel(s), set in motion by the trainee. METHODS: Ten middle-aged (30-53 years) men and women performed four sets of seven maximal, unilateral (left limb) knee extensions two or three times weekly for 5 weeks. Knee extensor force and electromyographic (EMG) activity of the three superficial quadriceps muscles were measured before and after this intervention. In addition, with the use of magnetic resonance imaging (MRI), volume of individual knee extensor and ankle plantar flexor muscles was assessed. RESULTS: Over the 12 training sessions, the average concentric (CON) and eccentric (ECC) force generated during exercise increased by 11% (P < 0.05). Likewise, maximal isometric strength (maximal voluntary contraction, MVC) at 90 and 120 degrees knee angle increased by (P < 0.05) 11 and 12% respectively, after training. Neither individual quadriceps muscle showed a change (P > 0.05) in maximal integrated EMG (iEMG) activity. Quadriceps muscle volume increased by 6.1% (P < 0.05). Although the magnitude of response varied, all individual quadriceps muscles showed increased (P < 0.05) volume after training. As expected, ankle plantar flexor volume of the trained limb was unchanged (P > 0.05). Likewise, MVC, CON and ECC force, iEMG and knee extensor and plantar flexor muscle volume were unaltered (P > 0.05) in the right, non-trained limb. CONCLUSION: The results of this study show that the present RE regimen produces marked muscle hypertrophy and important increases in maximal voluntary strength and appears equally effective as RE paradigms using gravity-dependent weights, in this regard.     

Hypertrophy without increased isometric strength after weight training

Summary Eight men (20–23 years) weight trained 3 days week^–1 for 19 weeks. Training sessions consisted of six sets of a leg press exercise (simultaneous hip and knee extension and ankle plantar flexion) on a weight machine, the last three sets with the heaviest weight that could be used for 7–20 repetitions. In comparison to a control group (n = 6) only the trained group increased (P<0.01) weight lifting performance (heaviest weight lifted for one repetition, 29%), and left and right knee extensor cross-sectional area (CAT scanning and computerized planimetry, 11%, P<0.05). In contrast, training caused no increase in maximal voluntary isometric knee extension strength, electrically evoked knee extensor peak twitch torque, and knee extensor motor unit activation (interpolated twitch method). These data indicate that a moderate but significant amount of hypertrophy induced by weight training does not necessarily increase performance in an isometric strength task different from the training task but involving the same muscle group. The failure of evoked twitch torque to increase despite hypertrophy may further indicate that moderate hypertrophy in the early stage of strength training may not necessarily cause an increase in intrinsic muscle force generating capacity.     

EMG activity and voluntary activation during knee-extensor concentric torque generation

This study was designed to re-examine and compare the neural drive of the knee extensors during isokinetic concentric muscular actions by means of the twitch interpolation technique (activation level, AL) and surface electromyographic (EMG) recordings (root mean square, RMS). Torque, AL and RMS amplitudes of three knee extensors and one knee flexor were measured in nine subjects during maximal and sub-maximal voluntary contractions, performed under concentric (60°·s^–1 and 120°·s^–1; Con60 and Con120, respectively) and isometric (Iso) conditions. Mean (SD) maximal voluntary torque was significantly lower (P<0.01) during concentric contractions [Con60: 208.6 (26.8) Nm and Con120: 184.7 (26.4) Nm] compared with isometric contractions [327.4 (52.0) Nm]. A significantly lower AL (P<0.05) was recorded during Con60 [80.9 (8.8)%] compared with Iso [87.9 (5.1)%] and Con120 [88.2 (6.6)%] maximal contractions. Simultaneously, a lower knee extensor average RMS amplitudes (av.RMS) was measured during Con60 maximal contractions compared with Iso and Con120 maximal contractions. The antagonist biceps femoris RMS values were not different between maximal Iso, Con60 and Con120 contractions. During sub-maximal voluntary contractions, the RMS/torque relationships were similar whatever the muscle considered (vastus lateralis, vastus medialis or rectus femoris) and the AL/av.RMS relationships did not reveal any noticeable differences between each contractile condition. The results of the present study indicate that av.RMS and AL describe similarly the neural drive during maximal and sub-maximal efforts and indicate that during maximal voluntary efforts, neural drive is dependent upon concentric angular velocity (up to 120°·s^–1). Thus, our results suggest that when applying different contractile conditions, the torque output is regulated via complex interactions between intrinsic muscular properties and the neural drive. Electronic Publication     

Do knee injuries have long-term consequences for isometric and dynamic muscle strength?

To ascertain whether decrements in knee extensor muscle strength persist years after a traumatic ligamentous or meniscal knee injury, with reference to (1) the type of muscle activity, (2) the dominance of injury, and (3) the time since injury, 36 subjects with previous unilateral knee injuries were assessed. Peak voluntary quadriceps muscle strength was measured using the KinCom 500H dynamometer during isometric, concentric (30°·s^–1 and 120°·s^–1) and eccentric contractions (30·s^–1), and the findings for each type of contraction were compared. Significant differences in quadriceps muscle strength were seen between (1) the injured and uninjured limbs during maximal voluntary isometric (P=0.0003), concentric (P<0.0001) and eccentric (P<0.0001) contractions, and (2) the isometric and concentric decrements (P=0.004), and the isometric and eccentric decrements (P=0.012) within the same injured limb. The decrements in eccentric strength were significantly greater if they affected the dominant rather than the non-dominant limb (P=0.0186). No relationship was seen between the time since injury and the degree of isometric or dynamic decrement. Deficits in quadriceps muscle strength remained for a long time after traumatic knee injury, with exercise levels rarely returning to the previously uninjured state. The degree of decrement in muscle strength was dependent upon the type of muscle activity, with concentric and eccentric activity showing greater decrements than those seen with isometric activity. Deficits in the type of activity varied widely in the same individual, and eccentric decrements were significantly worse following dominant knee injuries. Possible explanations for these findings and the implications for rehabilitation following knee injury are discussed. Electronic Publication     

Effects of dietary supplementation with vitamins C and E on muscle function during and after eccentric contractions in humans

Reactive oxygen species may contribute to exercise-induced skeletal muscle damage, and antioxidants may protect against such damage. This study examined the effectiveness of prophylactic supplementation with vitamins C and E on symptoms of muscle damage in a single blind, two-group study design. Twelve male volunteers were randomly assigned to either treatment or control groups. The treatment group received 500 mg of vitamin C and 1,200 IU of alpha–tocopherol daily and the control group received glucose placebo for 37 days. After 30 days of treatment, volunteers performed 300 maximal eccentric contractions of the knee extensor muscles of one leg. Maximal voluntary isometric contraction force and electrically evoked force at a frequency of 20 Hz and 50 Hz were recorded before and after exercise, and on days 1, 2 and 7 after exercise. Muscle soreness questionnaires were completed and muscle girth recorded at the same time points. Eccentric contractile torque and work during the bout declined significantly in both groups (P<0.001), but this decline was smaller in the vitamin-supplemented group (P<0.05). Maximal voluntary isometric contraction force and 20:50 Hz force ratio declined significantly after exercise in both groups (P<0.01), but the decline was smaller in the treatment group on days 1 and 2 post-exercise (P<0.05). Both groups experienced similar significant muscle soreness and swelling after exercise. These data suggest that prior supplementation with dietary antioxidants ameliorates muscle functional decrements subsequent to eccentric muscle contraction.     

Carbon monoxide inhalation reduces skeletal muscle fatigue resistance

Aim: To determine whether inhalation of carbon monoxide (CO), resulting in carboxyhaemoglobin (COHb) levels observed in smokers, had an effect on muscle fatigue during electrically evoked and voluntary muscle contractions. Methods: Young non‐smoking males inspired CO from a Douglas bag until their COHb level reached 6%. During the control condition the same participants inspired ambient air from a Douglas bag for 6 min. Fatigue was assessed as the decline in torque in isometric knee extensions, during 2 min of electrically evoked contractions (30 Hz, 1 s on, 1 s off) and during 2 min of maximal isometric voluntary contractions (1 s on, 1 s off). A fatigue index (FI) was calculated as the ratio of final torque : initial torque. Time to peak torque (TPT) and half relaxation time (½RT) were also determined for the electrically evoked contractions. Results: The FI during both the voluntary fatigue test (control: 0.80 ± 0.09 vs. CO: 0.70 ± 0.08; mean ± SD) and that of the fatigue test with electrically evoked contractions (control: 0.61 ± 0.09 vs. CO: 0.53 ± 0.12) was significantly lower after CO inhalation than after inhalation of ambient air (P < 0.05). There was, however, no effect of CO on the changes in TPT or ½RT during the fatigue test. Conclusion: Carbon monoxide inhalation resulting in COHb levels found in smokers has an acute impact on the ability of the muscle to resist fatigue.     

Short-term effects of whole-body vibration on maximal voluntary isometric knee extensor force and rate of force rise

Whole-Body vibration (WBV) may lead to muscle contractions via reflex activation of the primary muscle spindle (Ia) fibres. WBV has been reported to increase muscle power in the short term by improved muscle activation. The present study set out to investigate the acute effects of a standard WBV training session on voluntary activation during maximal isometric force production (MVC) and maximal rate of force rise (MRFR) of the knee extensors. Twelve students underwent a single standard WBV training session: 5×1 min vibration (frequency 30 Hz, amplitude 8 mm) with 2 min rest in between. During vibration, subjects stood barefoot on the vibration platform with their knees at an angle of 110°. At 90 s following vibration, maximal voluntary knee extensor force was reduced to 93 (5)% [mean (SD), P<0.05] of baseline value and recovered within the next 3 h. Voluntary activation remained significantly depressed (2–4%). Neither the electrically induced MRFR nor voluntary MRFR were significantly affected by WBV. In addition, six WBV training sessions in 2 weeks (n=10) did not enhance either voluntary muscle activation during MVC [99 (2)% of the baseline value] or voluntary MRFR [98 (9)% of the baseline value]. It is concluded that in the short term, WBV training does not improve muscle activation during maximal isometric knee extensor force production and maximal rate of force rise in healthy untrained students. Electronic Publication