The effects of antagonist moment on the resultant knee joint moment during isokinetic testing of the knee extensors

The purpose of this study was to examine the effects of moment of antagonistic muscle on the resultant joint moment during isokinetic eccentric and concentric efforts of the knee extensors. Ten males performed maximum eccentric and concentric knee extension and flexion efforts on a Biodex dynamometer at 0.52 rad · s⁻¹ (30° · s⁻¹). Electromyographic (EMG) activity of vastus medialis and biceps femoris (hamstrings) was also recorded. The antagonistic moment of the hamstrings was determined by recording the integrated EMG (iEMG)/moment relationship at different levels of muscle effort. The iEMG/moment curves were fitted using second-degree polynomials. The polynomials were then used to predict the antagonistic moment exerted by the hamstrings from the antagonist iEMG. The antagonistic moment had a maximum of 42.92 Nm and 28.97 Nm under concentric and eccentric conditions respectively; paired t-tests indicated that this was a significant difference (P < 0.05). These results indicate that the resultant joint moment of knee extensors is the result of both agonist and antagonist muscle activation. The greater antagonist muscle activity under concentric activation conditions may be partly responsible for the lower resultant joint concentric moment of knee extensors compared with the corresponding eccentric activation. The antagonist moment significantly affects comparisons between the isokinetic moments and agonist EMG and in vitro force measurements under different testing (muscle action and angular velocity) conditions. Accepted: 25 February 1997     

Combined effect of heat stress, dehydration and exercise on neuromuscular function in humans

This study examined the combined effect of exercise induced hyperthermia and dehydration on neuromuscular function in human subjects. Six trained male runners ran for 40 min on a treadmill at 65% of their maximal aerobic velocity while wearing a tracksuit covered with an impermeable jacket and pants to impair the evaporation of sweat. These stressful experimental running conditions led the runners to a physiological status close to exhaustion. On average, the 40 min run ended at a heart rate of 196 (SD 8) beats · min⁻¹, a tympanic temperature of 40 (SD 0.3) °C and with a loss of body mass of 2 (SD 0.5)%. Pre- and post-running strength tests included measurements of maximal knee extension and flexion torques in both isometric and isokinetic (at 60 and 240° · s⁻¹) conditions. A 20 s endurance test at 240° · s⁻¹ was also performed. Surface electromyographic (EMG) activity was recorded from six knee extensor and flexor muscles during the entire protocol. The treadmill run led to clear decrements in maximal extension torque and EMG activity both in isometric and at the slowest isokinetic velocity (60° · s⁻¹). However, no differences in these parameters were observed at 240° · s⁻¹. Furthermore, the EMG patterns of the major knee extensor and flexor muscles remained remarkably stable during the treadmill run. These results demonstrate that the exercise-induced hyperthermia and dehydration in the present experiments had only minor effects on the neuromuscular performance. However, it is also suggested that high internal body temperature per se could limit the production of high force levels. Accepted: 26 September 2000     

Effects of a single habituation session on neuromuscular isokinetic profile at different movement velocities

Single training session (STS) may increase the power output (i.e., maximal torque) in different contraction types; however, little is known about the neuromuscular adaptations to reach this enhancement. In this way, the present study examined the differences between knee extensors EMG, kinematics, and dynamometry at 60 and 180° s⁻¹ before (PRE) and after (POST) a STS. Seventeen healthy males completed three different tasks: (1) 5-maximal isokinetic knee extensions, without previous habituation (PRE) at 60 and 180° s⁻¹; (2) in the same day and after a proper rest, two bouts of 5-maximal isokinetic contractions (STS) at 60 and 180° s⁻¹; and (3) in a new visit, POST consisted in new 5-maximal isokinetic contractions at 60 and 180° s⁻¹. The main parameters examined were: knee extensors peak torque (PT), total work (TW), EMG (prior to the movement onset, agonist and antagonist activation), rate of force (RFD), and velocity development (RVD). There was significant increase in PT [12% (60° s⁻¹) and 8.7% (180° s⁻¹)] and TW [13.5% (60° s⁻¹) and 10.7% (180° s⁻¹)] from PRE to POST sessions. Increases in RFD were found for both velocities (p < 0.05); however, RVD and vastus lateralis EMG prior to the movement onset were significantly higher for POST only at 60° s⁻¹. The RFD percentage of change (%change) was significantly correlated to %change for PT at 60° s⁻¹ (r ² = 0.53) and 180° s⁻¹ (r ² = 0.45). In conclusion, STS improves neural strategies to contract muscles stronger and faster at the slowest velocity, while higher velocities present different adaptations and might need more practice to further adaptations.     

Metabolic and cardiorespiratory responses to maximal intermittent knee isokinetic exercise in young healthy humans

There have been many studies on the effects of isokinetic exercise on muscle performance in training and rehabilitative programmes. On the other hand, the cardiovascular and metabolic responses elicited by this type of exercise have been poorly investigated. This study was specifically designed to describe the relationships, if any, between metabolic and cardiorespiratory responses and power output during maximal intermittent knee isokinetic exercise when a steady state is reached. A group of 18 healthy subjects (10 men and 8 women, age range 25–30 years) were requested to perform at maximal concentric isokinetic knee extensions/flexions 60° · s⁻¹ and 180° · s⁻¹ for 5 min, with a 5-s pause interposed between consecutive repetitions. The power output (W˙) was calculated; before and during the tasks heart rate (f _c) and arterial blood pressure (AP_a) were continuously monitored. Pulmonary ventilation (V˙ _E) and oxygen uptake (V˙O₂) were measured at the 4th and at the 5th min of exercise and blood lactate concentration at rest and at the 3rd min of recovery. From the 4th to the 5th min only a slight decrease in W˙ was observed, both at 60° · s⁻¹ and 180° · s⁻¹. The V˙O₂, V˙ _E, f _c and AP_a showed similar values in the last 2 min of exercise, suggesting that a steady state had been reached. The V˙O₂ increased linearly as a function of W˙, showing a significantly steeper slope at 60° · s⁻¹ than at 180° · s⁻¹. The f _c, in spite of a large interindividual variation, was linearly related to metabolic demand, and was not affected by angular velocity. Systolic and diastolic AP_a were not related either to V˙O₂ or to angular velocity. In conclusion it would appear that the metabolic response to maximal intermittent knee isokinetic exercise resembles that of dynamic exercise. Conversely, the cardiocirculatory responses would seem to reflect a relevant role of the isometric postural component, the importance of which should be carefully evaluated in each subject. Accepted: 21 September 1999     

Trunk muscle strength in eccentric and concentric lateral flexion

The aim of this study was to investigate the position and velocity dependency of the strength (torque) output of lateral flexor muscles of the trunk. Twelve male volunteers with no history of back pain participated. Movement was constrained to the frontal plane and the velocity was controlled by an isokinetic dynamometer. The eccentric and concentric strength of lateral flexor muscles on the left side was measured in a supine position at velocities of 15, 30, 45 and 60° · s⁻¹ and static strength at 20, 10, 0, −10 and −20° of lateral trunk flexion. Average peak torque values ranged between 211 and 218 Nm (eccentric) and between 66 and 140 Nm (concentric) over all tested velocities, and the average static torque ranged between 80 and 172 Nm over all tested positions. The shape of the torque–position curves was unaffected by speed and peak torque occurred at an average position of 11–15° to the contralateral (right) side in both eccentric and concentric actions. In eccentric actions, torque output was significantly higher than that during concentric and static actions. Increasing the speed of contraction did not affect eccentric torque values, whereas both peak and angle-specific concentric torque decreased with increasing speed. These results are in general accordance with earlier findings from other muscle groups, such as the knee extensors. However, they are partially at variance with results obtained in studies of lateral lifting and lowering, indicating that there are other limiting factors in complex tasks that do not just involve the trunk muscles. Accepted: 24 July 2000     

Critical power of knee extension exercises does not depend upon maximal strength

A possible dependence of critical power (CP) and the Y-intercept of the work/exhaustion time relationship (Y _intercept) on maximal muscular strength of the same muscle group has been studied in nine endurance-trained subjects, seven gymnasts, and seven weight-lifters. CP was calculated as being equal to the slope of the linear relationship between exhaustion time and the work performed at exhaustion on a knee extension ergometer. Y _intercept was equal to the intercept between this relationship and the work axis. The muscular strength of the knee was evaluated by measuring the torques exerted on a Biodex knee isokinetic dynamometer at four angular velocities: 0° · s⁻¹ (T₀), 90° · s⁻¹ (T₉₀), 180° · s⁻¹ (T₁₈₀) and 240° · s⁻¹ (T₂₄₀). The results of the present study do not support the hypothesis that CP depends upon maximal strength. Indeed, CP was not correlated with T₀, T₉₀, T₁₈₀ or T₂₄₀ (|r| < 0.01). Y _intercept was significantly and positively correlated only with T₉₀. Accepted: 1 November 1999     

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 ± ] practised 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° · s⁻¹. Before and after, the force:velocity relationship was determined bilaterally (angular velocities 0–300° · 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. Accepted: 19 September 2000     

Antagonist moment of force during maximal knee extension in pubertal boys: effects of quadriceps fatigue

The examination of the moment exerted by the hamstrings during maximum isokinetic knee extensor tests is useful when comparing isokinetic strength and muscle activity patterns between children and adults. The purpose of this study was to examine the effect of antagonist moment of the hamstrings on the isokinetic moment of the knee extensors in pubertal children and to determine whether this effect is altered following a fatigue task. Eighteen healthy pubertal males [age 14.3 (0.5) years] performed 34 maximal isokinetic concentric efforts of the knee extensors at 60°·s⁻¹. The average moment of force and electromyographic (aEMG) signal of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) at 11–30°, 31–50°, 51–70° and 71–90° of knee flexion were calculated for each repetition. The hamstrings antagonist moment was determined before and after the fatigue task by fitting the aEMG–moment relationship at different levels of muscle effort using second-degree polynomials. The percentage contribution of the antagonist moment to the resultant joint moment ranged from 7.1 % to 60.4 % throughout the range of motion, with the highest percentage observed close to full knee extension (11–30°). The antagonist effect was significantly greater during concentric tests of the knee extensors compared to the corresponding eccentric tests (p<0.05). Following the fatigue test, there was an overall decline of the resultant joint moment, but no changes in the predicted hamstrings moment were observed. These results indicate that when testing maximal knee extensor isokinetic strength in pubertal boys, activity of the hamstrings implies a reduction of the net extensor moment as compared to the isolated capacity of the knee extensors. However, this antagonist effect is not altered following the performance of an isokinetic fatigue knee extension task. Electronic Publication     

The effects of eccentric and concentric training at different velocities on muscle hypertrophy

The purpose of this study was to examine the effect of isokinetic eccentric (ECC) and concentric (CON) training at two velocities [fast, 180° s⁻¹ (3.14 rad s⁻¹) and slow,30° s⁻¹(0.52 rad s⁻¹)] on muscle hypertrophy. Twenty-four untrained volunteers (age 18–36 years) participated in fast- (n=13) or slow- (n=11) velocity training, where they trained one arm eccentrically for 8 weeks followed by CON training of the opposite arm for 8 weeks. Ten subjects served as controls (CNT). Subjects were tested before and after training for elbow flexor muscle thickness by sonography and isokinetic strength (Biodex). Overall, ECC training resulted in greater hypertrophy than CON training (P<0.01). No significant strength or hypertrophy changes occurred in the CNT group. ECC (180° s⁻¹) training resulted in greater hypertrophy than CON (180° s⁻¹) training and CON (30° s⁻¹) training (P<0.01). ECC (30° s⁻¹) training resulted in greater hypertrophy than CON (180° s⁻¹) training (P<0.05), but not CON (30° s⁻¹) training. ECC (180° s⁻¹) training resulted in the greatest increases in strength (P<0.01). We conclude that ECC fast training is the most effective for muscle hypertrophy and strength gain.     

Specificity of velocity in strength training

Summary Twenty-one male volunteers (ages 23–25 years) were tested pre- and post training for maximal knee extension power at five specific speeds (1.05, 2.09, 3.14, 4.19, and 5.24 rad·s⁻¹) with an isokinetic dynamometer. Subjects were assigned randomly to one of three experimental groups; group S, training at 1.05 rad·s⁻¹ (n=8), group I, training at 3.14 rad·s⁻¹ (n=8) or group F, training at 5.24 rad·s⁻¹ (n=5). Subjects trained the knee extensors by performing 10 maximal voluntary efforts in group S, 30 in group I and 50 in group F six times a week for 8 weeks. Though group S showed significant increases in power at all test speeds, the percent increment decreased with test speed from 24.8% at 1.05 rad·s⁻¹ to 8.6% at 5.24 rad·s⁻¹. Group I showed almost similar increment in power (18.5–22.4 at all test speeds except at 2.09 rad·s⁻¹ (15.4%). On the other hand, group F enhanced power only at faster test speeds (23.9% at 4.19 rad·s⁻¹ and 22.8% at 5.24 rad·s⁻¹).     

The effect of strength training on the apparent inhibition of eccentric force production in voluntarily activated human quadriceps

Ten male and ten female young adults trained the knee extensors of one leg eccentrically and those of the other concentrically for 6 weeks, using a gymnasium leg-extension machine. Before and after training, both legs of each subject were tested isometrically for maximum voluntary knee-extensor force, and in both eccentric and concentric isokinetic modes at 30–250° · s⁻¹. All limbs showed improvements in mean eccentric force (ranging from 18% in the concentrically trained legs of the females to 31% in the eccentrically trained legs of the males, P < 0.01–0.001). Upward trends in isometric and concentric forces were smaller and less- or non-significant. In three of the four groups, mean eccentric forces after training were significantly greater than mean isometric forces, a difference that was not evident before training. Ten further subjects of each gender, not trained but tested isometrically and isokinetically three times in 2 weeks, showed no significant improvement over the series of tests. The explanation suggested is that the increased percentage activation (“decreased inhibition”), often regarded as the main mechanism of strength gain in the early weeks of training, had been displayed particularly in the subjects' eccentric performance. This implies that the activation-shortfall, which is reduced by the initial phase of strength training, is largely or completely the same as that responsible for the fact that untrained, voluntary eccentric force is less than that of isolated muscle. Accepted: 18 February 2000     

Muscle strength and electromyogram in boys and girls followed through puberty

The main purpose of this study was to investigate the changes in anthropometric measures and muscle strength that occur during puberty in children from the age of 11 to 16 years. Special attention was paid to possible gender- and muscle action-type-specific alterations in torque/velocity and EMG/velocity characteristics. Sixteen children participated in the study (9 boys and 7 girls). Eccentric and concentric muscle strength was measured on an isokinetic dynamometer at angular velocities of 45, 90 and 180° · s⁻¹. Simultaneously, a surface electromyogram (EMG) was recorded from the quadriceps muscle. At the age of 11, the boys and girls exhibited equal anthropometric measures and strength performance. In both genders, body measures and muscle strength increased significantly during the 5-year period, with larger increases being recorded for the boys. In addition, the boys increased selectively their eccentric torque per body mass, indicating an action-type-specific change in muscle quality. The general shape of the torque/velocity relationship exhibited an adult-like pattern both before and after puberty, and did not differ between genders. Both pre- and postpuberty, myoelectric activity was generally lower during eccentric than concentric actions, the highest values occurring for both genders in the concentric 180° · s⁻¹ test. Ratios of eccentric to concentric torque per EMG, which reflect electromechanical efficiency, showed no significant changes with age. A significant velocity- and gender-specific change in electromechanical efficiency was observed at the highest speed at postpuberty, where the ratio for the girls was higher than for the boys. Accepted: 13 June 1999     

The use of magnetic resonance images to investigate the influence of recruitment on the relationship between torque and cross-sectional area in human muscle

The purpose of the present study was to investigate the effect of recruitment on the relationship between peak torque and physiological cross-sectional area (PCSA) in human muscle. A group of 11 healthy men participated in this study. Isokinetic knee extension torques at seven (0, 30, 60, 120, 180, 240, and 300° · s⁻¹) velocities were determined. Magnetic resonance imaging (MRI) was performed to calculate PCSA of right quadriceps femoris (QF) muscle. Exercise-induced contrast shifts in spin-spin relaxation time (T2)-weighted MRI were taken at rest and immediately after repetitive knee-extension exercise and T2 of QF were calculated. The MRI pixels with T2 values more than 1 SD greater than the means at rest were considered to represent QF muscle that had contracted. The area of activated PCSA within the total in QF was expressed as percentage activated PCSA and used as an index of muscle recruitment. The PCSA correlated with peak torque at 0° · s⁻¹ (r=0.615, P < 0.05); in contrast, activated PCSA correlated with peak torque at 120° · s⁻¹ (r=0.603, P < 0.05) and 180° · s⁻¹ (r=0.606, P < 0.05). Additionally, there was a significant difference in correlation coefficients between the activated PCSA-peak torque relationship and the PCSA-torque relationship (P < 0.05). These results suggested that muscle recruitment affects the PCSA-torque relationship. Accepted: 11 August 2000     

Face temperature and cardiorespiratory responses to wind in thermoneutral and cool subjects exposed to −10 °C

The effects of the thermal state of the body (slightly cool and neutral) and moderate wind speeds on face temperature, blood pressure, respiratory function and pain sensation during cold exposure were studied on eight healthy male subjects. They were dressed in cold-protective clothing and preconditioned at +20 °C (TN) and −5 °C (CO) for 60 min, then exposed to −10 °C and 0 m · s⁻¹ (NoW), 1 (W1) and 5 (W5) m · s⁻¹ wind for 30 min. Thus, each individual was exposed six times. The exposure to wind entailed a combination of strong cooling of the bare face and mild body cooling. The forehead, cheek and nose temperatures decreased during cold exposure, and the decrease was greater at higher air velocities (P < 0.0001). All subjects reported pain sensations at 5 m · s⁻¹. At the end of exposure only the nose temperature was significantly lower in CO than in TN subjects; it was about 2 °C and reached 0 °C in two experiments. The systolic and diastolic blood pressure (SBP and DBP, respectively) increased significantly by 7.7 and 5.9 mmHg, respectively, during preconditioning at −5 °C, but did not change at +20 °C. SBP and DBP increased during exposure to −10 °C in TN by approximately 9 mmHg. However, the total average increase of blood pressure (1–90 min) was similar in TN and CO (SBP 15 mmHg and DBP 13 mmHg). SBP and DBP increased more during exposure to 5 m · s⁻¹ at −10 °C than NoW. Blood pressure responses as observed in this study (SBP and DBP up to 51 and 45 mmHg, respectively) are potential health risks for hypertensive individuals and angina patients. Respiratory functions (FVC, FEV₁) were reduced by about 3% by the cold (−5 and −10 °C) compared to pre-experiment values. Furthermore, the Wind Chill Index seems to underestimate the cooling power of 5 m · s⁻¹ at −10 °C of bare skin (e.g. face). Therefore it needs to be revised and we suggest that it is expanded to include risk levels for pain sensation. Accepted: 29 May 2000     

Isokinetic contractile properties of the quadriceps with relation to fiber type

Summary Fifteen subjects were assigned to three groups on the basis of the proportion of fast-twitch fibers (%FT) in their vastus lateralis muscles. Torque production per unit of fat free thigh volume was then determined during knee extension on an isokinetic dynamometer at 60, 120, 180, 240, and 300‡·s⁻¹. Maximal isometric force was also obtained at 65‡ from horizontal. Subjects with predominantly fast twitch muscle fibers demonstrated significantly greater peak power, rate of power production and work than subjects with predominantly slow twitch fibers at all but the lowest velocity of 60‡·s⁻¹. Mean peak power for all subjects occurred at approximately 30% of maximal knee extension velocity (210‡·s⁻¹) and 34% of maximal voluntary isometric contraction. When work, peak power and rate of power production were correlated with %FT fibers, then the highest correlations of 0.69, 0.57, and 0.73 respectively, all occurred at 180‡·⁻¹. These observations suggest that: 1) the torque-velocity, power-velocity relationship of the quadriceps is similar to the force-velocity, power-velocity relationship found for excised muscle, 2) when using the knee extension exercise for the prediction of muscle fiber composition of the quadriceps the most appropriate speed for testing appears to be 180‡·s⁻¹. Supported by a grant from Lumex Inc.     

The relationships among peak torque, mean power output, mechanomyography, and electromyography in men and women during maximal, eccentric isokinetic muscle actions

The purpose of the present investigation was to examine the velocity-related patterns of peak torque (PT), mean power output (MP), mechanomyographic (MMG) amplitude, and electromyographic (EMG) amplitude of the vastus lateralis (VL) muscle in men and women during maximal, eccentric isokinetic muscle actions of the leg extensors. Eight women [mean (SD)] [aged 23 (3) years] and seven men [aged 23 (2) years] volunteered for this investigation. Their PT was measured on a calibrated dynamometer at randomly ordered velocities of 30, 90, and 150°·s^–1. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes (Ag-AgCl) over the VL muscle. Their MP was determined using the dynamometer software. The results indicated no sex-related differences (P>0.05) for the patterns of PT, MP, MMG amplitude, or EMG amplitude across velocity. The normalized values for MP and MMG amplitude increased (P<0.05) from 30 to 150°·s^–1 (30°·s^–1<90°·s^–1<150°·s^–1), while PT and EMG amplitudes remained unchanged across velocity. The results indicated close associations between the velocity-related patterns of MP and MMG amplitudes as well as PT and EMG amplitudes. Thus, MMG amplitude reflected the velocity-related changes in muscle power output, but not torque production, while EMG amplitude reflected torque production, but not muscle power output. During maximal, eccentric isokinetic muscle actions, EMG and MMG signals provided unique information regarding the electrical and mechanical aspects of muscle strength and power. Electronic Publication     

Human hip and knee torque accommodations to anterior cruciate ligament dysfunction

It has been postulated that the adaptations of lower extremity function exhibited by anterior cruciate ligament (ACL) deficient and post-ACL surgical patients represent early accommodations to the loss of ACL function after injury so that excessive anterior displacement of the tibia is prevented. Prior studies have suggested that compensation patterns in ACL deficient and post-ACL surgical subjects may affect joint moments of the knee as well as the hip. However, the variance in knee and hip forces between ACL deficient, post-surgical ACL and uninjured groups has not been clearly elucidated. The purpose of this study was to assess hip:knee extensor torque ratios relative to anterior tibia shear in pre-surgical-ACL deficient, post-surgical and uninjured subjects. Measurements of hip and knee joint moments and anterior tibia shear were recorded from 45 injured and uninjured subjects (21 men, 24 women) during lower extremity, variable resistance exercise. Anterior tibia shear was computed by decomposing joint moments and reaction forces according to a model derived from cadaver knee dissections and radiography, in combination, to estimate the tibio-femoral compressive and shear forces generated by the patellar tendon at various angles throughout the knee joint range. Three groups of subjects were studied: recently injured ACL deficient pre-surgical subjects who were scheduled for immediate surgery (PRE; n=15); post-surgical subjects who had undergone ACL reconstructive surgery at least 1 year prior to testing (POST; n=15); and uninjured controls (CON; n=15). All PRE and POST subjects had a normal contralateral limb. Tests were conducted under six conditions: 1 and 1.5 Hz cadence and maximal speed at 33% and 50% one repetition maximum resistance. The results revealed that the hip:knee ratios were significantly greater for the post-ACL surgical group than the PRE and CON groups (P < 0.01; P < 0.03). There were significant negative correlations between the hip extensor:knee extensor torque ratios and maximal anterior tibia shear. across all groups. The hip:knee extensor torque ratio increased with decreased anterior tibia shear in all groups with significant correlations ranging from −0.55 to −0.88 (P < 0.01) for the injured limbs of PRE and POST groups, and −0.64 to −0.78; (P < 0.01) for the CON group. The highest overall correlations were found for the post-surgical subjects. The results revealed that anterior tibia shear declined significantly with speed (P < 0.01) in all groups. However, the converse was true for the hip:knee extensor torque ratio across speeds. The ratio increased significantly with speed (P < 0.001) for all groups at the 33% and 50% resistances. The results suggest (1) that post-ACL surgical subjects appear to accommodate to ACL substitution by using hip extensors to a significantly greater extent than the uninjured controls in closed-chain lower extremity exercise; (2) that the hip:knee extensor torque ratio is significantly related to the magnitude of anterior tibia shear; and (3) that the anterior tibia shear is significantly reduced as speed increases in closed-chain lower extremity exercise. Accepted: 30 April 2000     

Time of day effects on isometric and isokinetic torque developed during elbow flexion in humans

 The aim of this study was, firstly, to confirm or refute the existence of circadian rhythms during several velocities of concentric action of the elbow flexor muscles and, secondly, to compare the characteristics of these circadian rhythms with those obtained during isometric actions. Eight volunteer subjects participated in this study. The circadian rhythms were obtained from six test sessions (TS) carried out at different times of day over 6 days with only one TS a day. During each TS, oral temperature and the torque of the muscle action were measured. The subjects made, on an isokinetic ergometer, two maximal isokinetic concentric elbow flexions at five angular velocities (60, 120, 180, 240 and 300° · s⁻¹) and at an angle of 60°. Torque-angular velocity relationships, which characterised the functioning of the muscle during concentric and isometric actions, were established for the different times of day. The values of the torque recorded at each of the angular velocities presented a clear circadian rhythm. After normalisation of the torque values, no significant differences were observed among the computed characteristics of the circadian rhythms obtained at different angular velocities. Since the circadian rhythms during isometric and concentric torque were the same, the characteristics of the circadian rhythms of the musculo-skeletal system can be studied using either type of muscle action. The results indicated that torque and temperature varied concomitantly during the day. Thus, the recording of body temperature allows one to estimate the times of occurrence of maximal and minimal values in the circadian rhythm of muscle torque. Accepted: 10 October 2000     

Potentiation of isometric and isotonic contractions during high-frequency stimulation

Activity dependent potentiation is an enhanced contractile response resulting from previous contractile activity. It has been proposed that even a maximal effort contraction may be enhanced by prior activity if there is an increase in the peak rate of force development. This should increase the peak active force during a very brief maximal effort contraction. The purpose of these experiments was to evaluate potentiation during brief sequential contractions with high-frequency stimulation. For this experiment, the rat medial gastrocnemius muscle was isolated in situ. Sequential stimulation (two contractions per second for 4 s) with 200, 300, or 400 Hz doublets, triplets, and quadruplets was applied. A small degree of force potentiation was observed in isometric contractions at the reference length (RL), but the activity dependent potentiation of isometric contractions was greater at short muscle length. For example, peak rate of force development for 200 Hz doublets increased significantly from the first to the eighth contraction (from 0.30 ± 0.02 to 0.34 ± 0.02 N·s⁻¹ at RL and from 0.18 ± 0.02 to 0.28 ± 0.01 N·s⁻¹ at RL-3 mm). During isotonic contractions, there were significant increases in peak shortening from the first to the eighth contraction. With 200 Hz doublet stimulation, shortening increased from 0.85 ± 0.14 to 1.14 ± 0.17 mm, and this corresponded with an increase in peak velocity (from 116 ± 18 to 136 ± 19 mm·s⁻¹). Remarkably, even 400 Hz quadruplets showed a significant increase in shortening during repeated contractions (2 s⁻¹). These observations indicate the possibility that activity dependent potentiation can result in significant improvement in both isometric and dynamic contractions, even when activated at very high frequency.     

Neuromuscular adaptations to 8-week strength training: isotonic versus isokinetic mode

Previous studies attempted to compare the effectiveness of isokinetic and isotonic training. However, they have provided conflicting results. The purpose of this study was to compare the effects of isotonic versus isokinetic standardized concentric strength training programs of the knee extensors on the neuromuscular system. The standardization of these two training programs was ensured by the equalization of the total external amount of work performed and the mean angular movement velocity. Thirty healthy male students were randomly assigned to the isotonic (IT; n = 11), the isokinetic (IK; n = 11) or the control (C; n = 8) group. Both IT and IK groups trained their dominant lower leg 3 sessions/week for 8 weeks on a dynamometer. The IT group exercised using a preset torque of 40% of the maximal voluntary isometric torque at 70° (0° = leg in horizontal position). The IK group exercised at a velocity ranging between 150° and 180° s^?1. Isotonic, isokinetic and isometric tests were performed on a dynamometer before and after strength training. Surface electromyographic activity of vastus lateralis, vastus medialis, rectus femoris, semitendinosus and biceps femoris muscles was recorded during the tests. Significant strength increases in both dynamic and static conditions were noticed for IT and IK groups without any significant difference between the two trained groups. Agonist muscle activity also increased with training but no change in antagonist muscle co-activity was observed. The two training methods could be proposed by clinicians and athletic coaches to improve concentric muscle strength in dynamic and static conditions.