Influence of age on concentric isokinetic torque and passive extensibility variables of the calf muscles of women

The purpose of this study was to investigate the influence of age on concentric isokinetic torque (CIT) and passive extensibility (PE) variables of the calf muscles of healthy women. Ten younger women [31.9 (SD 6.1) years] and ten older women [71.1 (SD 6.6) years] were tested using a KIN-COM 500H dynamometer. The PE was tested by stretching the muscles from relaxed plantarflexion to the maximal dorsiflexion (DF) angle at 5°·s^–1 without raw electromyogram (EMG) activity exceeding 0.05 mV. The maximal CIT was tested from the maximal DF angle 60° into plantarflexion at four randomly ordered velocities of 30, 60, 120, and 180°·s^–1. Separate analysis of variance (ANOVA) tests showed that the standardized (% body mass) concentric peak and mean torques were lower for the older women for all isokinetic velocities (p < 0.001). The angular delay from the onset of concentric activation to peak torque was smaller for the older women at 120 and 180°·s^–1 (p < 0.05). Age showed negative relationships (Pearson r) with all standardized peak torques (p 0.001) and mean torques (p < 0.001), and the angular delay at 120 and 180°·s^–1 (p 0.05). Independent t-tests showed that the maximal DF angle and the change in the PE angle from an initial angle (defined at 10% of the maximal passive torque) to the maximal DF angle were less for the older women (p < 0.05). Age was negatively related to the maximal DF angle and the change in the PE angle (p < 0.01). The results suggest an age-related decrease in calf muscle CIT, muscle length and PE. The smaller angular delay for the older women at 120 and 180°·s^–1 indicates that CIT testing at rapid velocities can be used to examine age-related changes in calf muscle contractile properties in relation to rapid velocities of movement.     

Viscoelastic properties of short calf muscle-tendon units of older women: effects of slow and fast passive dorsiflexion stretches in vivo

Changes in connective tissues of the skeletal muscle-tendon unit (MTU) of aging animal muscles have been associated with increased passive viscoelastic properties. This study examined whether similar changes in the viscoelastic properties were present in short calf MTUs of older women in vivo. Fifteen women 68–87 years of age with short calf MTUs, as represented by limited active dorsiflexion (DF) range of motion (ROM) of 5°, and 15 women 20–26 years of age without decreased DF ROM participated. A Kin-Com dynamometer stretched the MTU from plantarflexion to maximal DF at the slow velocity of 5° s^–1 (0.087 rad s^–1) and the fast velocity of 120° s^–1 (2.094 rad s^–1) with minimal surface electromyogram activity in the soleus, gastrocnemius, and tibialis anterior muscles. Two-way analysis of variance (ANOVA) tests for repeated measures (Velocity × Group) indicated that all women showed greater passive torque, average passive elastic stiffness, and total absorbed passive elastic energy for the fast stretch than for the slow stretch (P<0.001). The older women had greater percent increases for the average passive torque (30%) and total absorbed passive elastic energy (26%) for the fast stretch than the younger women (P<0.05), who had 17.5 and 13% increases, respectively. The older women had less maximal and average passive torque (Nm) and total absorbed passive elastic energy (°Nm), but greater average passive elastic stiffness (Nm°^–1) at both stretch velocities (P<0.001). The results indicated that short calf MTUs of older women have increased passive viscoelastic properties that could have implications for balance and ambulatory function.     

Is passive stiffness in human muscles related to the elasticity of tendon structures?

The purpose of this study was to examine in vivo whether passive stiffness in human muscles was related to the elasticity of tendon structures and to performance during stretch-shortening cycle exercise. Passive torque of plantar flexor muscles was measured during passive stretch from 90° (anatomical position) to 65° of dorsiflexion at a constant velocity of 5°·s^–1. The slope of the linear portion of the passive torque-angle curve during stretching was defined as the passive stiffness of the muscle. The elongation of the tendon and aponeurosis of the medial gastrocnemius muscle (MG) was directly measured using ultrasonography during ramp isometric plantar flexion up to the voluntary maximum. The relationship between the estimated muscle force of MG and tendon elongation was fitted to a linear regression, the slope of which was defined as the stiffness of the tendon. In addition, the dynamic torques during maximal voluntary concentric plantar flexion with and without prior eccentric contraction were determined at a constant velocity of 120°·s^–1. There were no significant correlations between passive stiffness and either the tendon stiffness (r=0.19, P>0.05) or the relative increase in torque with prior eccentric contraction (r=–0.19, P>0.05). However, tendon stiffness was negatively correlated to the relative increase in torque output (r=–0.42, P<0.05). The present results suggested that passive stiffness was independent of the elasticity of tendon structures, and had no favourable effect on the muscle performance during stretch-shortening cycle exercise. Electronic Publication     

A stretching program increases the dynamic passive length and passive resistive properties of the calf muscle-tendon unit of unconditioned younger women

This study examined the effects of a 6-week stretching program on the dynamic passive elastic properties of the calf muscle-tendon unit (MTU) of unconditioned younger women. After random assignment of 12 women (age 18-31 years) to a stretching group (SG) or to a control group (CG), six subjects in the SG and four subjects in the CG completed the study. For the initial tests, a Kin-Com dynamometer moved the ankle from plantarflexion to maximal dorsiflexion (DF) with negligible surface EMG activity in the soleus, gastrocnemius and tibialis anterior muscles. Angular displacement, passive resistive torque, area under the curve (passive elastic energy) and stiffness variables were reduced from the passive DF torque curves. The SG then completed ten static wall stretches held 15 s each, five times a week for 6 weeks, the CG did not. The tests were repeated and the changes between the tests and retests were examined for group differences (Mann-Whitney U). The SG had significant increases in the maximal passive DF angle (7 degrees +/- 4 degrees ), maximal passive DF torque (11.2 +/- 8.3 N m), full stretch range of motion (23 degrees +/- 24 degrees ), full stretch mean torque (3.4 +/- 2.1 N m), and area under the full stretch curve (22.7 +/- 23.5 degrees N m) compared to the CG (P < or = 0.019). The passive stiffness did not change significantly. The results showed that a stretching program for unconditioned calf MTUs increased the maximal DF angle and length extensibility, as well as the passive resistive properties throughout the full stretch range of motion. The adaptations within the calf MTU provide evidence that stretching enhances the dynamic passive length and passive resistive properties in unconditioned younger women.     

Gender-specific knee extensor torque,flexor torque,and muscle fatigue responses during maximal effort contractions

The purpose of this study was to examine gender differences in knee extensor and flexor peak torque, work, power, and muscle fatigue during maximal effort isokinetic contractions. Subjects included 19 healthy male and 20 healthy female volunteers. Following a dynamic warm-up period, subjects performed 30 reciprocal, concentric maximal knee extension and flexion contractions at a pre-set angular velocity of 3.14 rad·s^–1 on the Biodex Isokinetic Dynamometer. Values for knee extensor peak torque, work, and power were calculated for each repetition over an angular displacement of 1.05 rad for each repetition. The single highest repetition value for knee extensor and flexor peak torque, work, and power was then calculated relative to body mass (N·m·kg^–1, J·kg^–1, W·kg^–1) and allometric-scaled (N·m·kg^{– n} , J·kg^{– n} , W·kg^{– n} ) units. The allometric-scaled units were derived from a log–log transformation and linear regression analysis to calculate the exponent to which body mass is raised. The rate of quadriceps femoris muscle fatigue was calculated as the decline in each isokinetic variable by the linear slope from the single highest repetition value through the 30th repetition, and by two different fatigue indexes. The results demonstrate higher knee extension and flexion peak torque, work, and power in absolute, relative, and allometric-scaled units for males compared to females. Males exhibited higher fatigue rates for both muscle groups of each isokinetic variable than females, as described by the slope and the fatigue index, except when adjusted for peak values via analysis of covariance. The findings suggest that during maximal-effort muscle contractions, males exhibit a higher susceptibility to muscle fatigue than females, a phenomenon that may be related to an inherent ability to generate higher knee extensor and flexor torque. Electronic Publication     

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     

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     

Maximal isokinetic and isometric muscle strength of major muscle groups related to age,body mass,height, and sex in 178 healthy subjects

The main objective of this study was to establish normative values for maximal concentric isokinetic strength and maximal isometric strength of all major muscle groups in healthy subjects applying sex, age, height, and body mass-adjusted statistical models. One hundred and seventy-eight (178) (93 male and 85 female) healthy non-athletic Danish volunteers aged 15–83 years were recruited. Eighteen test protocols for each sex were applied to determine isokinetic and isometric muscle strength at knee, ankle, hip, shoulder, elbow, and wrist using a dynamometer (Biodex System 3 PRO). Multiple linear regressions were performed with maximal muscle strength (peak torque) as dependent variable and age, height, and body mass as independent variables. Muscle strength significantly related to age in 24, to height in 13 and to body mass in 27 out of the 36 models. In gender-specific analyses, the variables age, height and body mass accounted for 25% (20–29) (95% confidence interval) of the variation (r ²) in strength for men and 31% (25–38) for women. The r ² was similar for the isokinetic models and the isometric models [31% (22–40) vs. 28% (23–34)]. Age, height, and body mass related to strength in most muscle groups and gender-specific models with estimated prediction intervals were established for maximal strength of major muscle groups.     

Adaptation to chronic eccentric exercise in humans: the influence of contraction velocity

We compared changes in muscle fibre composition and muscle strength indices following a 10 week isokinetic resistance training programme consisting of fast (3.14 rad^.s^–1) or slow (0.52 rad^.s^–1) velocity eccentric muscle contractions. A group of 20 non-resistance trained subjects were assigned to a FAST (n=7), SLOW (n=6) or non-training CONTROL (n=7) group. A unilateral training protocol targeted the elbow flexor muscle group and consisted of 24 maximal eccentric isokinetic contractions (four sets of six repetitions) performed three times a week for 10 weeks. Muscle biopsy samples were obtained from the belly of the biceps brachii. Isometric torque and concentric and eccentric torque at 0.52 and 3.14 rad^.s^–1 were examined at 0, 5 and 10 weeks. After 10 weeks, the FAST group demonstrated significant [mean (SEM)] increases in eccentric [29.6 (6.4)%] and concentric torque [27.4 (7.3)%] at 3.14 rad^.s^–1, isometric torque [21.3 (4.3)%] and eccentric torque [25.2 (7.2)%] at 0.52 rad^.s^–1. The percentage of type I fibres in the FAST group decreased from [53.8 (6.6)% to 39.1 (4.4)%] while type IIb fibre percentage increased from [5.8 (1.9)% to 12.9 (3.3)%; P<0.05]. In contrast, the SLOW group did not experience significant changes in muscle fibre type or muscle torque. We conclude that neuromuscular adaptations to eccentric training stimuli may be influenced by differences in the ability to cope with chronic exposure to relatively fast and slow eccentric contraction velocities. Possible mechanisms include greater cumulative damage to contractile tissues or stress induced by slow eccentric muscle contractions. Electronic Publication     

Relationship between in vivo muscle force at different speeds of isokinetic movements and myosin isoform expression in men and women

In an attempt to explore the relationship between force production during voluntary contractions at different speeds of isokinetic movement and the myofibrillar protein isoform expression in humans, an improved isokinetic dynamometer that corrects for gravitation, controls for acceleration and deceleration, and identifies a maximum voluntary activation was used. Muscle torque recordings were compared at the same muscle length (knee angle) and the torque was calculated as the average torque at each angle over a large knee angle range (75°–25°) to reduce the influence of small torque oscillation on the calculated torque. Muscle torque at fast (240° s⁻¹) versus slow (30° s⁻¹) speeds of movement, torque normalized to muscle cross-sectional area (specific tension), and absolute torque at fast speeds of movement were measured in 34 young healthy male and female short-, middle-, and long-distance runners. The relationship between the different measures of muscle function and the expression of myosin heavy chain (MyHC) isoforms using enzyme–histochemical and electrophoretic protein separation techniques were investigated. A significant correlation between the 240° s⁻¹ vs 30° s⁻¹ torque ratio and the relative area of the type II fibers and type II MyHC isoforms were observed in both the men (r=0.74;P<0.001) and the women (r=0.81; P<0.05). Thus, the present results confirm a significant relationship between in vivo human muscle function and the MyHC isoform expression in the contracting muscle. Electronic Publication     

Interrelationships among various measures of upper body strength assessed by different contraction modes

Summary Two studies were conducted in 83 college men to determine the degree of generality of individual differences in upper body muscular strength assessed by different testing modes. In study 1 (N=43), correlations were computed between four measures of upper body strength using the bench press movement, maximal isokinetic (0.09 rad·s^–1), maximal fast (0.126 m·s^–1) and slow (0.037 m·s^–1) hydraulic, and one repetition maximum (1-RM) free weight bench press (BP). Compared to free weight BP, maximal strength during isokinetic and slow hydraulic BP was 29% and 8% larger, and fast hydraulic BP strength was 63% lower (p<0.05). Simple linear regression of isokinetic BP on 1-RM BP yieldedr=0.79, error of prediction (SE)=12%, and generality=81%. The corresponding averaged values for the regression of slow and fast hydraulic BP on free weight 1-RM BP werer=0.77, SE=13.5%, and generality=84%. In Study 2 (N=40), testing included maximal isokinetic concentric and eccentric arm flexion and extension at 0.524, 1.570, and 2.094 rad·s^–1. The ratio of concentric to eccentric torque at the 3 speeds averaged 0.68 (flexion) and 0.70 (extension), and eccentric torques were 32% and 30% greater than concentric torques (p<0.05). The linear regression between concentric vs. eccentric flexion and extension torques at the three velocities yielded an averager=0.80, SE=13.7%, and generality=73%. The findings from both studies provide evidence for generality of concentric muscle forces obtained during isokinetic, hydraulic, and 1-RM BP movement, and concentric and eccentric torque measured at 3 velocities for simple arm flexion and extension. Thus, individuals who performed well (or poorly) on one type of upper body strength test were able to achieve the same relative level of performance when tested by different contraction modes.     

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     

Influence of a low-level contractile response from the soleus, gastrocnemius and tibialis anterior muscles on viscoelastic stress-relaxation of aged human calf muscle-tendon units

Measuring viscoelastic stress-relaxation (VSR) as the decline in passive tension over time when the skeletal muscle-tendon unit (MTU) is held in a lengthened position may be difficult in older people who may have difficulty relaxing. This study examined the VSR of the aged calf MTUs with a low-level involuntary contractile response from the soleus (S), gastrocnemius (G) and tibialis anterior (TA) muscles. Calf MTUs of 29 men and women (65–90 years) were stretched to maximal dorsiflexion and held for 60 s while torque (Nm) and surface electromyograms (SEMG) were recorded. Subjects with normalized SEMG (% of maximal voluntary contraction SEMG) of the S, G or TA <1% were assigned to Group 1 (n=14) and subjects with SEMG of the S or G ≥1% and <10%, or in the TA ≥1% were assigned to Group 2 (n=15). Although the mean total percent torque decline for Group 1 (17.9%) and Group 2 (16.6%) did not differ statistically, multiple regression analysis within Group 2 indicated that the G and TA SEMG accounted for 58% of its variation (R ²=0.581). The G SEMG from 0–15 s accounted for 56% (r ²=0.563) of the variation in the percent decline normalized to total percent decline (100%). From 15–30 s Group 2 had less percent decline (2%) than Group 1 (14%) (P=0.043) and the S and G SEMG accounted for 67% of its variation (R ²=0.673). The results indicated that a low-level involuntary contractile response from the S, G and TA muscles may be related to the VSR process of aged calf MTUs. Measuring VSR within time intervals normalized to the total percent decline offers a new method to study VSR, which should be measured with SEMG in the muscles <1% MVC SEMG to ensure valid measurements.     

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     

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     

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     

The relationship between contraction and relaxation during fatiguing isokinetic shoulder flexions. An electromyographic study

Summary Knowledge of the strength, endurance and coordination of the shoulder muscles during dynamic contractions in healthy women would contribute to the understanding of symptoms in that part of the body in patients with myalgia. Twenty clinically healthy women performed single maximal forward shoulder flexions at four different angular velocities (0.57–3.14 rad·s⁻¹). The same subjects also took part in two endurance tests (at angular velocities of 0.57 and 2.09 rad·s⁻¹, respectively) consisting of 150 repeated maximal shoulder flexions. Electromyographic activity (EMG) was registered from four shoulder flexors using surface electrodes. Work was used as the mechanical variable. During the endurance tests subjects rated their perception of fatigue in the shoulder muscles. Work and the amplitude of the EMG signals decreased with angular velocity. The mean power frequency of the EMG was constant in the span of angular velocities investigated. During the endurance tests, work and the mean power frequency decreased during the initial 40–60 contractions followed by stable levels. The relative work level was higher at 2.09 than at 0.57 rad·s⁻¹. Greater relative increases of the signal amplitudes of EMG occurred at 2.09 than at 0.57 rad·s⁻¹. The EMG activity between the flexions (during the supposed passive extension) was higher at 2.09 than at 0.57 rad·s⁻¹. Such a high activity was associated with a low mechanical performance at 2.09 rad·s⁻¹. It is suggested that the initial sharp decreases in work and in mean power frequency reflect the fatiguing of the fast twitch motor units. Dynamic work consisting of continuous activity could predispose to muscle complaints.     

Effects of contract-relax stretching training on muscle performance in athletes

The effects of an 8-week unilateral contract-relax (CR) stretching training program (passive stretch after isometric contraction) on muscular performance were investigated in a group of 16 athletes. The flexibility, maximum torque and angular position as well as contraction work in movements of the knee joint were determined before training and after 4 and 8 weeks of training. The torque measurements were performed under isokinetic conditions, eccentrically at angular velocities of 60°?·?s^?1 and 120°?·?s^?1, isometrically at five different joint positions, and concentrically at angular velocities of 60, 120, 180 and 240°?·?s^?1 using an isokinetic dynamometer. A surface electromyogram (EMG) of the thigh muscles (quadriceps and hamstrings) was recorded simultaneously. As compared to untrained control limbs, significant improvements in active and passive flexibility (up to 6.3° in range of motion), maximum torque (up to 21.6%) and work (up to 12.9%) were observed, and these were especially pronounced under eccentric load conditions. A comparison between integrated EMG recordings during eccentric and concentric loads, as well as the interpretation of the training-induced changes in the EMG, suggest that muscular activity under eccentric loads may be impaired by mental processes.     

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).     

Contraction history affects the in vivo quadriceps torque-velocity relationship in humans

We hypothesized that the history of contraction would affect the in vivo quadriceps torque-velocity relationship. We examined the quadriceps torque-velocity relationship of the human knee extensors at the descending and ascending limb of the torque-position relationship by initiating the knee extension at a knee angle position of 1.39 rad (80°) or 0.87 rad (50°) over a 0.52 rad (30°) range of motion under conditions of constant or linearly increasing velocity. Maximal voluntary isometric knee extension torque (M₀) was measured at 1.87 rad, 0.87 rad, and 0.35 rad, and concentric torque was measured. The subjects carried out ten maximal knee extensions at ten distinct velocities, each velocity ranging between 0.52 rad·s^–1 to 5.24 rad·s^–1 in steps of 0.52 rad·s^–1. Peak concentric torque was measured and mean torque calculated from the respective torque-time curves. Peak or mean torque, computed from the individual torque-time curves, and velocity data were fitted to the Hill equation under the four experimental conditions and the curve parameters computed. The M₀ was similar at 0.87 rad and 1.39 rad, but it was significantly lower at 0.35 rad. In the low-velocity domain of the torque-velocity curve where a plateau normally occurs, peak torque was always lower than M₀. Peak and mean torque were significantly greater under linearly increasing velocity conditions and the 1.39 rad starting knee position. Mean torque but not peak torque data could be well fitted to the Hill equation and the two computations resulted in significantly different Hill curve parameters including the concavity ratio, peak power, and maximal angular velocity. We concluded that the history of contraction significantly modifies the in vivo torque-velocity relationship of the human quadriceps muscle. Muscle mechanics and not neural factors may have accounted for the inconsistencies in the human torque-velocity relationships reported previously. Electronic Publication