Quadriceps muscle function characteristics in severely obese and nonobese adolescents

The purpose of this cross-sectional study was to compare quadriceps muscle strength and fatigue between severely obese (body mass index 34 kg/m²) and nonobese adolescents. Maximal isokinetic torque and angle of peak torque as well as isometric torque at short (40° of knee flexion) and long (80° of knee flexion) muscle length were measured using an isokinetic dynamometer. Muscle fatigue was quantified as the percent torque loss during an isokinetic voluntary protocol and an electrical stimulation isometric protocol. Obese adolescents produced greater absolute isokinetic (+16%; P < 0.05) and isometric torque at short (+25%; P < 0.01) but not at long muscle length (P > 0.05) compared to their lean counterparts. The angle of peak torque was significantly lower in obese than in nonobese subjects (−11%; P < 0.05), i.e., obese produced their maximal strength at shorter muscle length. Isokinetic and isometric torque normalized to the fat-free mass were not significantly different between the two groups. No significant difference in voluntary and stimulated torque loss was observed between groups. Muscle strength per unit of fat-free mass and muscle fatigue were similar in the obese and nonobese adolescents tested in this study, therefore suggesting that obesity has little or no effect on quadriceps muscle function characteristics. On the other hand, it remains to be confirmed whether the observed quadriceps muscle length specificity contributes to the reduced functional capacity of obese adolescents during complex motor tasks involving deep knee flexion (squatting, kneeling).     

Effect of isometric and isokinetic muscle training on static strength and dynamic power

Summary This study investigated whether isokinetic strength training might induce changes in static and dynamic power already achieved as a result of isometric strength training. The subjects were twelve males. The isometric strength and dynamic power of elbow flexors were tested by means of an electric dynamometer and fly-wheel every two weeks. During the first 8 weeks all subjects trained the elbow flexors isometrically at four different positions of elbow joints. This training produced 27–36% gains in isometric strength and 34–46% in power. Thereafter the subjects were divided into two groups: the FG group who trained isokinetically at a fast velocity of 157·s^–1, and the SG group at slow velocity of 73·s^–1. After 6 weeks of training, the FG group produced a significant gain in power with light equivalent masses and the SG group did so with heavy equivalent masses. Neither group showed change in isometric strength.     

Human skeletal muscle fibre types and force: velocity properties

It has been reported that there is a relationship between power output and fibre type distribution in mixed muscle. The strength of this relationship is greater in the range of 3–8 rad · s^–1 during knee extension compared to slower or faster angular knee extensor speeds. A mathematical model of the force: velocity properties of muscle with various combinations of fast- and slow-twitch fibres may provide insight into why specific velocities may give better predictions of fibre type distribution. In this paper, a mathematical model of the force: velocity relationship for mixed muscle is presented. This model demonstrates that peak power and optimal velocity should be predictive of fibre distribution and that the greatest fibre type discrimination in human knee extensor muscles should occur with measurement of power output at an angular velocity just greater than 7 rad · s^–1. Measurements of torque: angular velocity relationships for knee extension on an isokinetic dynamometer and fibre type distribution in biopsies of vastus lateralis muscles were made on 31 subjects. Peak power and optimal velocity were determined in three ways: (1) direct measurement, (2) linear regression, and (3) fitting to the Hill equation. Estimation of peak power and optimal velocity using the Hill equation gave the best correlation with fibre type distribution (r > 0.5 for peak power or optimal velocity and percentage of fast-twitch fibres). The results of this study confirm that prediction of fibre type distribution is facilitated by measurement of peak power at optimal velocity and that fitting of the data to the Hill equation is a suitable method for evaluation of these parameters.     

Allometric scaling of strength measurements to body size

For comparative purposes, normalisation of strength measures to body size using allometric scaling is recommended. A wide range of scaling exponents have been suggested, typically utilising body mass, although a comprehensive evaluation of different body size variables has not been documented. Differences between force (F) and torque (T) measurements of strength, and the velocity of measurement might also explain some of the variability in the scaling exponents proposed. Knee extensor strength of 86 young men was assessed with measurement of torque at four velocities (0–4.19 rad s⁻¹) and force measured isometrically. Body size variables included body mass, height and fat-free mass. Scaling exponents for torque were consistently higher than for force, but the velocity of torque measurement had no influence. As the confounding effects of fat mass were restricted, scaling exponents and the strength of the power-function relationships progressively increased. Fat-free mass determined a surprisingly high proportion of the variance in measured strength (F, 31%; T, 52–58%). Absolute force and torque measurements, and even torque normalised for body mass, were significantly influenced by height, although strength measures normalised to fat-free mass were not. To normalise strength measurements to body mass, for relatively homogenous lean populations (body fat <20%), exponents of 0.66 (F) and 1.0 (T) are appropriate. For more adipose populations (body fat >20%) lower body mass exponents appear more suitable (F, 0.45; T, 0.68). Nevertheless, fat-free mass is the recommended index for scaling strength to body size, and higher exponents (F, 0.76; T, 1.12) are advocated in this case.     

Relationship between myoelectric and mechanical manifestations of fatigue in the quadriceps femoris muscle group

Fatigue is commonly defined as “the failure to maintain the required force”. As such, it may be argued that the use of electromyographic (EMG) power spectral statistics to monitor muscle fatigue is inappropriate, because, during the maintenance of a submaximal force of contraction, EMG changes are readily observable in the absence of any decline in the muscle's mechanical output. However, it is possible that the EMG changes reflect the changing metabolic status of the muscle and hence its inability to generate its normal maximal force. The present study sought to examine whether the decline in EMG median frequency, which occurs during the maintenance of a submaximal force, is correlated with a reduction in the muscle's maximum force-generating capacity. The maximum voluntary contraction (MVC) of the knee extensors in ten young, healthy subjects was determined. On five separate occasions, randomly assigned forces of 20, 30, 40, 50 and 60% MVC were held to the limit of endurance. At intervals throughout the sustained contractions, subjects were required to rapidly generate an MVC for 1–2 s, then return to the fixed submaxial target force. Surface EMG signals were recorded throughout the contractions from the rectus lemons and vastus lateralis muscles, from which the power spectrum median frequency was calculated. Regression analysis revealed highly significant relationships between the rate of decline in MF and the rate of decline in MVC, and between each of these parameters and endurance time to fatigue (P = 0.0001, in each case). It is concluded that the decline in MF can be used to monitor fatigue, where fatigue is defined as the inability to generate the maximum force that can be produced by the muscle in its fresh state.     

Electrically evoked isometric and isokinetic properties of the triceps surae in young male subjects

Summary The relationship between electrically evoked isometric and isokinetic properties of the triceps surae have been studied in 11 healthy male subjects. The results showed that the time to peak tension (TPT) and half relaxation time (1/2 RT) of the maximal twitch were 110±11 ms and 82±11 ms respectively, and the peak rates of rise of contraction (P ₅₀, P ₂₀₀) and relaxation (P _R50, P _R200) at 50 and 200 Hz were 0.36±0.07, 0.48±0.08 and 1.27±0.33, 1.25±0.27% Po ms^–1 respectively. The decline in force during a fatigue test was significantly (P<0.02) associated with the decrease in maximal relaxation rate (r=0.79). The TPT was significantly (P<0.05) and inversely related to P ₅₀ and P ₂₀₀. The mean angle specific torque-velocity relationship for the 11 subjects was adequately described by the empirical exponential equation of the form: V=16.5 (e ^–p/30.8–e ^–84.3/30.8) where V=velocity (rads s^–1) P=torque (Nm). The only significant association found between the isometric and isokinetic properties of the muscle was between P _R200 and the torque expressed at a given velocity of 4 rads s^–1. This lack of association between the two variables is difficult to explain with certainty but it is suggested that it may be due to the differential effects of Ca²⁺ release and uptake and cross-bridge turnover rate in the two situations.     

Isometric dynamometry,dependent on knee angle,is a suitable alternative to isokinetic dynamometry when evaluating quadriceps strength symmetry in patients following anterior cruciate ligament reconstruction

BackgroundKnee extensor strength deficits increase re-injury risk following anterior cruciate ligament reconstruction (ACLR). This study investigated whether isometric strength testing methods are a suitable alternative to isokinetic assessment for identifying knee extensor strength asymmetry.MethodsThis study recruited 22 patients at 9–12 months after ACLR and 22 healthy controls. The single hop for distance (SHD) and knee extensor strength via isokinetic (60°/s and 120°/s) and isometric (positions of 90°, 60° and 30° of flexion, from full knee extension) methods were assessed. Absolute scores (normalized to body weight) and limb symmetry indices (LSIs) were calculated, with t-tests employed for statistical comparisons.ResultsThe SHD LSI was significantly higher (p < 0.01) than both isokinetic speeds and the 30° isometric position. No significant LSI differences (p > 0.01) existed within isokinetic or isometric test conditions. In ACLR patients, only the 60°/s isokinetic condition was significantly lower (p = 0.005) than the 60° isometric condition. When normalized to body weight, the operated limb in ACLR patients was significantly weaker than the non-operated limb during peak isokinetic strength testing at 60°/s (p = 0.001) and 120°/s (p = 0.010), as well as isometric testing at 30° (p = 0.009). Compared with controls, ACLR patients demonstrated significantly lower (p < 0.01) mean LSIs across most measures.ConclusionsAssessment of knee extensor strength via isometric methods appears suitable in the absence of isokinetic testing equipment, though consideration of test angle (30° and 90° knee angles better detect asymmetries similar to isokinetic testing) is important.     

Muscle strength and functional capacity in 78–81-year-old men and women

Summary Muscle strength was measured in 23 men and 29 women regarded as representative of the healthy urban population of about 80 years of age. Isometric and isokinetic strengths of right knee-extension, plantar, and dorsal flexion, and the isometric strength of some upper extremity functions were measured. For knee-extension there was a decrease in strength of about 30% from the values in a population study of 70-year-old subjects (Aniansson et al. 1980). Muscle strength was significantly lower in women than in men in all muscle groups except for plantar and dorsal flexion of the foot. Body cell mass was reduced compared to younger age groups by about the same extent as muscle strength, and correlated to knee-extension and hand grip strengths.Functional examinations such as step tests and walking tests were performed. A correlation between the results of these tests and muscle strength was not found except for knee-extension at 60 degrees/s in men. The rather low speed in comfortable walking (men 1.0 m·s^–1, women 0.9 m·s^–1) and the reduced capacity for step climbing diminish the possibility of using public transport and pedestrian street intersections in relatively healthy old people.     

Comparison of muscle cross-sectional area and strength between untrained women and men

The cross-sectional areas (CSA) of fat, muscle and bone tissues of the limb as well as maximal voluntary isokinetic strength were measured in untrained men (n=27) and women (n=26) aged 18–25 years. Anatomical CSA of the three tissues were determined by ultrasound on the upper arm and thigh. The isokinetic strength of the elbow and knee extensor and flexor muscles were measured by an isokinetic dynamometer (Cybex 11) at 1.05 rad · s^–1. The women had significantly (P<0.001) larger fat CSA and smaller bone and muscle CSA than the men in both the upper arm and thigh. Among tissue CSA, the largest difference between the women and men was found in fat CSA regardless of the measurement sites. The sex differences in bone and muscle CSA were found largely in the upper arm compared to the thigh, even when expressed per unit second power of the limb length. Regression analyses of the data for respective samples for the men and women showed significant correlations (r=0.411–0.707, P < 0.05–P < 0.001) between CSA and strength in all muscle groups except for the elbow extensors of the men (r=0.328, P>0.05) and the elbow flexors of the women (r=0.388, P>0.05). No significant difference between sexes was observed when strength was expressed per unit of muscle CSA (F · CSA^–1) for the elbow flexors and extensors. However, the men showed significantly higher F · CSA^–1 than the women for the knee flexors and extensors (P < 0.001). These results would indicate that, although the difference between sexes in muscle CSA is smaller in the thigh than in the upper arm, differences in the ability to develop dynamic strength proportional to the CSA appeared mainly in the thigh muscles compared to the upper muscles.     

The relationship between muscle myosin ATP-ase activity and isometric endurance in untrained male subjects

Summary The influence of variations in muscle fibre composition on isometric endurance capacity was measured in 23 young healthy untrained male volunteers. After determination of the maximum voluntary force of contraction (MVC), subjects sustained to fatigue contractions at forces of 80%, 50% and 20% of MVC with a 5-min rest between each. A needle biopsy was obtained from m. vastus lateralis and used for histochemical determination of fibre composition based on myosin ATP-ase activity, and fibre are a based on succinate dehydrogenase (SDH) activity. Endurance times were 21±9 s (mean±SD) at 80% of MVC, 56±17 s at 50% of MVC and 203±89 s at 20% of MVC. A wide range of muscle fibre compositions was observed with Type I fibres accounting for 48.0±10.5% of the total, corresponding to 45.0±11.5% of the total muscle area. Muscle fibre composition, whether expressed as the proportions of the different fibre types present, or as the fraction of total muscle cross-sectional area occupied by each fibre type was not correlated with isometric endurance capacity at any of the three forces studied. Endurance time was also unrelated to MVC. In contrast to the results of previous studies where trained subjects were used, or where different muscle groups were compared, these results suggest that isometric endurance is not influenced by muscle fibre composition.     

The bilateral leg strength deficit is present in old, young and adolescent females during isokinetic knee extension and flexion

The bilateral limb deficit (BLD) describes the difference in maximal or near maximal force generating capacity of muscles when they are contracted alone or in combination with the contralateral muscles. A deficit occurs when the summed unilateral force is greater than the bilateral force. This study examined the presence of the BLD during isokinetic knee extensions and flexions in a group of adolescent females (n = 8, mean of 15 ± 1 years) and compared with previously reported data by this researcher of adult and older females. Data were collected from subjects during slow (45 deg/s) isokinetic knee extensions and flexions and it was found that a BLD exists during both extension and flexion regardless of age. Furthermore, this study is the first to examine the presence of the deficit in an adolescent population. Myoelectric signal (MES) data showed that there is no difference between bilateral and unilateral isokinetic knee extensions and flexions regardless of age group.     

Activity-induced GLP-1 release in lean and obese subjects

The aim of the study was to determine whether physical activity stimulates GLP-1 release on the short-term in normal weight and in obese subjects compared to rest and, furthermore, whether modest weight loss affects GLP-1 release or sensitivity in the obese. Normal weight (n=28; 12 males, 16 females; BMI 22.9+/-1.4; age 35+/-12.7), as well as obese subjects (n=27; 21 males, 6 females; BMI 30.9+/-2.7; age 47.1+/-11.86) were tested in a resting and a physical activity condition. Obese subjects were matched over two groups for a weight loss period of 3 months. After weight loss, the tests were repeated. The area under the curve (AUC pmol/lxmin) for GLP-1 concentrations was significantly increased in the physical activity condition compared to rest in lean subjects (P=0.05) as well as in the obese subjects after weight loss (P<0.05), but not in the obese subjects before weight loss. Physical activity-stimulated GLP-1 release in lean and obese subjects after a weight loss period supports the idea of a neuroendocrine loop in addition to distal-intestinal stimulation of GLP-1 release. Modest weight loss might be effective for increasing GLP-1 sensitivity to acute stimulation.     

Maximal peak torque as a predictor of angle-specific torques of hamstring and quadriceps muscles in man

Summary This study assessed the relationship between the isokinetic peak torque (PT) (speed of movement 1.05 and 3.14 rads^–1) and the angle-specific torques (ASTs) at 0.26 and 1.31 rad of knee flexion in multiple contractions of the quadriceps and hamstrings in 70 individuals with a chronic anterior cruciate ligament (ACL) insufficiency and 78 individuals with a chronic medial collateral ligament (MCL) insufficiency in one knee. At every test speed, the Pearson product moment correlation coefficients (r) between the PT and ASTs were highly significant (P<0.001) in the uninjured knees (r=0.61–0.93) as well as in the knees with ACL (r=0.61–0.87) and MCL (r=0.74–0.91) insufficiency. In addition, in both groups the majority of the correlation coefficients exceeded 0.80, which is generally regarded as the threshold for the relationship to be considered clinically significant. Furthermore, using regression analysis, both extremities showed completely non-systematic distribution of the residuals. It is concluded that in healthy knees or knees with ACL or MCL insufficiency, the predictability of ASTs from PT was good, and, therefore, that AST analyses may offer little additional information about thigh muscle function to that obtained from a simpler and more commonly used measurement, the PT analysis.     

Bilateral isokinetic training reduces the bilateral leg strength deficit for both old and young adults

The bilateral limb deficit (BLD) describes the difference in maximal or near-maximal force generating capacity of muscles when they are contracted alone and in combination with the contralateral muscles. This study examined the effects of a 6-week (three times per week) bilateral leg strength training programme on BLD in younger and older adults. Data were collected from 33 subjects during slow (45°/s) isokinetic knee extensions and flexions before and after the training programme. After training, the BLD was reduced for extension (73.3–86.9%; P<0.001) but not for flexion (67.5–71.2%; P=0.13) regardless of age and gender. This study suggests that difficulty in recruiting all muscle units during a task involving bilateral activation can be improved by training, although such an effect appears to depend on the muscle group appreciated.     

Surface EMG and mechanomyogram disclose isokinetic training effects on quadriceps muscle in elderly people

Maximum voluntary contraction (MVC) and cross-sectional area (CSA) of fast and slow twitch fibers are reduced in the lower limb muscles of elderly subjects. Isokinetic training at medium and high velocities has been widely used to improve muscle performance and force in young as well as elderly subjects. EMG and mechanomyogram (MMG) are compound signals in which the electrical and mechanical activities of recruited motor units (MUs) are summated. The aim of the present study was to verify the hypothesis that isokinetic training in the elderly induces changes in EMG and MMG parameters, compatible with a functional retrieval of fast twitch fiber MUs. In ten sedentary males (62–78 years), the surface EMG and MMG were recorded from the vastus lateralis muscle during isometric contractions at 20, 40, 60, 80 and 100% of the MVC, before and after 12 weeks of isokinetic training (six series of ten repetitions, each at an angular velocity of 2.09 rad s^–1 and 4.19 rad s^–1, two times a week). With training: (a) MVC and CSA increased by about 35±5% and 8±1%, respectively (P<0.05); (b) the ratio MVC/CSA increased significantly in all subjects by 25±5%; (c) the EMG root mean square and MMG spectral mean frequency increased significantly at the highest workloads. In conclusion, our data indicate that isokinetic training in the elderly improved muscle size and performance significantly. The EMG and MMG changes suggest that these results may be due to a retrieval of the fast twitch fiber MUs, contributing to muscle action.     

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     

Mechanical and electrical correlates of isometric muscle fatigue in skeletal muscle in the cat

The amplitude and duration of motor unit action potentials, the rise time, peak tension, and half relaxation time of an isometric twitch, and forcevelocity relationship, and tetanic tension were measured at the beginning (fresh muscle) and at the end of a fatiguing isometric contraction at a tension of either 40 or 70% of the initial strength in the soleus (a slow twitch muscle) and the medial gastrocnemius (a fast twitch muscle) of the cat. These same parameters were also measured at set intervals following these contractions to assess their rate of recovery to pre-exercise values. At The end of a fatiguing contraction examined,V _mx, twitch tension and tetanic tension, were all reduced while there was a prolonged twitch duration and duration of the motor unit action potential for both types of muscle. The height of the motor unit action potential was only marginally effected by muscle fatigue. Following the fatiguing contraction, the endurance required several hours to recover in the medial gastrocnemius muscle but recovered fully within 15 min after either tension in the soleus muscle. Tetanic tension and twitch tension both required less than 10 min for full recovery in the medial gastrocnemius muscle but recovered fully to the pre-exercise values within 3 min following fatiguing isometric contractions in the soleus muscles.V _mx, and the height and duration of the motor unit action potential both recovered within 1 min following the end of the exercise.     

Skeletal muscle growth in lean and obese Zucker rats

Obese Zucker rats exhibit marked hyperphagia when compared to lean littermates but deposit a smaller percentage of total dietary energy as body protein. This study was designed to determine the roles of skeletal muscle protein synthesis, protein degradation, RNA, or DNA in producing the lower muscle mass of obese rats. At 44 days, 3 hindlimb muscles, the extensor digitorum longus (EDL), the gastrocnemius and the plantaris were significantly smaller in the obese animals. At 72 days, the differences in weights of these muscles were more pronounced. Protein synthesis and degradation were determined in the soleus at 44 days of age using an in vitro whole muscle incubation technique. Protein synthesis rate was significantly decreased in the obese animals. These changes were accompanied by reductions in both RNA and DNA levels. Significant changes in nucleic acid levels were observed in both the red and white portions of the gastrocnemius muscle. These changes in the anabolic process of protein accretion appear to be sufficient to account for the reduced muscle mass in the obese Zucker rat.     

Regression-based indices of fatigue in paraplegics'' electrically stimulated quadriceps

We studied muscle fatigue in 19 paraplegic patients during an isometric sustained contraction. Both quadriceps (n = 38) were maximally stimulated for 126 s. We defined a nonlinear equation with four coefficients to fit the data and extract fatigue indices carrying information on the amount (asymptotic values, torque coordinate at the inflection point, total decrement from initial value) and the rate of torque decrease (maximum slope, time constant, time coordinate at the inflection point, time at which the torque reached 50% of initial value). Most of the indices reported in the literature provide information limited to the amount of torque decrease. Three of the coefficients we defined could be graphically estimated. The goodness of fit was very high (r² = 0.993 ± 0.001) but none of the indices provided information on both the amount and the rate of decrease. Major differences were encountered among the patients. Therefore, some of the computed indices varied to a great extent. We studied the curves exhibiting the same fatigue indices and defined a simple general fatigue index able to distinguish from muscles having different resistance to fatigue (asymptotic value × the time constant). This index is carrying both types of information and is expressed in relative torque × second (range from 1.81 to 22.82).     

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.