Properties Of Human Skeletal Muscle Research Articles

Higher Myonuclei Density in Muscle Fibers Persists Among Former Users of Anabolic Androgenic Steroids.

No information exists on the long-lasting effects of supraphysiological anabolic androgenic steroids (AASs) usage on the myocellular properties of human skeletal muscle in previous AAS users. We hypothesized that former AAS users would demonstrate smaller myonuclei domains (ie, higher myonuclei density) than matched controls. A community-based cross-sectional study in men aged 18-50 years engaged in recreational strength training. Muscle biopsies were obtained from the m. vastus lateralis. Immunofluorescence analyses were performed to quantify myonuclei density and myofiber size. Twenty-five males were included: 8 current and 7 previous AAS users and 10 controls. Median (25th-75th percentiles) accumulated duration of AAS use was 174 (101-206) and 140 (24-260) weeks in current and former AAS users, respectively (P = .482). Geometric mean (95% CI) elapsed duration since AAS cessation was 4.0 (1.2; 12.7) years among former AAS users. Type II muscle fibers in former AAS users displayed higher myonuclei density and DNA to cytoplasm ratio than controls, corresponding to smaller myonuclei domains (P = .013). Longer accumulated AAS use (weeks, log2) was associated with smaller myonuclei domains in previous AAS users: beta-coefficient (95% CI) -94 (-169; -18), P = .024. Type I fibers in current AAS users exhibited a higher amount of satellite cells per myofiber (P = .031) than controls. Muscle fibers in former AAS users demonstrated persistently higher myonuclei density and DNA to cytoplasm ratio 4 years after AAS cessation suggestive of enhanced retraining capacity.

Design and Assist-as-Needed Control of Flexible Elbow Exoskeleton Actuated by Nonlinear Series Elastic Cable Driven Mechanism

Exoskeletons can assist the daily life activities of the elderly with weakened muscle strength, but traditional rigid exoskeletons bring parasitic torque to the human joints and easily disturbs the natural movement of the wearer’s upper limbs. Flexible exoskeletons have more natural human-machine interaction, lower weight and cost, and have great application potential. Applying assist force according to the patient’s needs can give full play to the wearer’s remaining muscle strength, which is more conducive to muscle strength training and motor function recovery. In this paper, a design scheme of an elbow exoskeleton driven by flexible antagonistic cable actuators is proposed. The cable actuator is driven by a nonlinear series elastic mechanism, in which the elastic elements simulate the passive elastic properties of human skeletal muscle. Based on an improved elbow musculoskeletal model, the assist torque of exoskeleton is predicted. An assist-as-needed (AAN) control algorithm is proposed for the exoskeleton and experiments are carried out. The experimental results on the experimental platform show that the root mean square error between the predicted assist torque and the actual assist torque is 0.00226 Nm. The wearing experimental results also show that the AAN control method designed in this paper can reduce the activation of biceps brachii effectively when the exoskeleton assist level increases.

Quantitative Estimation of Passive Elastic Properties of Individual Skeletal Muscle in Vivo Using Normalized Elastic Modulus-Length Curve.

Precise characterization of the passive elastic properties of human skeletal muscle could provide valuable information for functional assessment and medical diagnosis. Using normalized elastic modulus-length curve based on a piecewise exponential model, a non-invasive ultrasonography (US) method was developed for estimating three physiologically meaningful parameters, termed as passive-elastic coefficient k, slack length l0 and slack elastic modulus G0, to quantify the passive elastic properties of human gastrocnemius (GM) muscle, and correlate different findings with healthy and atrophic subjects. Dynamic B-mode US and shear wave elastography (SWE) videos of right GM muscles were performed in 32 healthy subjects and 18 atrophic patients with their ankle angles from 40° plantarflexion to 30° dorsiflexion by an isokinetic dynamometer. During muscle passive stretching induced by ankle rotation, consecutive muscle lengths were measured by automatically tracking the distal muscle-tendon junction positions in B-mode imaging frames using Hough transform and an active contour method. The corresponding elastic moduli were obtained using SWE imaging frames. The Gauss-Newton algorithm was used to estimate the parameters k, l0 and G0 from normalized elastic modulus-length curve. In-vivo experimental results showed that the measured muscle elastic modulus-length data fitted well to the proposed model with three estimated parameters for all subjects (0.942 < R2 < 0.997). The passive-elastic coefficient k was significantly higher for atrophic subjects compared with normal subjects (105.5 ± 45.3 versus 48.4 ± 16.0, p < 0.001). The proposed parameters allow further characterization of muscle essential mechanical properties and have a potential to become effective indexes for muscular disease diagnosis.

Age-related changes in human single muscle fibre passive elastic properties are sarcomere length dependent.

Studies have revealed an age-related decrease in the ability to produce force as well as an increase in the contractile stiffness and passive stress of single muscle fibres. However, further insight into age-related changes to the passive properties of human skeletal muscles is needed. The aim of this study was to characterize single muscle fibre passive properties from young and old males across a physiologic range of sarcomere lengths (SLs). Ten young ([YM]: mean age: 25.4years) and ten old ([OM]: mean age: 68.9years) males participated. Vastus lateralis muscles were biopsied and 182 fibres were tested and analyzed (90 YM and 92 OM) using a cumulative stretch-relaxation protocol. Passive mechanical characteristics of each fibre were obtained by fitting the integral of the logistic function to experimental stress-SL data. Muscle fibres from older individuals had significantly greater passive elastic moduli at short SL (1.9-2.65μm) compared with young (p<0.05). This resulted in a significantly larger passive stress for SLs between 2.1μm and 3.55μm for older individuals when compared with young (p<0.05). These results provide novel insight into the length dependency of changes in single muscle fibre passive mechanical properties with age.

Mechanisms of in vivo muscle fatigue in humans: investigating age‐related fatigue resistance with a computational model

Muscle fatigue can be defined as the transient decrease in maximal force that occurs in response to muscle use. Fatigue develops because of a complex set of changes within the neuromuscular system that are difficult to evaluate simultaneously in humans. The skeletal muscle of older adults fatigues less than that of young adults during static contractions. The potential sources of this difference are multiple and intertwined. To evaluate the individual mechanisms of fatigue, we developed an integrative computational model based on neural, biochemical, morphological and physiological properties of human skeletal muscle. Our results indicate first that the model provides accurate predictions of fatigue and second that the age-related resistance to fatigue is due largely to a lower reliance on glycolytic metabolism during contraction. This model should prove useful for generating hypotheses for future experimental studies into the mechanisms of muscle fatigue. During repeated or sustained muscle activation, force-generating capacity becomes limited in a process referred to as fatigue. Multiple factors, including motor unit activation patterns, muscle fibre contractile properties and bioenergetic function, can impact force-generating capacity and thus the potential to resist fatigue. Given that neuromuscular fatigue depends on interrelated factors, quantifying their independent effects on force-generating capacity is not possible in vivo. Computational models can provide insight into complex systems in which multiple inputs determine discrete outputs. However, few computational models to date have investigated neuromuscular fatigue by incorporating the multiple levels of neuromuscular function known to impact human in vivo function. To address this limitation, we present a computational model that predicts neural activation, biomechanical forces, intracellular metabolic perturbations and, ultimately, fatigue during repeated isometric contractions. This model was compared with metabolic and contractile responses to repeated activation using values reported in the literature. Once validated in this way, the model was modified to reflect age-related changes in neuromuscular function. Comparisons between initial and age-modified simulations indicated that the age-modified model predicted less fatigue during repeated isometric contractions, consistent with reports in the literature. Together, our simulations suggest that reduced glycolytic flux is the greatest contributor to the phenomenon of age-related fatigue resistance. In contrast, oxidative resynthesis of phosphocreatine between intermittent contractions and inherent buffering capacity had minimal impact on predicted fatigue during isometric contractions. The insights gained from these simulations cannot be achieved through traditional in vivo or in vitro experimentation alone.

Metallic muscles and beyond: nanofoams at work

In this contribution for the Golden Jubilee issue commemorating the 50th anniversary of the Journal of Materials Science, we will discuss the challenges and opportunities of nanoporous metals and their composites as novel energy conversion materials. In particular, we will concentrate on electrical-to-mechanical energy conversion using nanoporous metal-polymer composite materials. A materials system that mimic the properties of human skeletal muscles upon an outside stimulus is coined an ‘artificial muscle.’ In contrast to piezoceramics, nanoporous metallic materials offer a unique combination of low operating voltages, relatively large strain amplitudes, high stiffness, and strength. Here we will discuss smart materials where large macroscopic strain amplitudes up to 10 % and strain-rates up to 10−2 s−1 can be achieved in nanoporous metal/polymer composite. These strain amplitudes and strain-rates are roughly 2 and 5 orders of magnitude larger than those achieved in common actuator materials, respectively. Continuing on the theme of energy-related applications, in the summary and outlook, we discuss two recent developments toward the integration of nanoporous metals into energy conversion and storage systems. We specifically focus on the exciting potential of nanoporous metals as anodes for high-performance water electrolyzers and in next-generation lithium-ion batteries.

Effects of salbutamol on the contractile properties of human skeletal muscle before and after fatigue

The study examined the effects of an oral acute administration of the β2-agonist salbutamol (Sal) (6 mg) vs. placebo on muscle strength and fatigability in 12 non-asthmatic recreational male athletes in a randomized double-blind protocol. Contractile properties of the right quadriceps muscle were measured during electrical stimulations, i.e. twitch, 1-s pulse trains at 20 (P(20) ) and 80 Hz (P(80) ) and during maximal voluntary isometric contraction (MVIC) before (PRE) and after (POST) a fatigue-producing protocol set by an electromyostimulation (30 contractions, frequency: 75 Hz, on-off ratio: 6.25-20s). In addition, the level of muscle voluntary activation was measured. In PRE and POST conditions, the peak torque (PT) of twitch, P(80) and MVIC were not modified by the treatment. The PT in POST P(20) was slightly, although not significantly, less affected by fatigue in Sal compared with placebo condition. Moreover, twitch half-relaxation time at PRE was smaller under Sal than under placebo (P < 0.05). No significant changes in the degree of voluntary activation were observed with Sal treatment in PRE or POST condition. Although these findings did not exclude completely an effect of Sal on peripheral factors of human skeletal muscle, oral acute administration of the β2-agonist Sal seems to be without any relevant ergogenic effect on muscle contractility and fatigability in non-asthmatic recreational male athletes.

Neuromuscular electrical stimulation training induces atypical adaptations of the human skeletal muscle phenotype: a functional and proteomic analysis

The aim of the present study was to define the chronic effects of neuromuscular electrical stimulation (NMES) on the neuromuscular properties of human skeletal muscle. Eight young healthy male subjects were subjected to 25 sessions of isometric NMES of the quadriceps muscle over an 8-wk period. Needle biopsies were taken from the vastus lateralis muscle before and after training. The training status, myosin heavy chain (MHC) isoform distribution, and global protein pattern, as assessed by proteomic analysis, widely varied among subjects at baseline and prompted the identification of two subgroups: an "active" (ACT) group, which performed regular exercise and had a slower MHC profile, and a sedentary (SED) group, which did not perform any exercise and had a faster MHC profile. Maximum voluntary force and neural activation significantly increased after NMES in both groups (+∼30% and +∼10%, respectively). Both type 1 and 2 fibers showed significant muscle hypertrophy. After NMES, both groups showed a significant shift from MHC-2X toward MHC-2A and MHC-1, i.e., a fast-to-slow transition. Proteomic maps showing ∼500 spots were obtained before and after training in both groups. Differentially expressed proteins were identified and grouped into functional categories. The most relevant changes regarded 1) myofibrillar proteins, whose changes were consistent with a fast-to-slow phenotype shift and with a strengthening of the cytoskeleton; 2) energy production systems, whose changes indicated a glycolytic-to-oxidative shift in the metabolic profile; and 3) antioxidant defense systems, whose changes indicated an enhancement of intracellular defenses against reactive oxygen species. The adaptations in the protein pattern of the ACT and SED groups were different but were, in both groups, typical of both resistance (i.e., strength gains and hypertrophy) and endurance (i.e., a fast-to-slow shift in MHC and metabolic profile) training. These training-induced adaptations can be ascribed to the peculiar motor unit recruitment pattern associated with NMES.

Viscoelasticity-based MR elastography of skeletal muscle

An in vivo multifrequency magnetic resonance elastography (MRE) protocol was developed for studying the viscoelastic properties of human skeletal muscle in different states of contraction. Low-frequency shear vibrations in the range of 25–62.5 Hz were synchronously induced into the femoral muscles of seven volunteers and measured in a cross-sectional view by encoding the fast-transverse shear wave component parallel to the muscle fibers. The so-called springpot model was used for deriving two viscoelastic constants, μ and α, from the dispersion functions of the complex shear modulus in relaxed and in loaded muscle. Representing the shear elasticity parallel to the muscle fibers, μ increased in all volunteers upon contraction from 2.68 ± 0.23 kPa to 3.87 ± 0.50 kPa. Also α varied with load, indicating a change in the geometry of the mechanical network of muscle from relaxation (α = 0.253 ± 0.009) to contraction (α = 0.270 ± 0.009). These results provide a reference for a future assessment of muscular dysfunction using rheological parameters.

Passive Properties of Single Skeletal Muscle Fibers are Altered in Heart Failure Patients

We previously demonstrated that myosin heavy chain (MHC) content is decreased in skeletal muscle fibers from heart failure patients, thereby altering Ca2+-activated (pCa 4.5) contractile properties. In this study, we examined chemically-skinned, single human skeletal muscle fibers under relaxed conditions (low [Ca2+], pCa 8) using small amplitude sinusoidal analysis to determine whether the loss of MHC content affects their viscoelastic properties. We obtained vastus lateralis (quadriceps) muscle from needle biopsies of 9 patients and 5 sedentary controls. Surprisingly, Type I (slow twitch) and Type IIA (fast twitch) fibers produced slight but significant positive oscillatory work under relaxed conditions at 25°C, indicating the presence of cross-bridge cycling. This positive oscillatory work persisted when [Ca2+]levels were lowered even further (to pCa 9). Addition of 40 mM 2,3-butanedione monoxime (BDM) or 100 μM blebbistatin, inhibitors of skeletal muscle myosin ATPase, to a subset of Type I and IIA fibers eliminated their positive work output and reduced their high frequency elastic and viscous moduli. Type I fibers from heart failure patients had lower isometric tensions as well as lower elastic and viscous moduli compared to controls; whereas, Type IIA fibers from patients had tension values similar to controls, while the frequency of maximum work output (an indicator of myosin kinetics) was greater. Taken together, these results demonstrate that cross-bridges contribute to the viscoelastic properties of human skeletal muscle in the ‘relaxed’ state (low intracellular [Ca2+]) and that cycling myosin heads may contribute to resting muscle tone. As under Ca2+-activated conditions, heart failure affects relaxed muscle properties differentially depending upon fiber type. Decreased resting muscle tone in heart failure patients may decrease postural stability and contribute to a reduced ability to perform activities of daily living.

Relationship of physical function to vastus lateralis capillary density and metabolic enzyme activity in elderly men and women

There are no data showing whether or not age-related declines in physical function are related to in vitro properties of human skeletal muscle. The purpose of this study was to determine whether physical function is independently associated with histologic and metabolic properties of skeletal muscle in elderly adults. The study was a cross-sectional observational study of 39 sedentary, older (60-85 yrs) men and women. A needle biopsy of the vastus lateralis for assessment of muscle fiber type, fiber area, capillary density and citrate synthase and aldolase activities was performed. Physical function tests included the Short Physical Performance Battery (balance, walking speed, and chair rise time), as well as self-reported disability. Total fiber area (R=-0.41, p=0.02), number of Type II fibers (R=-0.33, p=0.05), and aldolase activity (R=-0.54, p=0.01) were inversely related to age. Persons who reported greater difficulty with daily activities had lower capillary density (R=-0.51, p=0.03) and lower citrate synthase activity (R=-0.66, p=0.03). Walking speed was directly related to fiber area (R=0.40, p=0.02), capillary density (R=0.39, p=0.03), citrate synthase (R=0.45, p=0.03) and aldolase (R=0.55, p<0.01) activities, even after adjustment for age, BMI and disease status. In older adults, skeletal muscle capillary density and metabolic enzymatic activity are independent predictors of lower extremity physical function.

Quantitative sonoelastography for the in vivo assessment of skeletal muscle viscoelasticity

A novel quantitative sonoelastography technique for assessing the viscoelastic properties of skeletal muscle tissue was developed. Slowly propagating shear wave interference patterns (termed crawling waves) were generated using a two-source configuration vibrating normal to the surface. Theoretical models predict crawling wave displacement fields, which were validated through phantom studies. In experiments, a viscoelastic model was fit to dispersive shear wave speed sonoelastographic data using nonlinear least-squares techniques to determine frequency-independent shear modulus and viscosity estimates. Shear modulus estimates derived using the viscoelastic model were in agreement with that obtained by mechanical testing on phantom samples. Preliminary sonoelastographic data acquired in healthy human skeletal muscles confirm that high-quality quantitative elasticity data can be acquired in vivo. Studies on relaxed muscle indicate discernible differences in both shear modulus and viscosity estimates between different skeletal muscle groups. Investigations into the dynamic viscoelastic properties of (healthy) human skeletal muscles revealed that voluntarily contracted muscles exhibit considerable increases in both shear modulus and viscosity estimates as compared to the relaxed state. Overall, preliminary results are encouraging and quantitative sonoelastography may prove clinically feasible for in vivo characterization of the dynamic viscoelastic properties of human skeletal muscle.

Age and Exercise Effects on Myosin Heavy Chain Isoform mRNA Expression in Human Muscle

0389 Aging is associated with changes in the contractile and metabolic properties of human skeletal muscle. Among these changes is a shift in fiber type towards a predominance of Type I fibers. It was reported that mRNA levels of myosin heavy chain (MHC) isoforms, the determinant of contractile phenotype, are altered with age in a manner consistent with fiber type results, although two studies found no changes in MHC mRNA expression. Further, a predominance of Type I fibers has been observed in highly aerobically-trained athletes but the effect of aerobic training on MHC mRNA expression and the potential interaction with age remains to be determined. PURPOSE: To determine the effect of aging and a moderate intensity aerobic exercise training program on mRNA levels of MHC isoforms in human muscle. METHODS: Previously sedentary, healthy men and women aged 22–88 y were studied before and after 16 weeks of bicycle training (30–45 min at 70–80% peak heart rate, 3–4 days/week, N = 62) or control (stretching, N = 37) activity. Muscle biopsies of the vastus lateralis were analyzed for MHC mRNA isoforms I, IIA and IIX using real-time PCR. RESULTS: At baseline MHC-I mRNA was unchanged with age, while levels of IIA (r = −0.45) and IIX (r = −0.34) declined by 14% and 10% per decade, respectively (p < 0.001). After training, the mRNA levels of MHC-I and IIA increased 61 and 99%, respectively, while IIX decreased 50% (all p < 0.001). The increase in MHC-I mRNA was positively associated with age (r = 0.30, p < 0.01). There was no interaction between age and exercise responses for MHC-IIA or IIX, and MHC mRNA did not change in the control group. CONCLUSION: This study demonstrates that MHC isoform expression changes with aging consistent with reported changes in fiber type. Despite baseline differences, older muscle retains the ability to alter expression of MHC isoforms at the mRNA level in response to aerobic training. This agrees with previous data from this study demonstrating that training-induced enhancement of mitochondrial genes and functions are similar among younger and older people. Thus, skeletal muscle plasticity, at least at the transcript level, remains intact in older people. Future work will determine if training-induced changes in MHC mRNA are also reflected at the protein level. Supported by NIH AG-RO109531, MO1-RR00585 and Mayo Foundation.

Alterations in contractile properties of human skeletal muscle induced by joint immobilization.

The effects of joint immobilization on the contractile properties of human skeletal muscle were examined using the first dorsal interosseous (FDI) muscle. The middle finger, index finger and thumb were immobilized for a period of 6 weeks, and the contractile properties of FDI were tested before immobilization, after 3 and 6 weeks of immobilization, and after a 6 week recovery period. Twitch and tetanic contractions of FDI were evoked by per-cutaneous electrical stimulation. The peak twitch tension (Pt), contraction time (CT) and half-relaxation time (1/2RT) were measured from twitch contractions, while the stimulus frequency-force relationship was obtained from the tetanic contractions (2 s) evoked using various frequencies of stimulation (10-100 Hz). The fatigability of FDI was tested using Burke's fatigue protocol.Pt was significantly increased after 6 weeks of immobilization (P < 0.05) but little alteration was observed in CT or 1/2RT. No change was noted in the FDI fatigue index throughout the immobilization period. The stimulus frequency-force relationship was shifted to the left by immobilization, indicating that a larger percentage of maximal force was evoked by the lower rates of stimulation. Indeed, the tetanic force evoked by a stimulus frequency of 10 Hz was enhanced after immobilization (P < 0.05). On the other hand, the force evoked by frequencies above 50 Hz, including maximal tetanic tension, was decreased (P < 0.05). As a result, the twitch/tetanus ratio was increased (P < 0.01) after immobilization. The changes induced by immobilization in the FDI twitch/tetanus ratio and the estimated maximal firing rate of FDI motoneurones showed a significant correlation (r = 0.80, P < 0.05). It is suggested that the changes in the contractile properties of the FDI muscle seen after joint immobilization are causally linked to the changes in firing rate modulation of FDI motoneurones.

Myotonic dystrophy kinase modulates skeletal muscle but not cardiac voltage-gated sodium channels

Altered modulation of skeletal muscle voltage-gated sodium channels by myotonic dystrophy kinase (DMPK) has been proposed as a possible mechanism underlying myotonia in this disease. We examined the effect of a recombinant mouse DMPK on the functional properties of human skeletal muscle (hSkM1) and cardiac (hH1) voltage-gated sodium channels in the Xenopus oocyte expression system. Co-expression of DMPK with hSkM1 in oocytes resulted in significantly lower peak sodium current amplitude as compared to cells expressing hSkM1 alone in agreement with a previous report. By contrast, DMPK had no effect on the level of expressed sodium current in cells expressing hH1. Similarly, there were no measurable effects of the kinase on the kinetics or steady-state properties of activation or inactivation. Our findings support the previous observations made with rat muscle sodium channels and demonstrate that the effect of DMPK on sodium channels is isoform specific despite conservation of a putative phosphorylation site between the two isoforms.

Effects of aging on shortening velocity and myosin isoform composition in single human skeletal muscle cells.

Maximum unloaded shortening velocity (V0) and maximum force normalized to cross-sectional fiber area (specific tension) were determined in 400 single quadriceps muscle cells of young (n = 4, 25-31 yr) and old (n = 4, 73-81 yr) men. Two of the old men were physically very active, and the subjects were divided into young control, old control, and old physically active groups. The expressions of types I, IIa, and IIb myosin heavy chains (MHC) and essential and regulatory myosin light chains were determined by 6 and 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. V0 was highly dependent on the MHC isoform composition, and a gradient from slow to fast was observed in the sequence I, I-IIa, IIa, IIab, and IIb MHC cells. The V0 values and specific tensions of types I and IIa MHC fibers in the young control group were significantly higher than those in the old control and old physically active groups. Thus the present results provide evidence of qualitative changes in contractile properties of human skeletal muscle in old age, which probably play an important role in the age-related impairment of skeletal muscle function.

Contractile properties of human ankle muscles determined by a systems analysis method for the EMG-force relationship

In order to determine painlessly and non-invasively the contractile properties of human skeletal muscles, the systems analysis method was applied to the relationship between the electromyogram (EMG) of agonist muscles and the isometric force produced by the voluntary contraction of the ankle plantarflexors and dorsiflexors. The subject's task was to produce force impulses at six different ankle positions for both muscles. The EMG(input)-force(output) relationship was approximated using the linear second-order system, and the contraction time (CT), half-relaxation time (HRT) and impulse response (IMP) were estimated. CT, HRT and IMP were found to increase as the muscles were stretched. The CT and HRT were longer for the plantarflexors than for the dorsiflexors, except at extremely plantarflexed positions. These results show qualitative agreement with results obtained by supramaximal electrical stimulation. It is suggested that the method of systems analysis is useful in identifying differences in muscle contractile properties under various force-production conditions.

An Approach to a Visco-elastic Actuator with Properties of Human Skeletal Muscle.

An Approach to a Visco-elastic Actuator with Properties of Human Skeletal Muscle.

The relation between the resultant moments at a joint and the moments measured by an isokinetic dynamometer

Isokinetic dynamometers have frequently been used to determine in vivo properties of human skeletal muscles. In these investigations the moments recorded by the dynamometer were often (implicitly) assumed to be the same as the joint moments produced by muscles. This assumption may have contributed to the conflicting results about in vivo muscle properties of human skeletal muscles that are reported in the literature. The purpose of this paper is to present a method for calculating the resultant joint moments from the moments recorded using a Cybex II dynamometer and to determine the differences between these two moments for some typical exercise conditions. The differences between these moments are shown to be due to (a) gravitational effects, (b) inertial effects, and (c) non-rigidity of the Cybex arm/shank-foot system. The results obtained suggest that the moments recorded using the dynamometer should not be used to derive muscle properties without taking into account the relationship between the resultant joint moments and the moments measured using an isokinetic dynamometer.

Mechanical properties of human skeletal muscle from in vitro studies of biopsies.

The steady-state and small-scale dynamic mechanical properties of human rectus abdominis and intercostal muscle have been investigated by testing small biopsiesin vitro, taken during normal surgery. It has proved possible to obtain valid data on the tension/length/stimulation-rate relationship at body temperature for up to 24 hours after removal. The twitch and tetanus parameters, i.e. maximum tension, rise time and decay time, show comparability with those found by other workersin vivo. The steady-state tension/length/stimulation-rate relationship is similar to those found for other mammalian muscles; the tension/stimulation-rate relationship being dependent on muscle length. The responses to small-scale dynamic inputs show the importance of dynamic properties at physiological rates of change of parameters. Responses to single-parameter inputs appear to add when both inputs are applied simultaneously.