Isometric Properties Research Articles

An Evaluation of the Length-Tension Relationship in Elderly Human Plantarflexor Muscles

The purpose of this study was to determine the effect of aging on the muscle length-tension relationship in the plantarflexor muscles of 10 subjects aged 20-30 yr (Mean = 23; 5 males, 5 females), 10 subjects aged 60-80 yr (Mean = 72.3; 5 males, 5 females), and 10 subjects over 80 yr (Mean = 84.1, 5 males, 5 females). Isometric twitch properties, maximum voluntary strength, passive tension, and range of motion were measured at five different joint angles [20 degrees dorsiflexion (DF), 10 degrees DF, 0 degree, 10 degrees plantarflexion (PF), and 20 degrees PF]. Active (evoked and voluntary) and passive torque production were maximal when the ankle was rotated into the DF positions for all three age groups, whereas the lowest values were recorded when the ankle was rotated into 20 degrees PF. Males were stronger than females at all joint angles (p < .01). Also, young adults were stronger than both elderly adult groups (p < .01). These results illustrate that despite the considerable age-associated loss in both voluntary and evoked strength in the plantarflexors, the optimal angle for torque production remains the same for younger and older adults.

Mechanism of specific force deficit in the senescent rat diaphragm

Aging is associated with a decline in the maximal in vitro specific force in the rat costal diaphragm. The purpose of this study was to determine if this force deficit is associated with a decrease in the concentration of myofibrillar protein in diaphragm fibers of senescent rats. Isometric twitch and tetanic contractile properties were measured on diaphragm strips from young adult (9-month-old; n=12) and senescent (26-month-old; n=13) male specific pathogen free-barrier protected Fischer 344 rats. Maximal tetanic force (P o) normalized to the cross-sectional area (CSA) of the in vitro diaphragm strips was 16.4% lower in the senescent diaphragms (21.03±0.4 N/cm 2) compared to the young adult (25.16±0.5 N/cm 2) ( p<0.001). Diaphragm water content was significantly higher in the senescent group (75.9% of total wet mass) compared to the young adult (72.1% of total wet mass, p<0.05). Subtracting the contribution of water from the CSA of the diaphragm strips significantly reduced ( p<0.05) the senescent specific P o deficit (from −16.4 to −6.4%). Further, correcting P o for the contribution of myofibrillar protein to CSA resulted in no age group differences in specific force. These data indicate that the age-related decline in diaphragm in vitro maximal specific P o can be explained by an age-related increase in the water content of the diaphragm muscle. Future experiments are necessary to determine the mechanism(s) responsible for this observation.

The Effects of Experimental Hyperthyroidism and Hypothyroidism on the Isometric Contraction Properties of Plantaris Muscle in Rats, the Relationship Between These Properties and Capillarity

In this study, the isometric properties and the capillarity of isolated plantaris muscle in hyperthyroid and hypothyroid rats and the relation between these parameters have been investigated. The experiment was carried out in 3 groups as control, hyperthyroidism and hypothyroidism. Each group comprised of 20 Wistar albino adult female rats. In anaesthetized rats, single and tetanic stimulations were applied directly to the plantaris muscle in an isolated tissue bath. From the recordings obtained by an isometric transducer and a polygraph, contraction time, half relaxation time, twitch tension and tetanic force were measured. The plantaris muscle of the other leg was prepared in paraffin blocks after fixation in neutral formalin. 5 \mum sections taken from the blocks were stained by haematoxylin and eosin and their capillary densities were determined under light microscope. No significant difference was found in the contraction time between the hyperthyroid and the hypothyroid groups. Half relaxation time decreased in the hyperthyroid group while it increased in the hypothyroid group. Twitch tension and tetanic force decreased in the hyperthyroid group and increased in the hypothyroid group. No difference was found in the capillary density of plantaris muscle between the groups. There was also a lack of correlation between the capillary density of the muscle and the twitch tension and the tetanic force of the muscle in both the hyperthyroid and the hypothyroid groups.

Isometric and isotonic muscle properties as determinants of work loop power output.

The power output of rabbit latissimus dorsi muscle was calculated under isotonic conditions and during oscillatory work. Isotonic shortening studies yielded a maximum power output of 120 W . kg-1 at a P/P0 of 0.4 compared to a maximum power output of 32 W . kg-1 obtained using the work loop technique. This difference can largely be explained by comparing actual work loops with those constructed using force velocity (P/V) and isometric data. At low cycle frequencies, work loop power output is quite close to that predicted from P/V and isometric data. However, at higher frequencies other dynamic muscle properties appear to exert a more marked effect.

Hydrogen peroxide disrupts Ca2+ release from the sarcoplasmic reticulum of rat skeletal muscle fibers.

Reactive oxygen species such as superoxide (O2-) and H2O2 are produced at low levels in resting muscles and at substantially higher levels in exercising muscles. Increased respiratory activity with exercise leads to O2- production by the NADPH oxidase reaction and the subsequent generation of H2O2 from O2- by spontaneous dismutation or by the superoxide dismutase reaction. The long-lasting (24-h) depression of contractile function after exercise has been linked to damage of one or more proteins important in the excitation-contraction coupling process. We studied mechanically and chemically skinned fibers from the extensor digitorum longus muscle of the rat to evaluate the effects of a 5-min exposure to 1.0 mM H2O2 on muscle function. We found that H2O2 had no effect on the isometric force-producing properties of the contractile apparatus or on Ca2+ uptake by the sarcoplasmic reticulum. It did, however, significantly affect Ca2+ release from the sarcoplasmic reticulum. Maximum depolarization-induced Ca2+ release was inhibited, and the sensitivity to depolarization was decreased. Ca(2+)-induced release was completely blocked. We conclude that elevated levels of H2O2 with exercise are capable of damaging one or more proteins of the excitation-contraction coupling process to produce a disruption in function that can account, at least in part, for the long-lasting effects of fatiguing stimulation.

Strain-dependent cross-bridge cycle for muscle. II. Steady-state behavior

Quantitative predictions of steady-state muscle properties from the strain-dependent cross-bridge for muscle are presented. With a stiffness of 5.4 x 10(-4) N/m per head, a throw distance of 11 nm, and three allowed actin sites/head, isometric properties and their dependence on phosphate and nucleotide levels are well described if the tension-generating step occurs before phosphate release. At very low ATP levels, rigorlike states with negative strain are predicted. The rate-limiting step for cycling and ATP consumption is strain-blocked ADP release for isometric and slowly shortening muscle. Under rapid shortening, ATP hydrolysis on detached heads is the rate-limiting step, and the ratio of bound ATP to bound ADP.Pi increases by a factor of 7. At large positive strains, bound heads must be forcibly detached from actin to account for tension in rapid extension, but forced detachment in shortening has no effect without destroying isometric attached states. Strain-blocked phosphate release as proposed produces modest inhibition of the ATPase rate under rapid shortening, sufficient to give a maximum for one actin site per helix turn. Alternative cross-bridge models are discussed in the light of these predictions.

The Effects of Tapering on Strength Performance in Trained Athletes

The optimum pre-competition taper procedure for "strength athletes" is not known. We examined voluntary strength and evoked contractile properties of the elbow flexors over a 10 day rest only (ROT) and a 10 day reduced volume taper (RVT) in 8 resistance trained males (23 +/- 2.1 years). Following 3 wks of standardized training of the elbow flexors, subjects were randomly assigned to one of the tapers. Upon completion, they resumed training for 3 wks and completed the other taper. No arm training was performed during the ROT, while high intensity, low volume training was done every second day during the RVT. Maximum isometric (MVC), low (0.52 rad.s-1; LV) and high velocity (3.14 rad.s-1; HV) concentric peak torque, and evoked isometric twitch contractile properties were measured before and after each training phase and every 48 h during each taper. ANOVA comparison of the tapers revealed that MVC increased (p < or = 0.05) over pre-taper values throughout the RVT (measurement days 2, 4, 6, 8 and 10), as did LV at 2, 4, 6, and 8 d. MVC did not change over the ROT but LV was significantly higher on day 2 and lower on days 8 and 10. LV was also greater on days 4, 6, 8 and 10 during the RVT compared to the ROT. The evoked contractile properties remained largely unchanged. The data indicate that resistance-trained athletes can improve low velocity concentric strength for at least 8 days by greatly reducing training volume, but maintaining training intensity.

Large pulmonary arteries and the control of pulmonary vascular resistance in the newborn

In the immediate neonatal period the pulmonary vascular resistance is higher than later in life. The role of maturational differences in the smooth muscle mechanical properties of large capacitance vessels in this response has not been previously studied. To this end, we studied the smooth muscle isometric and isotonic mechanical properties, as well as the myogenic response of large extralobar pulmonary arteries in newborn and adult guinea pigs. Compared with the adult, the newborn pulmonary vascular smooth muscle generates less force and has a similar shortening capacity but longer isometric and isotonic relaxation half times. The myogenic response could be elicited in 80% of the newborn and 70% of the adult vessels. A 20% increase in vessel diameter resulted in force generation equivalent to 46 +/- 5.1% of maximal isometric tension in the newborn. Such a response was significantly greater than observed for the adult vessels (14 +/- 2.8%; p < 0.001). Our results showed significant maturational differences in the mechanical properties of the large pulmonary artery smooth muscle in the newborn. We speculate that the presence of myogenic response in large pulmonary capacitance vessels and the observed greater magnitude of stretch-induced force generation in the newborn may play an important role in the maintenance of a higher pulmonary vascular resistance in the neonatal period.

The effect of Trichinella spiralis and Trichinella pseudospiralis on the mechanical properties of mammalian diaphragm muscle.

The isometric mechanical properties of diaphragm muscle were studied in mice infected with either Trichinella spiralis or Trichinella pseudospiralis. Measurements of muscle stress were taken at 15, 20, 30 and 40 days post-infection. Infected diaphragm muscle showed a significant (P < 0.001) reduction in muscle stress during both twitch and tetanic contractions when compared with muscle from control animals. T. spiralis caused a significant reduction in resistance to muscle fatigue. The consequences of these changes in muscle function to host pathology are discussed, and related to previous work on the effects of Trichinella on host biochemistry and the immune response.

The force-velocity relation of the rabbit inferior oblique muscle; influence of temperature

The contractile properties of the rabbit inferior oblique muscle (IO) were studied in vitro with direct stimulation at temperatures between 20 and 35 degrees C. Isovelocity releases were used to determine the force/velocity relation. Cooling the muscle from 35 degrees C to 20 degrees C increased contraction and half-relaxation times of single twitches with a temperature coefficient (Q10) of 0.4, but did not affect significantly the twitch tension. The tetanic tension increases with increasing temperature (Q10 = 1.32). Cooling decreased the maximum shortening velocity of the IO with a Q10 of 1.6 and the maximum mechanical power with a Q10 of 2.3. At 35 degrees C, the maximum speed of shortening of the muscle (19 +/- 2 muscle lengths/s, mean +/- SEM) corresponded to a maximum shortening velocity of the sarcomeres of 57 +/- 6 microns/s. This value is similar to data obtained for extraocular muscles (EOM) of smaller rodents (mice and rats). In comparison with mammalian limb muscles the isometric and force-velocity properties of mammalian EOM appear to be virtually independent of the size of the animal. Thus, IO is a fast-twitch muscle endowed with a maximum velocity of shortening higher than that of fast-twitch skeletal muscle, but using a tetanic mechanical power lower than that produced by slow-twitch muscle: the combination of these properties makes it ideally suited to move an ocular globe of low mass at high velocity.

Effects of age and training on skeletal muscle physiology and performance.

Aging skeletal muscle exhibits decreases in muscle mass and force and changes in contractile properties. The effects of aging on the physiological characteristics of skeletal muscle are fiber type specific. This review describes the aging process in skeletal muscle; specifically, the effects of aging on the biochemical, morphological, and physiological characteristics of type I (slow-twitch) fibers and type II (fast-twitch) fibers. The effects of training on specific fiber types are also reviewed. The age-related decrease in maximum isometric force may be due, in part, to a decline in muscle mass. Decreases in muscle mass appear to occur in weight-bearing muscles and are most marked in those with a high proportion of type II fibers. The age-related fiber atrophy contributes to the decline in muscle mass. The decline in fiber size is prominent in type II fibers, whereas type I fibers are less affected. The age-related prolongation in isometric twitch properties may be due, in part, to alteration in the capacity of the sarcoplasmic reticulum for calcium release and recapture. Resistance and endurance training appear to attenuate the age-related alterations in skeletal muscle properties if the stimulus is of a sufficient intensity and duration.

Isometric properties of lower extremity muscles in cyclists determined by least squares fitting of muscle models

Isometric properties of lower extremity muscles in cyclists determined by least squares fitting of muscle models

Matching between motoneurone and muscle unit properties in rat medial gastrocnemius.

1. Electrical and contractile (isometric) properties were studied for sixty-six motoneurone-muscle unit combinations from rat medial gastrocnemius (MG). The animals were anaesthetized with pentobarbitone. 2. The muscle units were classified into S (slow) and F (fast) on the basis of measurements of speed and fatigue resistance: the 'slow' category comprised units with an initial twitch contraction time exceeding those found among fatigue-sensitive units (border value 20 ms). 3. Twitch speed was assessed by three different measures: (i) contraction time (time to peak, range 11.4-28.0 ms), (ii) half-relaxation time (8.4-56.5 ms), and (iii) total twitch duration (34-116 ms). All three measures were mutually highly correlated and their respective values showed a continuous and unimodal distribution across the unit population. 4. The motoneurones were investigated with regard to their time course and amplitude of post-spike after-hyperpolarization (AHP; range of total durations 30-116 ms, amplitudes 0.9-8.0 mV), rheobase (0.8-17.1 nA), input resistance (0.8-5.1 M omega) and axonal conduction velocity (33-85 m/s). 5. Motoneurones of slow-twitch muscle units (type S) had, on average, a significantly slower time course of AHP, a smaller rheobase, a higher input resistance and more slowly conducting axons than those innervating fast-twitch muscle units. 6. Across the whole neuronal sample, input conductance (reciprocal of input resistance) correlated well with rheobase (r = 0.74). However, the differences in rheobase did not seem to be caused exclusively by the associated differences in input conductance. 7. Throughout the sampled population, the relative slowness of AHP showed a continuous and highly significant correlation with the relative slowness of the corresponding unit twitch. The absolute duration of AHP was close to that of the twitch. In the Discussion it is argued that this 'speed match' between motoneurone and muscle unit would help ensure that barely recruited motoneurones start firing at a frequency that is optimally suited for the subsequent rate gradation of force. 8. AHP amplitude was, on average, significantly smaller for fast-twitch than for slow-twitch motoneurones. Calculations indicated that these differences were almost completely caused by the associated differences in input resistance; the computed value for the conductance change underlying the AHP was nearly the same for fast- and slow-twitch motoneurones. 9. A simple neurone model was used to calculate the consequences of the differences in AHP amplitude and duration for repetitive discharge properties of fast and slow cell categories.(ABSTRACT TRUNCATED AT 400 WORDS)

Contractile properties of the striated adductor muscle in the bay scallop Argopecten irradians at several temperatures.

The isometric and isotonic contractile properties of the cross-striated adductor muscle of the bay scallop (Argopecten irradians) were measured in vitro at 10, 15 and 20 degrees C. The length at which twitch force was maximal as a function of the closed length in situ (L0/Lcl) averaged 1.38 +/- 0.01 (mean +/- S.E.M.) at 10 degrees C. This length is very close to the typical length at maximum gape during natural swimming at this temperature. Passive force was very low over the range of lengths measured here; at L0, passive force averaged approximately 0.08 N cm-2, or only 0.5% of the corresponding peak twitch force. The mean peak isometric twitch force (Ptw,max) at 10 degrees C was 21.43 +/- 0.68 N cm-2 (S.E.M.), and the ratio of peak twitch force to tetanic force (Ptw,max/P0) averaged 0.89 +/- 0.01. Temperature did not affect either twitch force (Ptw), once fatigue was taken into account, or Ptw,max/P0. In contrast, the time-related properties of twitch contractions (latent period, tL; time to peak tension, tPtw; and time from peak tension to half-relaxation, t50%R) were positively modified by temperature at all temperatures measured (Q10 > 1.8). All three properties were more temperature-sensitive over the range 10-15 degrees C than over the range 15-20 degrees C. The force-velocity relationships of the striated adductor muscle were fitted to the hyperbolic-linear (HYP-LIN) equation. The force-velocity curves of the striated adductor muscle of the scallop were strongly influenced by temperature. Maximal velocity at zero force (Vmax), and therefore maximal power output, increased significantly with temperature. The Q10 over the temperature range 10-15 degrees C (1.42) was significantly lower than that over the range 15-20 degrees C (2.41). The shape of the force-velocity relationship, assessed through comparisons of the power ratio (Wmax/VmaxP0), was not influenced by temperature.

Developmental differences in vascular smooth muscle mechanics in pulmonary and systemic circulations

To evaluate the maturational changes in vascular muscle mechanics we studied and compared the isotonic half-time relaxation (t1/2P,CE) and maximal load-bearing capacity normalized to stress of pulmonary and systemic arterial muscle from perinatal and adult sheep. For the pulmonary and systemic vessels t1/2P,CE was significantly shorter in adult than in perinatal sheep (P < 0.01). In newborns t1/2P,CE of the pulmonary vessels was 185 +/- 31 (SE) s, longer than that of the systemic vessels (64 +/- 10 s; P < 0.01). In adults t1/2P,CE of the pulmonary vessels (101 +/- 14 s) was longer than that of the systemic vessels (37 +/- 5 s; P < 0.01). Maximal load-bearing capacities normalized to stress of pulmonary vessels of fetal and newborn sheep were twofold greater than those of adult sheep and of the systemic vessels of newborns and adults (P < 0.01). In conclusion, significant maturational changes in the isotonic and isometric mechanical properties of vascular pulmonary and systemic smooth muscle were observed in sheep.

Force-length properties and functional demands of cat gastrocnemius, soleus and plantaris muscles

The purpose of this study was to measure isometric force-length properties of cat soleus, gastrocnemius and plantaris muscle-tendon units, and to relate these properties to the functional demands of these muscles during everyday locomotor activities. Isometric force-length properties were determined using an in situ preparation, where forces were measured using buckle-type tendon transducers, and muscle-tendon unit lengths were quantified through ankle and knee joint configurations. Functional demands of the muscles were assessed using direct muscle force measurements in freely moving animals. Force-length properties and functiona demands were determined for soleus, gastrocnemius and plantaris muscles simultaneously in each animal. The results suggest that isometric force-length properties of cat soleus, gastrocnemius and plantaris muscles, as well as the region of the force-length relation that is used during everyday locomotor tasks, match the functional demands.

A novel remote-sensing isometric force transducer for micromechanics studies.

We have developed an innovative transducer for measuring force with femtonewton-to-micronewton resolution in biological systems. A magnetic microsphere is attached to the specimen being studied and is positioned between two electromagnets. Video microscopy and edge detection are used to monitor small movements of the microsphere that occur when the specimen generates force. An automatic control system adjusts the current through the electromagnets to keep the microsphere stationary. Measured force is a linear function of this current. This transducer is unique in its combination of sensitivity and isometric properties and its ability to measure force without direct connections to the specimen. That is, the transducer is "remote sensing" and can measure force through intervening membrane or tissue. The transducer is isometric at steady state to the limit at which displacement of the microsphere can be resolved, which can be as low as 19 nm. The completed system is being used to study the mechanics of interstitial connective tissue but may also be used to study molecular generation of force.

The time course of thyroid-hormone-induced changes in the isotonic and isometric properties of rat soleus muscle.

Rat thyroidectomy resulted in changes in a number of parameters used to characterise the mechanical and histochemical status of skeletal muscle. Thus thyroidectomy resulted in a prolongation of soleus slow-twitch muscle isometric contraction time and half-relaxation time with a reduced maximum velocity of shortening and maximum rate of development of tetanic tension but no significant change in twitch: tetanus ratio i.e. the ratio of twitch force/unit area to tetanic force/unit area. In addition the percentage of IIA fibres was reduced and the percentage of type I fibres increased. Triiodothyronine, administered to hypothyroid rats, brought about a speeding of these parameters again with no change in twitch: tetanus ratio. There was an increase in the percentage of IIA fibres with a concomitant reduction in the percentage of type I fibres. These changes were induced over 18 days and resulted in isotonic and isometric properties almost identical to those of soleus muscles from chronically hyperthyroid rats; speeding could be detected as early as 2 days after triiodothyronine had been given. Consideration is given to the possibility that changes in myosin isoforms and/or the kinetics of changes in intracellular calcium concentration in activation and relaxation could account for the time course of the observed changes.

Myocardial mechanics predict hemodynamic performance during normal function and alcohol-induced dysfunction in rats

To determine whether mechanical evaluation of muscle tissue removed from the myocardium can be employed as a direct indicator of cardiac contractile performance in situ, isometric and isotonic parameters of muscle mechanics in vitro were correlated with in vivo global functional characteristics of the same heart. Twelve-month-old animals maintained on standard food and water were employed as representative of normal cardiac function. Animals of identical age with left ventricular (LV) dysfunction induced by oral alcohol (30%) ingestion from 4 to 12 mo were utilized to represent depressed cardiac performance. Accordingly, 24 h after the establishment of the hemodynamic profile for a control or experimental heart, the LV posterior papillary muscle was removed from the same heart and examined isometrically and isotonically. Least squares regression analysis was employed to establish a correlation coefficient and P values between various in vitro and in vivo parameters. Hemodynamic measurements were performed under chloral hydrate anesthesia and LV pump performance was evaluated with respect to aortic and ventricular pressures and the rates of rise and decay of the LV pressure trace. Papillary muscles were evaluated with respect to timing parameters of the isometric and isotonic twitch, the first derivative of isometric tension development, and the speed of muscle shortening at increasing physiologic loads. LV peak rate of pressure rise and decay were then correlated with the various isometric and isotonic properties. Myocardial mechanics and hemodynamics revealed depressed function in the papillary muscles and hearts from alcoholic rats. Moreover, significant correlations were found between the LV rate of pressure change (peak +dP/dt and -dP/dt) and both isometric and isotonic twitch measurements.(ABSTRACT TRUNCATED AT 250 WORDS)

Isobaric and isometric large artery properties in hypertension demonstrate that vasodilatation attenuates greater decrease in arterial compliance

Isobaric and isometric large artery properties in hypertension demonstrate that vasodilatation attenuates greater decrease in arterial compliance