Force and surface mechanomyogram frequency responses in cat gastrocnemius

Muscle surface displacement is a mechanical event taking place simultaneously with the tension generation at the tendon. The two phenomena can be studied by the surface mechanomyogram signal (MMG) (produced by a laser distance sensor) and the force signal (from a load cell). The aim of this paper was to provide data on the reliability of the laser detected MMG in muscle mechanics research. To this purpose it was verified if the laser detected MMG was suitable to estimate a frequency response in the cat medial gastrocnemius and its frequency response was compared with the one retrieved by the force signal at the tendon level. The force and MMG from the exposed medial gastrocnemius of four cats were analysed. The frequency response was investigated by sinusoidally changing the number of orderly recruited motor units, in different trials, in the 0.4-6 Hz range. It resulted that it was possible to model the force and MMG frequency response by a critically damped second-order system with two real double poles and a pure time delay. On the average, the poles were at 1.83 Hz (with 22.6 ms delay) and at 2.75 Hz (with 38 ms delay) for force and MMG, respectively. It can be concluded that MMG appears to be a reliable tool to investigate the muscle frequency response during stimulated isometric contraction. Even though not statistically significant. the differences in the second-order system parameters suggest that different components of the muscle mechanical model may specifically affect the force or MMG.     

Velocity of ultrasound in active and passive cat medial gastrocnemius muscle.

The lengths and pinnation angles of muscle fibers in the medial gastrocnemius (MG) muscle have recently been measured in freely moving cats [Hoffer et al., Progr. Brain Res. 80, 75-85 (1989); Muscle Afferents and Spinal Control of Movement (1992)] using an ultrasound transit-time (USTT) technique. This method assumed that the velocity of ultrasound through intact muscles was constant, independent of fiber orientation, muscle activity, load, belly shape, or fiber movement. However, the velocity of ultrasound along and across the fibers has been reported to depend on the state of muscle activation in frog muscle experiments in vitro [Hatta et al., J. Physiol. 403, 193-209 (1988)]. In the present study, the assumption of constant velocity of ultrasound in the cat MG muscle was evaluated. In acute experiments, done in situ with intact blood supply, the USTT was measured along and across cat MG muscle fibers in the passive, reflexly activated and tetanically activated states, with and without changes in muscle fiber length, for situations that reproduced the length and force ranges normally used by cats during locomotion. The velocity of ultrasound was found to be independent of the state of activation or motion of the muscle, and independent of the direction of the measurement with respect to the fiber orientation, within a measurement uncertainty less than or equal to 0.2%. These results validate the use of the USTT technique for the measurement of intramuscular dimensions in freely moving animals.     

Tetanic depression in fast motor units of the cat gastrocnemius muscle.

Ability of muscle fibers to generate force is decreased when higher frequency of stimulation of motor units immediately follows lower frequency. This phenomenon called tetanic depression was found in rat medial gastrocnemius. However, it was not clear whether tetanic depression occurred only in rat muscle or it concerns all mammals. This study was conducted on motor units of cat medial gastrocnemius. Analyses were made at three successive trains of stimulation: 30 Hz, 20 and 30 Hz and again 30 Hz (the first pattern) or 40 Hz, 25 and 40 Hz and 40 Hz (the second pattern). In all fast units force generated within the middle tetanus was lower than force generated at the same, but constant frequency of stimulation applied earlier or later. The mean tetanic depression in 30 Hz tetani amounted to 10.9% for fast fatigable (FF) and 15.9% for fast resistant (FR) motor units, whereas in 40 Hz tetani mean values were 5.6% and 7.3% for FF and FR motor units, respectively. In slow motor units tetanic depression was not observed. These results proved the existence of tetanic depression in the feline muscle and indicated that its intensity depends on the fusion of tetanus. It has been concluded, that the tetanic depression is a general property of fast motor units in mammals.     

Enzymatic responses of cat medial gastrocnemius fibers to chronic inactivity.

The role of neuromuscular activity in maintaining the normal enzyme heterogeneity found in a predominantly fast mixed muscle was studied. Enzymatic profiles of single fibers in the adult cat medial gastrocnemius (MG) were examined after almost complete elimination of neuromuscular activity for 6 mo. Inactivity was achieved by spinal cord isolation (SI), i.e., spinal transection at T12-T13 and L7-S1 combined with bilateral dorsal rhizotomy between the two transection sites. Cross-sectional area and succinate dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPD) activities were determined in a population of fibers identified in frozen serial cross sections. Each fiber was categorized as light or dark on the basis of its staining characteristics for qualitative myosin adenosinetriphosphatase (ATPase), alkaline preincubation, and its reaction to fast and slow myosin heavy chain (MHC) antibodies. SI resulted in a conversion of nearly all light (approximately 36% in the control) to dark ATPase fibers. Virtually all MG fibers in the SI cats reacted with the fast MHC antibody, whereas very few fibers reacted with slow MHC antibody. On the basis of fiber cross-sectional area, it was estimated that the MG atrophied by approximately 10% after SI. Compared with the mean of the dark and light ATPase fibers in control (weighted by the percent fiber type distribution), mean SDH activity was significantly lower (approximately 70%) and mean GPD activity was significantly higher (approximately 120%) in the SI cats. These data indicate that prolonged electrical silence of a mixed fast hindlimb extensor results in virtually all fibers expressing fast MHC as well as oxidative and glycolytic enzyme profiles normally observed in fast glycolytic fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Low-frequency depression of tension in the cat gastrocnemius muscle after eccentric exercise.

Subjecting a muscle to a series of eccentric contractions in which the contracting muscle is lengthened results in a number of changes in its mechanical properties. These include a fall in isometric tension that is particularly pronounced during low-frequency stimulation, a phenomenon known as low-frequency depression (LFD). Reports of LFD have not taken into account the shift in optimum length for active tension generation to longer muscle lengths that takes place after eccentric contractions. Given the length dependence of the stimulation frequency-tension curve, we tested the hypothesis that the change in this relationship after eccentric exercise is due to the shift in optimum length. We measured LFD by recording tension in response to a linearly increasing rate of stimulation of the nerve to medial gastrocnemius of anesthetized cats, over the range 0-100 pulses per second. Tension responses were measured before and after 50 eccentric contractions consisting of 6-mm stretches starting at 3 mm below optimum length and finishing at 3 mm above it. An index of LFD was derived from the tension responses to ramp stimulation. It was found that LFD after the eccentric contractions was partly, but not entirely, due to changes in the muscle's optimum length. An additional factor was the effect of fatigue. These observations led to the conclusion that the muscle length dependence of LFD was reduced by eccentric contractions. All of this means that after eccentric exercise the tension deficit at low rates of muscle activation is likely to be less severe than first thought.     

Multi-functionality of the cat medical gastrocnemius during locomotion

The functional role of biarticular muscles was investigated based on direct force measurement in the cat medial gastrocnemius (MG) and analysis of hindlimb kinematics and kinetics for the stance phase of level, uphill, and downhill walking. Four primary functional roles of biarticular muscles have been proposed in the past. These functional roles have typically been discussed independently of each other, and biarticular muscles have rarely been assigned more than one functional roles for different phases of the work cycle. The purpose of this study was to elucidate the functional role of the biarticular cat MG during locomotion. It was found that MG forces were primarily associated with the moment requirements at the ankle for most of the stance phase, but also helped to satisfy the moments at the knee in the initial phase of stance. In the second half of stance, MG transferred mechanical energy from the knee to the ankle from the knee to the ankle, while simultaneously producing a substantial amount of mechanical work. Based on these results, we hypothesize that MG's primary function is that of an ankle extensor. However, because of the coupling of the ankle extensor moment with a knee flexor moment in the initial, and a knee extensor moment in the final phase of stance, MG satisfies two joint moments in early stance, and transfers mechanical energy from the knee to the ankle in late stance. We conclude that cat MG has multiple functional roles during the stance phase of locomotion, and speculate that such multi-functionality also exists in other bi- and multi-articular muscles.     

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 functional 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.     

Characteristics of surface mechanomyogram are dependent on development of fusion of motor units in humans.

The purpose of this study was to test whether surface mechanomyogram (MMG) recorded on the skin reflects the contractile properties of individual motor units in humans. Eight motor units in the medial gastrocnemius muscle were identified, and trains of stimulation at 5, 10, 15, and 20 Hz were delivered to each isolated motor unit. There was a significant positive correlation between the duration of MMG and twitch duration. MMG amplitude decreased with increasing stimulation frequency. Reductions in MMG amplitude were in parallel with the reductions in force fluctuations, and the rate of change in both was positively correlated across the motor units. Rate of change in MMG amplitude against force was negatively correlated to half relaxation time and twitch duration. Similar negative correlations were found between force fluctuations and contractile properties. These results provide evidence supporting a direct relation between MMG and contractile properties of individual motor units within the gastrocnemius muscle, indicating that surface MMG is dependent on the contractile properties of the activated motor units in humans.     

Estimation of cat medial gastrocnemius fascicle lengths during dynamic contractions

In typical muscle models, it is often assumed that the contractile element (fascicle) length depends exclusively on the instantaneous muscle-tendon length and the instantaneous muscle force. In order to test whether the instantaneous fascicle length during dynamic contractions can be predicted from muscle-tendon length and force, fascicle lengths, muscle-tendon lengths, and muscle forces were directly measured in cat medial gastrocnemii during isometric and dynamic contractions. Two theoretical muscle models were developed: model A was based on force-time data obtained during the activation phase and model D on force-time data obtained during the deactivation phase of isometric contractions. To test the models, instantaneous fascicle lengths were predicted from muscle-tendon lengths and forces during dynamic contractions that simulated cat locomotion for speeds ranging from 0.4 to 1.6m/s. The theoretically predicted fascicle lengths were compared with the experimentally measured fascicle lengths. It was found that fascicle lengths were not uniquely associated with muscle-tendon lengths and forces; that is, for a given muscle-tendon length and force, fascicle lengths varied depending on the contractile history. Consequently, models A and D differed in fascicle length predictions; model D (maximum average error=8.5%) was considerably better than model A (maximum average error=22.3%). We conclude from this study that it is not possible to predict the exact fascicle lengths from muscle-tendon lengths and forces alone, however, adequate predictions seem possible based on such a model. The relationship between fascicle length and muscle force and muscle-tendon length is complex and highly non-linear, thus, it appears unlikely that accurate fascicle length predictions can be made without some reference contractions in which fascicle length, muscle-tendon length, and force are measured simultaneously.     

Interspecies differences in the force-frequency relationship of the medial gastrocnemius motor units.

Single, functionally isolated motor units were studied in the medial gastrocnemius (MG) muscle of cats and rats. Axons of their motoneurons were stimulated with trains of pulses at frequencies increasing from 1 to 150 Hz and forces developed by muscle fibers were measured and force-frequency curves were compared between species. The following observations were made: (1) the most steep parts of curves (related to unfused tetani of motor units) begun at lower frequencies of stimulations in all types of feline motor units, (2) for fast motor units, the same relative values of force of unfused tetani were achieved at significantly lower frequencies of stimulations in the cat than in the rat. Twitch time parameters of both species influenced the course of force-frequency curves. It was showed that the contraction times of feline units varied in the wide range (21-81 ms), and these units reached 60% of the maximum force at stimulation frequencies between 10 and 38 Hz. On the other hand, contraction times of rat units ranged from 10 to 34 ms, whereas stimulation frequencies necessary to reach 60% of the maximum force varied considerably, from 12 to 65 Hz. The correlations between the above parameters were found for motor units of each species. However, the regression lines drown for the collected population of cat and rat units did not form linear continuity. Thus it seems that interspecies differences in the twitch contraction times do not fully explain different force-frequency relationships in mammalian skeletal muscles.     

Force depression decays during shortening in the medial gastrocnemius of the rat

Force depression due to shortening of activated skeletal muscles has previously been described to be long lasting during isometric contractions following the shortening. In the present study, using the medial gastrocnemius of the rat, effects of force depression have been made apparent during shortening by computationally partially compensating for the direct effect of shortening velocity due to the tension–velocity relation. Evidence was found for the decay and complete disappearance of force depression already during continuation of the shortening contraction to short muscle lengths.     

Interspecies differences of motor units properties in the medial gastrocnemius muscle of cat and rat

The purpose of the study was to analyze the interspecies differences of motor unit contractile properties in two most frequently studied mammals: cats and rats. A total sample of 166 motor units (79 in cats and 85 in rats) was investigated in the medial gastrocnemius muscle. Considerable differences were found in composition of the studied muscle. In cats, fast fatigable, fast resistant and slow units formed 68, 18 and 14% of the investigated population, whereas in rats 36, 52 and 12%, respectively. The contraction and relaxation times of motor units in the cat muscle were evidently longer than in the rat and the border values for fast/slow motor units division in these species were 44 and 20 ms, respectively. The mean values of twitch and tetanic forces appeared to be 7-8 times lower in rats, for fast, while 2-5 times for slow motor units. Also variability between the strongest and the weakest units within each type revealed differences 10-60 times in cats, whereas only 3.5-14 times in rats. The summation of twitches into tetanus for fast units was comparable in both species, but for S units was evidently more effective in the cat. In fast motor units' tetanic contractions evident interspecies differences concerned sag appearance and profiles of unfused tetani of FF and FR units. Differences in contractile properties described in the study may depend on the size, number and innervation ratio of motor units in the muscle of cat and rat, as well as their biochemical variability. Differences in composition of motor unit types and uneven mechanisms of force development may reflect biological adaptation to variable behaviour of cats and rats.     

The effect of skinfold on frequency of human muscle mechanomyogram.

The aim of the study was to assess the effect of skinfold thickness on median and peak frequency of mechanomyographic (MMG) signal in relation to subject's age, gender and force during voluntary contraction of elbow flexor and extensor muscles. Seventy-nine healthy subjects participated in the study: 22 young females (age 20.1+/-1.1 years), 22 young males (age 23.4+/-1.1 years), 17 elderly females (age 64.9+/-5.1 years), and 18 elderly males (age 67.4+/-6.2 years). Three identical MMG probes were used to record MMG signals from above the triceps brachii (TB), biceps brachii (BB), and brachioradialis (BR) muscles simultaneously with the force signal. The results showed that the tissue between the muscle and the skin surface has a major contribution to the median and a minor contribution to the peak MMG frequencies independent of subjects' age (with force having more than 2 folds a smaller effect). During antagonistic function of the main elbow flexors and extensors, there is a decreasing effect of skinfold thickness and an increasing effect of force on the MMG frequency, and the relative contribution of both factors to the MMG signal is age related, especially in the TB and BR muscles. The BR muscle differs from the TB and BB muscles in regard to the effects of skinfold thickness and force on the MMG frequency, as well as in the effect of age on the relationship between the MMG frequency and skinfold thickness and force. The effect of age on the relative contribution of skinfolds and force to MMG frequency is specific for muscle and its function. It was concluded that studies that report MMG frequency with different values of skinfold thickness cannot be easily compared, especially when maximally activated prime movers are tested. A use of force and skinfold thickness as covariates is recommended when an MMG frequency is analyzed in subjects differing in the skinfold thickness.     

Damage to different motor units from active lengthening of the medial gastrocnemius muscle of the cat.

Slow-twitch motor units in the medial gastrocnemius muscle of the anesthetized cat were found to have an average optimum length for active tension that was 0.8 +/- 0.5 (SE) mm longer than the whole muscle optimum. For fast-twitch units (time to peak < 50 ms), the average optimum was 1.3 +/- 0.3 mm shorter than the whole muscle optimum. After the muscle had been subjected to 10 stretches while maximally activated, beginning at the whole muscle optimum length, the optimum lengths of the 27 fast-twitch motor units shifted significantly further in the direction of longer muscle lengths (mean 4.3 +/- 0.3 mm) than for the eight slow-twitch units (2.1 +/- 0.4 mm). A shift in the muscle's length-tension relation was interpreted as being due to sarcomere disruption. Statistical analysis showed that a motor unit's optimum length for a contraction, relative to the whole muscle optimum, was a better indicator of the unit's susceptibility to damage from active lengthenings than was motor unit type.     

Influence of motor units recruitment and firing rate on the soundmyogram and EMG characteristics in cat gastrocnemius

The separate contributions of the recruitment level and of the firing rate of the motor units on the soundmyogram and electromyogram time domain parameters were investigated during stimulation of the motor nerve of the cat gastrocnemius muscle. Upon orderly increase in the number of active motor units at a fixed firing rate, both the peak to peak amplitude (P-P_max) and the root mean square (RMS) of the sound myogram increased. At full recruitment the increase in firing rate from 2.5 to 50 Hz induced an exponential decline in the P-P_max. The RMS, however, followed this trend only from 15 to 50 Hz while showing an increase from 2.5 to 10 Hz. During simultaneous changes of recruitment and firing rate, the effect of increasing the number of motor units on the P-P_max and RMS is dampened by the increasing firing rate. The peak to peak amplitude of the EMG compound action potential increased with the number of active motor units. Moreover, its amplitude was not influenced by the firing rate. The EMG RMS, however, increases as a function of the firing rate. The results indicate that both the number and the firing rate of the active motor units contribute to the determination of the soundmyogram characteristics. Moreover, the peculiar changes of the soundmyogram time domain properties, compared to the ones of the EMG, allow one to differentiate the influence of the motor units number and firing rate on the electrical and mechanical performance of the muscle when stimulated.     

Time course of stretching-induced changes in mechanomyogram and force characteristics

To evaluate the time-course of stretching-induced changes in mechanical properties of the muscle-tendon unit (MTU), 11 participants (age 22 ± 1 yr; body mass 77 ± 5 kg; stature 1.78 ± 0.05 m; mean ± SD) underwent tetanic electrical stimulations of the medial gastrocnemius muscle before and after (up to 2 h) stretching administration. During contractions, surface electromyogram (EMG), mechanomyogram (MMG) and force were recorded simultaneously. From MMG, peak-to-peak (p–p) and root mean square (RMS) were calculated during the on-phase and plateau phase of tetanic contraction, respectively. After stretching: (i) no differences were found in EMG parameters; (ii) MMG p–p and slope decreased (−16% and −10%, respectively; P < 0.05) and remained depressed for the entire recovery period; (iii) MMG RMS increased (+20%; P < 0.05), returning to pre-stretching values within 15 min; and (iv) peak force (pF), with its first (dF/dt) and second (d²F/dt²) derivative, decreased significantly by 32%, 35% and 54%, respectively, and remained depressed for the entire recovery period. The lack of MMG p–p and pF recovery could be ascribable to a reduced muscle force generating capacity due to persisting changes in viscoelastic characteristics of series elastic components. The early return of MMG RMS to pre-stretching values suggests that changes in viscoelastic parallel components recovered after few minutes.     

The motor units of cat medial gastrocnemius: electrical and mechanical properties as a function of muscle length.

The effects of changing muscle length on the mechanical properties of 89 motor units from adult cat medial gastrocnemius have been studied in eight experiments. Few differences were found between the effects of length on tetanic tension, twitch tension, twitch-tetanus ratio, twitch contraction time, twitch half relaxation time, rate of force development and electrical activity for fast contracting (twitch contraction time less than or equal to 45 msec) and slowly contracting (greater than 45 msec) units. Those differences that did appear did not persist when these two groups were matched by tetanic tension. It is concluded that the biophysical mechanisms responsible for the changes in mechanical and electrical properties with length must be similar for fast and slow twitch units and not related to potential differences in their muscle fiber type. The effects of changing muscle length on the mechanical properties of the eight whole muscles suggest that changes in force output with length are of minor importance during normal movements as the muscle is found to be electrically active over a relatively narrow range of lengths close to the optimum length for tetanus of the whole muscle. The very shortest muscle lengths at which there is only minimal force development are not used in natural movements, while the declining limb of the length tension curve is at muscle lengths beyond the maximum in situ length.