Biomedical subjects

S A Binder-Macleod

Publications and source records attributed to S A Binder-Macleod.

31 records · Page 2Linked to original sources

PubMed1997

A mathematical model that predicts skeletal muscle force.

This study demonstrates the validity of a mathematical model that predicts the force generated by rat skeletal muscles during brief subtetanic and tetanic isometric contractions. The model consists of three coupled differential equations (ODE's). The first two equations represent the calcium dynamics and the third equation represents force dynamics. The model parameters were identified from brief trains of regularly spaces pulses [constant-frequency trains (CFT's)] that produce subtetanic muscle responses. Using these parameters, the model was able to predict isometric forces from other stimulation patterns. For the gastrocnemius muscles predictions were made for responses to CFT's with interpulse intervals (IPI's) ranging from 10 to 50 ms and variable-frequency trains (VFT's), where the initial IPI = 10 ms and the remaining IPI's were identical to those used for the CFT's. For the soleus muscles predictions were made for 10-100-ms CFT's. The shape of the predicted responses closely match the experimental data. Comparisons between experimental and modeled force-time integrals, peak forces, and time-to-peak also suggest excellent agreement between the model and the experiment data. Many physiological parameters predicted by the model agree with values obtained independently by others. In conclusion, the model accurately predicts isometric forces generated by rat gastrocnemius and soleus muscles produced by brief stimulation trains.

Actins↗

PubMed1997

Variable-frequency train stimulation of canine latissimus dorsi muscle during shortening contractions.

In cardiomyoplasty, the latissimus dorsi muscle (LDM) is wrapped around the heart ventricles and electrically activated with a constant-frequency train (CFT). This study tested the hypotheses that increased mechanical performance from the LDM could be achieved by activating the muscle with variable-frequency trains (VFTs) of shorter duration or containing fewer stimulus pulses than the CFT now used. The mechanical performance of the canine LDM (n = 7) during shortening contractions was measured while the muscle was stimulated with 5- and 6-pulse CFTs (of duration 132 and 165 ms, respectively) and 5- and 6-pulse VFTs (of duration 104 and 143 ms, respectively) that were designed to take advantage of the catchlike property of skeletal muscle. Measurements were made from fresh and fatigued muscles. For the fresh muscles, the VFTs elicited significantly greater peak power than did the 6-pulse CFT. When the muscles were fatigued, VFT stimulation significantly improved both the peak and mean power produced compared with stimulation by CFTs. These results show that stimulation of the LDM with shorter duration VFTs is potentially useful for application in cardiomyoplasty.

Animals↗

PubMed1996

Catchlike property of human muscle during isovelocity movements.

This study examined the catchlike property of skeletal muscle during eccentric and concentric isovelocity contractions of fresh and fatigued quadriceps femoris muscles of 10 healthy subjects. During concentric contractions of fresh muscles, stimulation trains that elicited a catchlike response (CITs) produced greater force outputs and rates of rise force than comparable constant-frequency trains. These enhancements became more pronounced during fatigue. CITs were less effective in enhancing forces during eccentric contractions but did improve the rates of rise of force. Overall, the CIT that produced the greatest augmentation had a 5-ms initial interpulse interval. Proposed mechanisms for the catchlike property involve enhanced muscle stiffness for more efficient transmission of tension and increased calcium release. These results suggest that stimulation trains that take advantage of the catchlike property of skeletal muscle may be helpful during clinical applications where neuromuscular electrical stimulation is used to restore function in patients with damaged central nervous systems.

Adult↗

PubMed1995

Variable-frequency stimulation patterns for the optimization of force during muscle fatigue. Muscle wisdom and the catch-like property.

Muscle wisdom is the process whereby the activation rates of motor units are modulated by the central nervous system to optimize the force during sustained voluntary contractions. During maximal voluntary contractions the activation rates decline as the muscle fatigues. No similar decline has been observed during submaximal contractions. Subsequent chapters explore the potential mechanisms for muscle wisdom. In this chapter, a historical background on the development of ideas on muscle wisdom is first presented. Next, artificial wisdom, the procedure used to optimize force during an electrically imposed tetanus by progressively reducing the stimulation frequency as the muscle fatigues, is discussed. Finally, recent studies are described in which fatigue was delayed and reduced by the use of variable-frequency stimulus trains that elicit the catch-like property of muscle.

Evoked Potentials↗

PubMed1995

Effects of stimulation intensity on the physiological responses of human motor units.

Quadriceps femoris muscles were studied in 50 healthy subjects to determine the physiological responses of the motor units recruited at different force levels during transcutaneous electrical stimulation. During one set of experiments force-frequency relationships were compared at stimulation intensities that produced tetanic contraction of 20%, 50%, or 80% of the maximum voluntary isometric contraction (MVC). No differences in the normalized force-frequency relationship were observed between the 20% and 50% of MVC conditions and only a slight shift to the left was observed at 80% of MVC. The other set of experiments measured the responses to electrically elicited fatigue tests using frequencies of 20, 40, or 60 pps and, at each frequency, intensities that produced 20% or 50% of MVC. Fatigue was greater for the 50% than 20% MVC force conditions. Within each force level fatigue increased with increasing frequency. However, though the differences in the level of recruitment needed to produce the two forces varied for each frequency, the differences in the amount of fatigue produced at each force did not vary between the three stimulation frequencies. This suggests that the fatigue characteristics of the recruited motor units were similar at all intensities tested. We posit, therefore, that the physiological recruitment order during transcutaneous electrical stimulation is less orderly than previously suggested.

Adult↗

PubMed1993

Muscle fatigue: clinical implications for fatigue assessment and neuromuscular electrical stimulation.

Muscle fatigue can be defined as a decrease in the force-generating ability of a muscle that resulted from recent activity. Recent studies of muscle fatigue are reviewed that are relevant to two areas of interest to physical therapists: clinical assessment of muscle fatigue and neuromuscular electrical stimulation. Volitional and electrical tests have been used to quantify muscle fatigue. Several variations on each type of test are discussed, as are the possible sites in which fatigue might occur. The rate of fatigue during the therapeutic application of electrical stimulation of skeletal muscle is much greater than that seen during volitional contractions. Factors contributing to this phenomenon are examined. The unique requirements affecting how stimulus variables can be manipulated to minimize muscle fatigue in three specific therapeutic uses of neuromuscular electrical stimulation are addressed.

Electric Stimulation↗

PubMed1993

Fatigability of human quadriceps femoris muscle following anterior cruciate ligament reconstruction.

The responses of quadriceps femoris muscles to an electrically elicited fatigue test were recorded from both lower extremities of 18 patients who had recently undergone unilateral, anterior cruciate ligament reconstruction. The fatigue test consisted of 40 pps, 13-pulse electrical trains that were repeated once per second for 3 min. The intensity of stimulation was set for each extremity to produce 20% of the maximum voluntary isometric contraction of the uninvolved muscle. The uninvolved quadriceps femoris muscle showed a significantly greater rate of decline in force over the first minute than the involved muscle (0.803%.s-1 for uninvolved muscle vs 0.620%.s-1 for involved muscle). Similarly, the average forces produced over the last minute were significantly lower for the uninvolved than the involved quadriceps femoris muscle (uninvolved = 42.6%, involved = 50.4% of their original forces). These surprising results showed that the involved quadriceps femoris muscles were more endurant than the uninvolved muscles. It is suggested that the increases in endurance of the involved muscle may have been due, in part, to greater recruitment of Type I fibers with electrical stimulation or selective Type II fiber atrophy in the involved muscle.

Adolescent↗

PubMed1992

Changes in the force-frequency relationship of the human quadriceps femoris muscle following electrically and voluntarily induced fatigue.

The purpose of this study was to identify the changes in the force-frequency relationship (FFR) of the human quadriceps femoris muscle following electrically and voluntarily induced fatigue. Twenty nondisabled subjects each participated in one experimental session to test the effects of electrically induced fatigue on the FFR; 10 of these subjects participated in a second session in which voluntarily induced fatigue was produced. Fatigue was induced by having subjects perform repeated, 8-second, isometric contractions followed by 12-second rests until 50% of the initial force was produced. Markedly decreased forces were seen at all frequencies tested following fatigue. Low frequency fatigue was observed following both fatiguing protocols. The frequencies needed to produce near-maximum forces did not shift with fatigue. These results suggest that the most appropriate stimulation frequency to use when activating skeletal muscle depends on both the percentage of tetanic force desired and the fatigue state of the muscle. This study also provides the clinician with data on the FFR of healthy human quadriceps femoris muscle prior to fatigue. [Binder-Macleod SA, McDermond LR. Changes in the force-frequency relationship of the human quadriceps femoris muscle following electrically and voluntarily induced fatigue.

Adult↗

PubMed1992

Force response of rat soleus muscle to variable-frequency train stimulation.

1. The purpose of this study was to study the effects of a high-frequency burst of pulses at the onset of a subtetanic train of pulses on the force output of the rat soleus muscle. 2. The soleus muscle was studied in eight rats deeply anesthetized with urethan. The effects of two-, three-, or four-pulse bursts at the onset of subtetanic trains containing a total of 12 pulses were studied in detail. 3. The results showed that two-pulse bursts at the onset of the train produced approximately 20% augmentation in average force and nearly a 50% reduction in the time required to reach a targeted level of force, compared with a comparable 12-pulse subtetanic constant-frequency train; three- or four- pulse bursts produced progressively less additional improvement. In contrast, the two-pulse bursts produced approximately 13% increase in the force-time integral (Area), the three-pulse burst did not significantly further increase the Area gain, and the use of four-pulse bursts markedly decreased the gain in Area. 4. For all three bursts, the observed force augmentation rapidly declined over the 12-pulse trains. Extrapolation beyond the actual data suggested that the force augmentation should last for between approximately 16 and 19 interpulse intervals. 5. To describe the characteristics of the contractile response of the muscle that explains or predicts the amount of force augmentation seen, we made three measurements of the response to the burst of pulses: 1) the peak force produced by the initial burst of pulses (PeakBURST), 2) the force at the time of arrival of the pulse that followed the burst (CatchBURST), and 3) the rise in force produced by the pulse that followed the burst (PotBURST). Of these three measurements, the CatchBURST was the best predictor of the force augmentation seen. 6. The present results showed 1) the importance of the stimulation pattern on the force output of skeletal muscle; 2) that the force-frequency relationship is multivalued, with force depending on both the stimulation history and stimulation frequency; and 3) that a relatively simple discharge strategy, where each train of pulses begins with one or two brief interpulse interval durations, will produce the maximum force from the muscle and result in a predictable force-frequency relationship.

Animals↗

PubMed1991

Use of a catchlike property of human skeletal muscle to reduce fatigue.

This study compared the force output produced by variable frequency, short-duration trains (VFTs) of electrical pulses with the forces produced by constant frequency, short-duration trains (CFTs). Human quadriceps femoris muscle was stimulated with a 300-msec train of pulses once every second for 180 seconds. Each subject (n = 12) participated in 4 randomly assigned experimental sessions. During 3 sessions, a CFT of 80, 40 or 20 pps was used. During a fourth session, a VFT, which consisted of all 3 of the above frequencies, was used. The force at 100 msec, average force of each contraction and peak force were calculated for every 30th contraction. By the 90th contraction, the force at 100 msec and the average force were significantly greater for the VFT than for each CFT. Thus, the VFT, by using a catchlike property, may provide significant advantages over any CFT when using electrical stimulation for functional electrical stimulation.

Adult↗

PubMed1990

Preservation of force output through progressive reduction of stimulation frequency in human quadriceps femoris muscle.

The purpose of this study was to determine the effects of a reduction in the pulse frequency on the fatigue rate of human quadriceps femoris muscle during intermittent (8-second) contractions. Twelve healthy subjects each participated in two experimental sessions. Thirty cycles (cycle time: 8 seconds "on"/12 seconds "off") were applied during each session. During one session, a frequency of 60 pulses per second (pps) was used for all trains. During the other session, the subjects were stimulated with 60 pps for the first train. The stimulating frequency of each train was then progressively reduced, in 5-pps steps, for contractions 2, 3, 5, 8, 12, and 20. By the fifth contraction, the differences in average force produced by the 60-pps trains and the reduced-frequency trains were significant. The difference between the two conditions increased, with the variable-frequency protocol producing 46% more force than the constant-frequency protocol during the last contraction. These results showed that, compared with a constant pulse frequency, reducing the pulse frequency during a fatiguing contraction can markedly decrease the rate of force fatigue of skeletal muscle. This finding suggests that a variable-frequency protocol, similar to the one used in this may prove to be a more effective pattern of stimulation for activation of skeletal muscle than the traditionally used constant-frequency protocol.

Adult↗

PubMed1989

Force output of cat motor units stimulated with trains of linearly varying frequency.

1. A relation between stimulation frequency and muscle force is usually determined with stimulus trains of constant frequency and described as a single-valued sigmoid curve. This relationship fails to explain a number of features of rate coding. 2. Single motor units were isolated in medial gastrocnemius or soleus muscles of cats deeply anesthetized with pentobarbital sodium. Motor units were classified as fast or slow. Each unit was stimulated with a train whose frequency varied linearly from less than 3 pulses per second (pps) to 20% above the unit's fusion frequency and back to about 3 pps with a period of 5 s. 3. All motor units showed a marked hysteresis during frequency-varying stimulation. A greater force was produced when frequency was decreasing than when it was increasing. The force output of each unit remained nearly maximal as stimulus frequency declined from its maximum to about one-half of the unit's fusion frequency; force rapidly declined with further decreases in frequency. The force-frequency relation could change with each trial as frequency increased but was highly reproducible when frequency decreased. This suggested a strategy by which central nervous system (CNS) control could maximize the force at any discharge rate and produce a predictable force-frequency relation. 4. Posttetanic potentiation, motor unit slowing, and a preload which causes a motor unit to operate on the negatively sloping portion of the length-tension curve may each contribute to the observed hysteresis under certain circumstances. None can explain why hysteresis was consistently seen in all motor units. A time-dependent rate of tension development and decay together with a catchlike property can account for all of the properties of hysteresis and appeared to be the primary cause of hysteresis in fully potentiated motor units.

Animals↗

PubMed1982

Use of the Krusen Limb Load Monitor to quantify temporal and loading measurements of gait.

The purposes of this investigation were to determine whether the temporal and force measurements from the Krusen Limb Load Monitor produced clinically reliable data and to begin identifying the factors that determine the monitor's reliability. Temporal and loading measurements were made from the output of the Krusen Limb Load Monitor and compared to values obtained from a calibrated force platform. Such comparisons were made for 30 steps taken by two subjects on three separate occasions and from the same two subjects plus a third subject for 100 consecutive steps. For most measures, mean values from the limb-load monitor were significantly different from those recorded from the force platform. From a clinical perceptive, however, the range of measures was narrow for the 95 percent confidence level of the observed differences for the temporal components of stance between the limb-load monitor and force platform, with the narrowest range of measures related to the appropriateness of "fit" of the limb-load monitor force plate within the shoe. The loading components of stance showed a relatively wide 95 percent confidence interval that appeared unrelated to fit. Thus, given a "good fitting" force plate insert, the therapist can make clinically meaningful measurements of the temporal components of the stance phase of gait using the limb-load monitor.

Biomechanical Phenomena↗