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Biomedical subjects

F E Zajac

Publications and source records attributed to F E Zajac.

At least 55 records · Page 3Linked to original sources
PubMed1990

Restoring unassisted natural gait to paraplegics via functional neuromuscular stimulation: a computer simulation study.

Functional neuromuscular stimulation (FNS) of paralyzed muscles has enabled spinal-cord-injured patients to regain a semblance of lower-extremity control, for example to ambulate while relying heavily on the use of walkers. Given the limitations of FNS, specifically low muscle strengths, high rates of fatigue, and a limited ability to modulate muscle excitations, it remains unclear, however, whether FNS can be developed as a practical means to control the lower extremity musculature to restore aesthetic, unsupported gait to paraplegics. A computer simulation of FNS-assisted bipedal gait shows that it is difficult, but possible to attain undisturbed, level gait at normal speeds provided the electrically-stimulated ankle plantarflexors exhibit either near-normal strengths or are augmented by an orthosis, and at least seven muscle-groups in each leg are stimulated. A combination of dynamic programming and an open-loop, trial-and-error adjustment process was used to find a suboptimal set of discretely-varying muscle stimulation patterns needed for a 3-D, 8 degree-of-freedom dynamic model to sustain a step. An ankle-foot orthosis was found to be especially useful, as it helped to stabilize the stance leg and simplified the task of controlling the foot during swing. It is believed that the process of simulating natural gait with this model will serve to highlight difficulties to be expected during laboratory and clinical trials.

Computer Simulation↗
PubMed1990

Biomechanical analysis of the Chiari pelvic osteotomy. Preserving hip abductor strength.

Although the Chiari osteotomy is usually effective in reducing pain, many patients are left with a long-term limp. The postoperative limp can at times be caused by hip abductors that have strength insufficient to counteract the torque from body weight during single-leg stance. To study how the surgical technique affects the hip abductor muscles, a biomechanical model was developed that computes the postsurgery pelvic geometry and the resulting hip abductor torque given three surgical parameters: angulation of the osteotomy, distance of medical displacement, and angle of internal rotation. The computer simulations of the Chiari osteotomy showed that some sets of surgical parameters conserve abductor torque while others greatly reduce it. Simulated surgeries with high angulation and large medial displacement reduce gluteus medius abductor torque by up to 65%. Therefore, this combination of surgical parameters may account for some instances of the postoperative limp. In the model, high angulation reduces the length of the gluteus medius and is the primary cause of reduced abductor strength. Simulated horizontal osteotomies (0 degrees to 10 degrees) were found to best conserve both muscle length and abductor torque.

Biomechanical Phenomena↗
PubMed1989

A planar model of the knee joint to characterize the knee extensor mechanism.

A simple planar static model of the knee joint was developed to calculate effective moment arms for the quadriceps muscle. A pathway for the instantaneous center of rotation was chosen that gives realistic orientations of the femur relative to the tibia. Using the model, nonlinear force and moment equilibrium equations were solved at one degree increments for knee flexion angles from 0 (full extension) to 90 degrees, yielding patellar orientation, patellofemoral contact force and patellar ligament force and direction with respect to both the tibial insertion point and the tibiofemoral contact point. The computer-derived results from this two-dimensional model agree with results from more complex models developed previously from experimentally obtained data. Due to our model's simplicity, however, the operation of the patellar mechanism as a lever as well as a spacer is clearly illustrated. Specifically, the thickness of the patella was found to increase the effective moment arm significantly only at flexions below 35 degrees even though the actual moment arm exhibited an increase throughout the flexion range. Lengthening either the patella or the patellar ligament altered the force transmitted from the quadriceps to the patellar ligament, significantly increasing the effective moment arm at flexions greater than 25 degrees. We conclude that the levering action of the patella is an essential mechanism of knee joint operation at moderate to high flexion angles.

Biomechanical Phenomena↗
PubMed1989

Paraplegic standing controlled by functional neuromuscular stimulation: Part I--computer model and control-system design.

We have developed a planar computer model to investigate paraplegic standing induced by functional neuromuscular stimulation. The model consists of nonlinear musculotendon dynamics (pulse train activation dynamics and musculotendon actuator dynamics), nonlinear body-segmental dynamics, and a linear output-feedback control law. The model of activation dynamics is an analytic expression that characterizes the relation between the stimulus parameters (pulse width and interpulse interval) and the muscle activation. Hill's classic two-element muscle model was modified into a musculotendon actuator model in order to account for the effects of submaximal activation and tendon elasticity on development of force by the actuator. The three body-segmental, multijoint model accounts for the anterior-posterior movements of the head and trunk, the thigh, and the shank. We modeled arm movement as an external disturbance and imposed the disturbance to the body-segmental dynamics by means of a quasistatic analysis. Linearization, and at times linear approximation of the computer model, enabled us to compute a constant, linear feedback-gain matrix, whose output is the net activation needed by a dynamical joint-torque actuator. Motivated by an assumption that minimization of energy expenditure lessens muscle fatigue, we developed an algorithm that then computes how to distribute the net activation among all the muscles crossing the joint. In part II, the combined feedback control strategy is applied to the nonlinear model of musculotendon and body-segmental dynamics to study how well the body ought to maintain balance should the feedback control strategy be employed.

Biomechanical Phenomena↗
PubMed1989

Paraplegic standing controlled by functional neuromuscular stimulation: Part II--Computer simulation studies.

We simulated two types of body motion. First, the body position is assumed to be initially perturbed from the upright position, and all muscles are assumed inactive at the initial position. The control law developed in the preceding paper drives the body segments to the standing position. Arm movements are then applied to the body to investigate how performance is affected by an external disturbance. Simulated body motion indicated that the current output-feedback control law functions well. The body can recover upright posture from a highly flexed position, and the controller can then maintain the body near the vertical during arm movements. The simulation results showed three consistent activation patterns based on energy minimization: 1) no antagonistic muscle pairs are coactivated, 2) strong muscles are recruited before weak ones, and 3) fast muscles are recruited before slow ones. The reason for the second and third observations is that energy liberation rate depends heavily on the relative amount of muscle activation. Since the current control law requires muscles to generate specific joint torques at a prescribed time, strong muscles do not have to be activated as much as weak ones, and recruiting a fast muscle at low activation level consumes less energy than recruiting a slow one at high activation level. Although the output-feedback control law functions well according to our simulation results, the static optimization process would, in practice, take too much computational time to make it practical. Based on the consistent activation patterns found in our simulations, we therefore developed a simpler (suboptimal) activation-distribution scheme that takes much less time and still gives nearly identical performance.

Biomechanical Phenomena↗
PubMed1989

Muscle and tendon: properties, models, scaling, and application to biomechanics and motor control.

Skeletal muscles transform neural control signals into forces that act upon the body segments to effect a coordinated motor task. This transformation is complex, not only because the properties of muscles are complex, but because the tendon affects the transmission of muscle force to the skeleton. This review focuses on how to synthesize basic properties of muscle and tendon to construct models applicable to studies of coordination. After a review of the properties of muscle and tendon, their integrated ability to generate force statically and dynamically is studied by formulating a generic model of the "musculotendon actuator", which has only one parameter, the ratio of tendon length at rest to muscle fiber length at rest. To illustrate the utility of the model, it is analyzed to show how this one parameter specifies whether excitation-contraction or musculotendon contraction is the rate-limiting process of force generation, whether elastic energy is stored in tendon or muscle, and whether hip- and knee-extensor actuators function as springs or dashpots during walking.

Biomechanical Phenomena↗
PubMed1988

A mechanically decoupled two force component bicycle pedal dynamometer.

A design is presented for a bicycle pedal dynamometer that measures both normal and tangential forces (i.e. driving forces). Mechanical decoupling is used to reduce the cross-sensitivity of the dynamometer to loads doing no work to propel the bicycle. This obviates the need to measure all six loads for accurate data reduction. A compact strain ring is the transducer element, and a monolithic design eliminates mechanical hysteresis between the strain ring and the dynamometer frame. The angular orientation of the dynamometer with respect to the crank arm is determined with a continuous-rotation potentiometer. Design criteria and design implementation are discussed, sample data are presented, and the performance of the dynamometer is evaluated.

Biomechanical Phenomena↗
PubMed1985

Thigh muscle activity during maximum-height jumps by cats.

Cats were trained to jump from a force plate and touch a cotton ball suspended as high as 1.6 m. Force-plate reaction forces and double-joint hamstring muscle activity observed early in propulsion varied from one maximal jump to another. This variability is consistent with theory (31, 32, 42); that is, different coordination strategies can be implemented prior to the heels losing contact with the force plate (heel-off). Single-joint hip extensor and double-joint posterior thigh (hip extensor-knee flexor) muscles were coactivated prior to heel-off. This coactivation is probably partially responsible for the observed backward rotation of the trunk. Forepaws, observed to contact the force plate prior to heel-off, probably assist the hindlimbs in generating trunk rotation. Both single-joint knee extensor and hip extensor muscles exhibited greatest activation between heel-off and body lift-off. Single-joint flexor muscles were inactive throughout propulsion. Double-joint posterior thigh muscles were deactivated at heel-off and remained inactivated until lift-off. These observations agree with the theoretical notion that muscles should be either fully activated, inactivated, or switched from one extreme to the other (i.e., bang-bang control) between heel-off and body lift-off (31, 32, 42, 44). All seven muscles studied shortened while activated. Using computations based on muscle geometry, fiber architecture, and joint angle trajectories, I propose that sarcomeres shorten along the flat and ascending regions of the force-length curve. De- and inactivation of double-joint posterior thigh muscles between heel-off and lift-off coincided with muscle stretch. The reason for inactivation of these muscles is that the negative work that would have been generated had these muscles stayed activated would have hindered propulsion. Contractions preceded by active stretch were not observed. Enhancement of positive work by previous storage of energy in elastic musculotendinous structures is thus not used by cat thigh musculature in jumps starting from the squat. Adductor femoris, semimembranosus anterior, and biceps femoris anterior muscles were activated synergistically as one group yet differently from the synergistic activation of gracilis, semitendinosus, and biceps femoris posterior muscles. The separation of these muscles into two groups based on their activation patterns during jumping is compatible with the classification of these muscles into hip extensor and knee flexor muscle groups, respectively, based on their reflex patterns (37), spinal cord reflex connectivity (18, 30), and firing patterns during locomotion (20).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

Relationship among recruitment order, axonal conduction velocity, and muscle-unit properties of type-identified motor units in cat plantaris muscle.

A strict interpretation of the size-principle hypothesis (37, 39-41) is that a muscle's motor units should be recruited in an ascending order according to both the size of their motoneurons and the size of their innervated muscle units (for reviews see Refs. 9, 39, 73). Studies of large mixed muscles in the cat hindlimb, however, have shown that motor axonal conduction velocity and tetanic tension, which are frequently considered indices of motoneuron and muscle-unit size, respectively, are uncorrelated in the fast-twitch (type F) motor-unit subpopulation (12, 13, 23, 24, 30, 32, 63, 71, 79). Attempting to focus on type F units, we compared the recruitment order of 42 pairs of cat plantaris (PL) motor units with both axonal conduction velocity and tetanic tension as well as with other muscle-unit properties. Single PL alpha-motor axons were functionally isolated in intact L7 ventral root filaments of decerebrate cats. Tension responses produced by stimulating each isolated motor axon were used to find the tetanic tension of the muscle unit and to classify the unit (12) as either type S (slow twitch, fatigue resistant), type FR (fast twitch, fatigue resistant), type FI (fast twitch, intermediate fatigability), or type FF (fast twitch, highly fatigable). Conduction velocity of each isolated axon was computed from measurements of axonal conduction time and length. The axon's reflex discharges were recorded from the proximal end of the cut filament and compared with the discharges of another PL axon residing in a different, previously cut filament of the same cat. The recruitment order of each motor-unit pair studied was found during reflexes elicited by homonymous muscle stretch, tendon taps, or single shocks at group I intensity to the PL nerve. If either axon of the pair failed to discharge, as often was the case with the high-threshold type F units, the monosynaptic reflex was facilitated by a 500-pps conditioning train applied proximal to a complete reversible cooling block of the PL nerve. In all 42 pairs studied, the weaker motor unit had the lower functional threshold for recruitment. Recruitment also invariably followed the order S greater than FR greater than FI greater than FF units. The motor unit of a pair with the higher resistance to fatigue thus always had the lower functional threshold. In 21 of the 22 pairs containing at least one type S motor unit, the unit with the slower-conducting motor axon had the lower functional threshold for recruitment.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Kinematic and EMG reactions to imposed interlimb phase alterations during bipedal cycling.

Strong 'neural constraints' synchronizing the step cycles of bilaterally paired limbs in decerebrate cats have been revealed by forcing those limbs to step at different speeds. To determine if humans operate under similar restrictions, we studied subjects pedaling a bicycle ergometer whose coupled cranks imposed continuous change upon interlimb phasing. Unlike the cat, the EMG timing patterns of each leg proved to vary little as a function of phase. Phase change did induce fluctuations in average EMG signal energy, but these were highly idiosyncratic to each muscle. Nevertheless, leg kinematic trajectories remained confined to narrow bounds well within biomechanical limitations. We thus suggest that the trajectory of each leg is actively regulated during pedaling, although the interlimb neural constraints apparent in decerebrate stepping are absent.

Adult↗
PubMed1984

Dependence of jumping performance on muscle properties when humans use only calf muscles for propulsion.

Using optimal control techniques, maximum height jumps were simulated for humans who held their body rigid except for the ankle. Three dynamic models of ankle torque generation based on known calf muscle properties were used. Force and kinematics obtained from the simulations using nominal and perturbed parameters were compared with data obtained from humans who had performed this type of jump. One torque model incorporated the series elastic, force-length and force-velocity properties of muscle. Our results suggest that higher jumps would be achieved by those who have the most compliant and fastest contracting muscles. It was also found that height attained depended much more on the ability of muscles to generate isometric force at long lengths than at short lengths. Studies of forward and strictly vertical jumps using similar computer methods suggest that for any maximal jump the optimal strategy is first to achieve a unique state (position, velocity and acceleration) with the feet flat on the ground, and then to maximally activate one's calf muscles until lift-off.

Adult↗
PubMed1981

Hindlimb muscular activity, kinetics and kinematics of cats jumping to their maximum achievable heights.

Cats were trained to jump from a force platform to their maximum achievable heights. Vertical ground reaction forces developed by individual hindlimbs showed that the propulsion phase consists of two epochs. During the initial "preparatory phase' the cat can traverse many different paths. Irrespective of the path traversed, however, the cat always attains the same position, velocity and momentum at the end of this phase. Starting from this dynamic state the cat during the subsequent "launching phase' (about 150 ms long) generates significant propulsion as its hindlimbs develop force with identical, stereotypic profiles. Cinematographic data, electromyographic data, and computed torques about the hip, knee and ankle joints indicate that during the jump proximal extensor musculature is activated before distal musculature. During terminal experiments when the hindlimb was set at positions corresponding to those in the jump, isometric torques produced by tetanic stimulation of groups of extensor and flexor muscles were compared with computed torques developed by the same cat during previous jumps. These comparisons suggest that extensor muscles of the hindlimb are fully activated during the maximal vertical jump.

Animals↗
PubMed1976

The effect of activation history on tension production by individual muscle units.

Isometric responses of individual gastrocnemius muscle units of both fast (F) and slow (S) twitch type were studied during repetitive intracellular stimulation of the innervating motoneurons in anesthetized cats. Paired stimuli usually produced maximum responses (measured in terms of tension-time area) with interpulse intervals between 5 and 10 msec. However, longer pulse trains produced maximum tension-time area per pulse with relatively long interpulse intervals (referred to as the optimum interval). For each unit, stimulus intervals around the optimum produced maximum modulation of tension during unfused tetani. Post-activation potentiation produced shifts in the optimum toward longer intervals. The optimum intervals for type F units were shorter (mean 49.4 msec) than those for type S units (mean 86.9 msec), but there was no difference between the groups when the mean intervals were expressed in ratio with the twitch contraction time, Tc (F units: 1.42 X Tc; S units: 1.31 X Tc). The range of optimum intervals for all gastrocnemius muscle units corresponded to motoneuron firing frequencies of 7-40 pulses/sec, well within the frequency ranges observed for gastrocnemius motoneurons firing under natural synaptic drive. Insertion of a single short interval (5--10 msec) at the onset of a relatively low frequency stimulus train produced a prolonged 'catch-like' enhancement of tension output, particularly in type S units. The duration of the catch-like effect depended on the interval between pulses in the underlying basic train, and was maximum for intervals between 1 and 2 X Tc. The catch-like effect was also seen in type F units but its duration was curtailed by another process called the 'sag' property. The 'sag' property occurred in type F units over a wide range of stimulus intervals (usually 0.5-3 X Tc) but was present in type S units only with relatively long intervals (greater than 2 X Tc). The 'sag' property appears to represent a disenhancement process intrinsic to muscle unit fibers activated in unfused tetani.

Animals↗