Biomedical subjects

R Blickhan

Publications and source records attributed to R Blickhan.

At least 19 recordsLinked to original sources

PubMed2005

Musculoskeletal support of lumbar spine stability.

Using a biomechanical model and experimental data the self-stabilising behaviour of antagonistic trunk muscles was analyzed. The biomechanical model is constituted of a pair of antagonistic Hill-type muscles, their geometric arrangement with respect to the spine, and the instantaneous centre of rotation in frontal plane. Using Ljapunov's theory, the stability of certain motion and loading situations was analyzed. Applying a sensitivity analysis, the influence of different muscle properties and the geometric arrangement on stability was investigated. The simulations revealed that the stability of spinal movements depended primarily on the geometrical arrangement of muscles and the position of the centre of rotation of the spine, the latter was affected in turn by the activities of the profound muscles. To stabilize the situations simulated oblique muscle arrangements were necessary. In order to define an instantaneous centre of rotation in the lower region of the spine negative attachment angles (medio-lateral decline) of muscles were necessary, corresponding to the real anatomy of obliquus externus muscles. More cranially located instantaneous centres of rotation required positive attachment angles for stability, corresponding to obliquus internus or multifidus muscles. Furthermore, the fibre-type distribution of muscles influenced the stability of the system, i.e. a high percentage of fast-twitch-fibres supported the stabilisation. Conclusions drawn from the simulations were supported by experimental data. Sudden loads and quick-release perturbations with two different amplitudes were applied to the upper body of ten male subjects. In comparison to sudden load situations preactivation of muscles due to an external load, i.e. quick-release perturbation, led to significantly less dependency of the amplitude of deflection on the amplitude of the perturbation. This observation relates to the self-stabilising properties of the musculoskeletal system. In conclusion, training seems to be advantageous if directed towards not only enhancing the endurance capacity of the muscles, but also increasing the cross-sectional area of oblique fast-twitch-fibres. Training should also improve the co-ordination of deep and superficial trunk muscles. These findings may influence physiotherapy and training programs for low back pain patients.

Journal Article↗

PubMed2005

ISOFIT: a model-based method to measure muscle-tendon properties simultaneously.

Estimation of muscle parameters specifying force-length and force-velocity behavior requires in general a large number of sophisticated experiments often including a combination of isometric, isokinetic, isotonic, and quick-release experiments. This study validates a simpler method (ISOFIT) to determine muscle properties by fitting a Hill-type muscle model to a set of isovelocity data. Muscle properties resulting from the ISOFIT method agreed well with muscle properties determined separately in in vitro measurements using frog semitendinosus muscles. The force-length curve was described well by the results of the model. The force-velocity curve resulting from the model coincided with the experimentally determined curve above approximately 20% of maximum isometric force (correlation coefficient R>0.99). At lower forces and thus higher velocities the predicted curve underestimated velocity. The stiffness of the series elastic component determined with direct experiments was approximately 10% lower than that determined by the ISOFIT method. Use of the ISOFIT method can decrease experimental time up to 80% and reduce potential changes in muscle parameters due to fatigue.

Algorithms↗

PubMed2001

Stable operation of an elastic three-segment leg.

Quasi-elastic operation of joints in multi-segmented systems as they occur in the legs of humans, animals, and robots requires a careful tuning of leg properties and geometry if catastrophic counteracting operation of the joints is to be avoided. A simple three-segment model has been used to investigate the segmental organization of the leg during repulsive tasks like human running and jumping. The effective operation of the muscles crossing the knee and ankle joints is described in terms of rotational springs. The following issues were addressed in this study: (1) how can the joint torques be controlled to result in a spring-like leg operation? (2) how can rotational stiffnesses be adjusted to leg-segment geometry? and (3) to what extend can unequal segment lengths and orientations be advantageous? It was found that: (1) the three-segment leg tends to become unstable at a certain amount of bending expressed by a counterrotation of the joints; (2) homogeneous bending requires adaptation of the rotational stiffnesses to the outer segment lengths; (3) nonlinear joint torque-displacement behaviour extends the range of stable leg bending and may result in an almost constant leg stiffness; (4) biarticular structures (like human gastrocnemius muscle) and geometrical constraints (like heel strike) support homogeneous bending in both joints; (5) unequal segment lengths enable homogeneous bending if asymmetric nominal angles meet the asymmetry in leg geometry; and (6) a short foot supports the elastic control of almost stretched knee positions. Furthermore, general leg design strategies for animals and robots are discussed with respect to the range of safe leg operation.

Animals↗

PubMed2000

A finite-element model for the mechanical analysis of skeletal muscles.

In the present paper, a finite-element model for simulating muscle mechanics is described. Based on nonlinear continuum mechanics an algorithm is proposed that includes the contractile active and passive properties of skeletal muscle. Stress in the muscle is assumed to result from the superposition of a passive and an active part. The passive properties are described by a hyperelastic constitutive material law whereas the active part depends on the fibre length, shortening velocity and an activation function. The constraint of approximate incompressibility of the muscle element is satisfied as a property of the constitutive equations. Because of the nonlinear behaviour of the material and the highly dynamical performance an incremental procedure including iterative methods is used. The advantage of the model over previous formulations is the possibility to integrate the element into an engineering standard finite-element programme ANSYS using advanced numerical tools. The model allows simulations of muscle recruitment, calculations of stress and strain distributions and predictions of muscle shape. Other possible applications are studies of the muscle architecture, the effect of inertia and impacts. First, simple examples are presented.

Animals↗

PubMed2000

The function of resilin in beetle wings.

This account shows the distribution of elastic elements in hind wings in the scarabaeid Pachnoda marginata and coccinellid Coccinella septempunctata (both Coleoptera). Occurrence of resilin, a rubber-like protein, in some mobile joints together with data on wing unfolding and flight kinematics suggest that resilin in the beetle wing has multiple functions. First, the distribution pattern of resilin in the wing correlates with the particular folding pattern of the wing. Second, our data show that resilin occurs at the places where extra elasticity is needed, for example in wing folds, to prevent material damage during repeated folding and unfolding. Third, resilin provides the wing with elasticity in order to be deformable by aerodynamic forces. This may result in elastic energy storage in the wing.

Animals↗

PubMed2000

Reduced muscle vascular resistance in intrauterine growth restricted newborn piglets.

It has been shown that asymmetrical intrauterine growth restriction is denoted by disproportional reduction of muscle mass compared to body weight reduction. However, the effects of IUGR on regional vascular resistance and blood flow of skeletal muscles and their contractile function have not been studied until now. Therefore, muscle blood flow (MBF) and isometric force output of serial stimulated hindlimb plantar flexors was measured in thiopental -anesthetized normal weight (NW; n = 9) and intrauterine growth restricted (IUGR; n = 9) one-day-old piglets. Additionally, muscle vascular resistance (MVR) and thyroid hormones were estimated. MBF was found to be markedly increased in IUGR piglets by 36% with a concomitant MVR reduction of 37% (p < 0.05). Isometric force of the plantar flexors was considerably higher in NW than in IUGR piglets (p < 0.05). However, amount of muscle fatigue was more pronounced in NW piglets (9.1+/-2.8%) than was in IUGR piglets (3.7+/-2.3%) (p < 0.05). Furthermore, specific tension of NW muscles (18.8+/-0.7 N/cm2) was significantly lower than for IUGR muscles (21.2+/-0.9 N/cm2) (P<0.05). IUGR newborn piglets exhibited increased plasma levels of thyroxine (T4) (p < 0.05), whereas triiodothyronine (T3) showed similar values in both animal groups. These data clearly indicate that muscle hemodynamics and contractile function are more developed in newborn IUGR piglets. Furthermore it is suggested that the improved tolerance to fatigue during isometric contractions may indicate an increased oxidative capacity of calf muscles due to intrauterine growth restriction.

Animals↗

PubMed2000

Accelerated contractile function and improved fatigue resistance of calf muscles in newborn piglets with IUGR.

Asymmetrical intrauterine growth restriction is denoted by disproportional reduction of muscle mass compared with body weight reduction. However, effects on contractile function or tissue development of skeletal muscles were not studied until now. Therefore, isometric force output of serial-stimulated hindlimb plantar flexors was measured in thiopental-anesthetized normal weight (NW) and intrauterine growth-restricted (IUGR) 1-day-old piglets under conditions of normal, reduced (aortic cross clamping), and reestablished (clamp release) blood supply (measured by colored microspheres technique). Furthermore, muscle fiber type distribution was determined after histochemical staining, specific muscle force of the plantar flexors [quotient from absolute force divided by muscle mass (N/g)] was calculated, and glycogen content and morphometric data of the investigated muscles were estimated. Regional blood flow of hindlimb muscles was similar in NW (6 +/- 2 ml. min(-1). 100 g(-1)) and IUGR piglets (8 +/- 1 ml. min(-1). 100 g(-1)). Isometric muscle contractions induced a marked increase in regional blood flow of 4.1-fold in NW and 5-fold in stimulated hindlimb muscles of IUGR piglets (baseline blood flow). Specific force of NW piglet muscles (5.2 +/- 0.2 N/g) was significantly lower than IUGR piglet muscles (6.1 +/- 0.6 N/g; P < 0.05). Isometric muscle contractions (NW: 32.7 +/- 4.7 N; IUGR: 21.7 +/- 4.0 N) resulted in a higher rate of force decrease in the calf muscles of NW animals compared with IUGR piglets (8 +/- 2 vs. 3 +/- 1%; P < 0. 01). Functional restoration of contractile performance after hindlimb recirculation was nearly complete in IUGR piglets (98 +/- 1%), whereas in NW piglets a deficit of 9 +/- 3% was found (P < 0. 01). Muscle fiber type estimation revealed an increased proportion of type I fibers in flexor digitalis superficialis and gastrocnemius medialis in IUGR piglets (P < 0.05). These data clearly indicate that contractile function is accelerated in newborn IUGR piglets.

Animals↗

PubMed2000

Optimum take-off techniques and muscle design for long jump.

A two-segment model based on Alexander (1990; Phil. Trans. R. Soc. Lond. B 329, 3-10) was used to investigate the action of knee extensor muscles during long jumps. A more realistic representation of the muscle and tendon properties than implemented previously was necessary to demonstrate the advantages of eccentric force enhancement and non-linear tendon properties. During the take-off phase of the long jump, highly stretched leg extensor muscles are able to generate the required vertical momentum. Thereby, serially arranged elastic structures may increase the duration of muscle lengthening and dissipative operation, resulting in an enhanced force generation of the muscle-tendon complex. To obtain maximum performance, athletes run at maximum speed and have a net loss in mechanical energy during the take-off phase. The positive work done by the concentrically operating muscle is clearly less than the work done by the surrounding system on the muscle during the eccentric phase. Jumping performance was insensitive to changes in tendon compliance and muscle speed, but was greatly influenced by muscle strength and eccentric force enhancement. In agreement with a variety of experimental jumping performances, the optimal jumping technique (angle of attack) was insensitive to the approach speed and to muscle properties (muscle mass, the ratio of muscle fibre to tendon cross-sectional area, relative length of fibres and tendon). The muscle properties also restrict the predicted range of the angle of the velocity vector at take-off.

Biomechanical Phenomena↗

PubMed1999

Stabilizing function of skeletal muscles: an analytical investigation.

Stability is the ability of a system to return to its original state after a disturbance. Taking vertical oscillations of the centre of mass of a human bending his legs as an example we prove that the intrinsic mechanical properties of musculature can stabilize the oscillatory movement (preflex) without reflexive changes in activation. The human is represented by a model consisting of a massless two-segment linkage system (knee) topped by a point mass. Conditions for stability are calculated analytically based on the theory of Ljapunov and the results are illustrated by numerical examples. In order to guarantee a self-stabilizing ability of the muscle-skeletal system, the muscle properties such as force-length relationship, force-velocity relationship and the muscle geometry must be tuned to the geometric properties of the linkage system.

Animals↗

PubMed1999

Dynamics of the long jump.

A mechanical model is proposed which quantitatively describes the dynamics of the centre of gravity (c.g.) during the take-off phase of the long jump. The model entails a minimal but necessary number of components: a linear leg spring with the ability of lengthening to describe the active peak of the force time curve and a distal mass coupled with nonlinear visco-elastic elements to describe the passive peak. The influence of the positions and velocities of the supported body and the jumper's leg as well as of systemic parameters such as leg stiffness and mass distribution on the jumping distance were investigated. Techniques for optimum operation are identified: (1) There is a minimum stiffness for optimum performance. Further increase of the stiffness does not lead to longer jumps. (2) For any given stiffness there is always an optimum angle of attack. (3) The same distance can be achieved by different techniques. (4) The losses due to deceleration of the supporting leg do not result in reduced jumping distance as this deceleration results in a higher vertical momentum. (5) Thus, increasing the touch-down velocity of the jumper's supporting leg increases jumping distance.

Biomechanical Phenomena↗

PubMed1994

NOTE ON THE CALCULATION OF PROPELLER EFFICIENCY USING ELONGATED BODY THEORY

The elongated body theory has been widely used for calculations of the hydrodynamic propulsive performance of swimming fish. In the biological literature, terms containing the slope of the amplitude function at the tail end have been neglected in the calculations of thrust and efficiency, and a slope of zero has been assumed. However, some fishes, such as saithe and trout, have non-zero values of the slope near the tail end and, when this term is taken into account, the efficiency may be reduced by as much as 20 % and approaches the result given by the three-dimensional waving plate theory. The inclusion of the slope in the efficiency considerations results in an optimum ratio of the swimming speed to the wave speed that is clearly less than 1. It is suggested that the slope terms should be included in the estimation of propulsive performance for fish swimming with variable amplitude.

Journal Article↗

PubMed1994

Dynamic and static stability in hexapedal runners.

Stability is fundamental to the performance of terrestrial locomotion. Running cockroaches met the criteria for static stability over a wide range of speeds, yet several locomotor variables changed in a way that revealed an increase in the importance of dynamic stability as speed increased. Duty factors (the fraction of time that a leg spends on the ground relative to the stride period) decreased to 0.5 and below with an increase in speed. The duration of double support (i.e. when both tripods, or all six legs, were on the ground) decreased significantly with an increase in speed. All legs had similar touch-down phases in the tripod, but the shortest leg, the front one, lifted off before the middle and the rear leg, so that only two legs of the tripod were in contact with the ground at the highest speeds. Per cent stability margin (the shortest distance from the center of gravity to the boundaries of support, normalized to the maximum possible stability margin) decreased with increasing speed from 60% at 10 cms-1 to values less than zero at speeds faster than 50 cms-1, indicating instances of static instability at fast speeds. The center of mass moved rearward or posteriorly with respect to the base of support as speed increased. Moments about the center of mass, as shown by the center of pressure (the equivalent of a single 'effective' leg), were variable, but were balanced by opposing moments over a stride. Thus, hexapods can exploit the advantages of both static and dynamic stability. Static or quasi-static assumptions alone were insufficient to explain straight-ahead, constant-speed locomotion and may hinder discovery of behaviors that are dynamic, where kinetic energy and momentum can act as a bridge from one step to the next.

Animals↗

PubMed1991

The time course and frequency content of hydrodynamic events caused by moving fish, frogs, and crustaceans.

In the present study the time course and spectral-amplitude distribution of hydrodynamic flow fields caused by moving fish, frogs, and crustaceans were investigated with the aid of laser-Doppler-anemometry. In the vicinity of a hovering fish sinusoidal water movements can be recorded whose velocity spectra peak below 10 Hz. Single strokes during startle responses or during steady swimming of fish, frogs, and crustaceans cause short-lasting, low-frequency (less than 10 Hz), transient water movements. Low-frequency transients also occur if a frog approaches and passes a velocity-sensitive hydrodynamic sensor. In contrast, transient water movements caused by a rapidly struggling or startled fish or water motions measured in the wake of a slowly swimming (less than or equal to 47 cm/s) trout can be broadbanded, i.e., these water movements can contain frequency components up to at least 100 Hz. High-frequency hydrodynamic events can also be measured behind obstacles submerged in running water. The possible biological advantage of the ability to detect high-frequency hydroynamic events is discussed with respect to the natural occurrence of high frequencies and its potential role in orientation and predator-prey interactions of aquatic animals.

Animals↗

PubMed1991

Hopping frequency in humans: a test of how springs set stride frequency in bouncing gaits.

The storage and recovery of elastic energy in muscle-tendon springs is important in running, hopping, trotting, and galloping. We hypothesized that animals select the stride frequency at which they behave most like simple spring-mass systems. If higher or lower frequencies are used, they will not behave like simple spring-mass systems, and the storage and recovery of elastic energy will be reduced. We tested the hypothesis by having humans hop forward on a treadmill over a range of speeds and hop in place over a range of frequencies. The body was modeled as a simple spring-mass system, and the properties of the spring were measured by use of a force platform. Our subjects used nearly the same frequency (the "preferred frequency," 2.2 hops/s) when they hopped forward on a treadmill and when they hopped in place. At this frequency, the body behaved like a simple spring-mass system. Contrary to our predictions, it also behaved like a simple spring-mass system when the subjects hopped at higher frequencies, up to the maximum they could achieve. However, at the higher frequencies, the time available to apply force to the ground (the ground contact time) was shorter, perhaps resulting in a higher cost of generating muscular force. At frequencies below the preferred frequency, as predicted by the hypothesis, the body did not behave in a springlike manner, and it appeared likely that the storage and recovery of elastic energy was reduced. The combination of springlike behavior and a long ground contact time at the preferred frequency should minimize the cost of generating muscular force.

Adult↗

PubMed1991

Leg design in hexapedal runners.

Many-legged animals, such as crabs and cockroaches, utilize whole-body mechanics similar to that observed for running bipeds and trotting quadrupedal mammals. Despite the diversity in morphology, two legs in a quadrupedal mammal, three legs in an insect and four legs in a crab can function in the same way as one leg of a biped during ground contact. To explain how diverse leg designs can result in common whole-body dynamics, we used a miniature force platform to measure the ground reaction forces produced by individual legs of the cockroach Blaberus discoidalis. Hexapedal runners were not like quadrupeds with an additional set of legs. In trotting quadrupedal mammals each leg develops a similar ground reaction force pattern that sums to produce the whole-body pattern. At a constant average velocity, each leg pair of the cockroach was characterized by a unique ground reaction force pattern. The first leg decelerated the center of mass in the horizontal direction, whereas the third leg was used to accelerate the body. The second leg did both, much like legs in bipedal runners and quadrupedal trotters. Vertical force peaks for each leg were equal in magnitude. In general, peak ground reaction force vectors minimized joint moments and muscle forces by being oriented towards the coxal joints, which articulate with the body. Locomotion with a sprawled posture does not necessarily result in large moments around joints. Calculations on B. discoidalis showed that deviations from the minimum moments may be explained by considering the minimization of the summed muscle forces in more than one leg. Production of horizontal forces that account for most of the mechanical energy generated during locomotion can actually reduce total muscle force by directing the ground reaction forces through the leg joints. Whole-body dynamics common to two-, four-, six- and eight-legged runners is produced in six-legged runners by three pairs of legs that differ in orientation with respect to the body, generate unique ground reaction force patterns, but combine to function in the same way as one leg of a biped.

Animals↗

PubMed1989

The spring-mass model for running and hopping.

A simple spring-mass model consisting of a massless spring attached to a point mass describes the interdependency of mechanical parameters characterizing running and hopping of humans as a function of speed. The bouncing mechanism itself results in a confinement of the free parameter space where solutions can be found. In particular, bouncing frequency and vertical displacement are closely related. Only a few parameters, such as the vector of the specific landing velocity and the specific leg length, are sufficient to determine the point of operation of the system. There are more physiological constraints than independent parameters. As constraints limit the parameter space where hopping is possible, they must be tuned to each other in order to allow for hopping at all. Within the range of physiologically possible hopping frequencies, a human hopper selects a frequency where the largest amount of energy can be delivered and still be stored elastically. During running and hopping animals use flat angles of the landing velocity resulting in maximum contact length. In this situation ground reaction force is proportional to specific contact time and total displacement is proportional to the square of the step duration. Contact time and hopping frequency are not simply determined by the natural frequency of the spring-mass system, but are influenced largely by the vector of the landing velocity. Differences in the aerial phase or in the angle of the landing velocity result in the different kinematic and dynamic patterns observed during running and hopping. Despite these differences, the model predicts the mass specific energy fluctuations of the center of mass per distance to be similar for runners and hoppers and similar to empirical data obtained for animals of various size.

Animals↗

PubMed1988

Muscle forces during locomotion in kangaroo rats: force platform and tendon buckle measurements compared.

The muscle forces and stresses occurring during normal locomotor activity in kangaroo rats are compared with the peak isometric force developed by the same muscles in situ. Two methods were used simultaneously to determine the stresses (force/cross-sectional area) acting in the ankle extensors during steady-speed hopping and during jumps when animals were startled: a direct measurement using a force buckle surgically implanted around a tendon; and an indirect measurement using a force platform/ciné analysis technique. We obtained essentially the same values with the two techniques. We found that at slow speeds (0.7 m s-1) the ankle extensor muscles of kangaroo rats exerted 20% of the maximum isometric force developed when the muscles were stimulated via the tibial nerve. This increased to 53% at higher speeds (1.9 m s-1). At the animals's preferred hopping speed (1.5 m s-1), peak force was approximately 40% of maximum isometric force. In jumps when animals were startled, peak forces as high as 175% of the maximal elicited isometric force were recorded. These high forces always occurred when the muscles were being stretched. It appears that kangaroo rats utilize nearly the entire range of muscle force possible during normal locomotor events (i.e. up to 175% of maximum isometric force when muscles are stretched).

Animals↗

PubMed1988

Preferred speeds in terrestrial vertebrates: are they equivalent?

Terrestrial animals have 'preferred speeds' within each gait, that are used much more frequently than others for moving along the ground. Energy costs reach minimal values at these speeds within each gait. In this study we asked whether these speeds are mechanically equivalent among different animals (i.e. speeds where the same levels of peak muscle stress occur). If so, this would help in establishing a link between the energetics and the mechanics of the active muscles at these speeds, providing a first step in understanding why energy costs are minimal. We also asked whether peak muscle stress reaches a similar fraction of the maximal isometric stress at these speeds. If so, this would suggest that muscles are structured so that a similar reserve capacity remains, with a similar safety factor for avoidance of injury in response to prolonged repetitive loading. We compared two species that use quite different locomotory methods at their preferred speeds: white rats that gallop and kangaroo rats that hop. We measured peak stress in the ankle extensor muscles of these two species, as they moved at their preferred speeds, using a force platform/ciné analysis technique. We also measured the maximum isometric force that this muscle group could develop in situ in the same individuals. We found the ankle extensors of white rats and kangaroo rats developed virtually identical levels of peak stress at their preferred speeds (70 +/- 6 kPa and 69 +/- 6 kPa, respectively, mean +/- S.E.), despite a fourfold difference in peak ground reaction force per unit body mass exerted on each limb. The values of peak isometric stress in situ were also virtually identical (206 +/- 17 kPa and 200 +/- 9 kPa, respectively). Our finding that the peak muscle stress is about one-third of maximum isometric stress at the preferred speeds is consistent with the idea that these are mechanically equivalent speeds, where the same fraction of available muscle fibres is recruited. Finding nearly identical values in two species that move in such different ways (galloping vs hopping), and have such large differences in ground reaction force exerted by each limb, suggests this may be true more generally for terrestrial vertebrates.

Animals↗