Identification of intrinsic and reflex contributions to human ankle stiffness dynamics
01 Jun 1997-IEEE Transactions on Biomedical Engineering (IEEE Trans Biomed Eng)-Vol. 44, Iss: 6, pp 493-504
TL;DR: It is concluded that reflex stiffness can be large enough to be important functionally, but that its effects will depend strongly upon the particular conditions.
Abstract: The authors have examined dynamic stiffness at the human ankle using position perturbations which were designed to provide a wide-bandwidth input with low average velocity. A parallel-cascade, nonlinear system identification technique was used to separate overall stiffness into intrinsic and reflex components. Intrinsic stiffness was described by a linear, second-order system similar to that demonstrated previously. Reflex stiffness dynamics were more complex, comprising a delay, a unidirectional rate-sensitive element and then lowpass dynamics. Reflex mechanisms were found to be most important at frequencies of 5-10 Hz. The gain and dynamics of reflex stiffness varied strongly with the parameters of the perturbation, the gain decreasing as the mean velocity of the perturbation increased. Under some conditions, torques generated by reflex mechanisms were of the same magnitude as those from intrinsic mechanisms. It is concluded that reflex stiffness can be large enough to be important functionally, but that its effects will depend strongly upon the particular conditions.
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TL;DR: The literature supports the notion that, although spasticity is multifactorial and neural in origin, significant structural alterations in muscle also occur and an understanding of the specific changes that occur in the muscle and extracellular matrix may facilitate the development of new conservative or surgical therapies for this problem.
Abstract: This review summarizes current information regarding the changes in structure or function that occur in skeletal muscle secondary to spasticity. Most published studies have reported an increase in fiber size variability in spastic muscle. There is no general agreement regarding any shift in fiber type distribution secondary to spasticity. Mechanical studies in whole limbs as well as in isolated single cells support the notion of an intrinsic change in the passive mechanical properties of muscle after spasticity in addition to the more widely reported neural changes that occur. Evidence is presented for changes within both the muscle cell and extracellular matrix that contribute to the overall changes in the tissue. Taken together, the literature supports the notion that, although spasticity is multifactorial and neural in origin, significant structural alterations in muscle also occur. An understanding of the specific changes that occur in the muscle and extracellular matrix may facilitate the development of new conservative or surgical therapies for this problem.
357 citations
TL;DR: The size of the maximum reflex contribution varied widely among subjects, in some it was so small that it would be unlikely to have any functional importance; however, in other subjects, reflex contributions were large enough to play a significant role in the control of posture and movement.
Abstract: A parallel-cascade system identification method was used to identify intrinsic and reflex contributions to dynamic ankle stiffness over a wide range of tonic voluntary contraction levels and ankle positions in healthy human subjects. Intrinsic stiffness dynamics were described well by a linear pathway having elastic, viscous, and inertial properties. A velocity-sensitive pathway comprising a delay, a static non-linearity, resembling a half-wave rectifier, followed by a low-pass filter, described reflex stiffness dynamics. The absolute magnitude of intrinsic and reflex stiffness parameters varied from subject to subject but the relative changes with contraction level and position were consistent. Intrinsic stiffness increased monotonically with contraction level while reflex stiffness was maximal at low contraction levels and then decreased. Intrinsic and reflex stiffness both increased as the ankle was dorsiflexed. As a result, reflex mechanics made their largest relative contributions near the neutral position at low levels of activity. The size of the maximum reflex contribution varied widely among subjects, in some it was so small (ca 1%) that it would be unlikely to have any functional importance; however, in other subjects, reflex contributions were large enough (as high as 55% in one case) to play a significant role in the control of posture and movement. This variability may have arisen because stretch reflexes were not useful for the torque-matching task in these experiments. It will be of interest to examine other tasks where stretch reflexes would have a direct impact on performance.
313 citations
TL;DR: It was concluded that lumbar spine stability increased with increased trunk load magnitude to the extent that this load brought about an increase in trunk muscle activation and suggested that muscle reflex response to sudden loading can augment the lumbary spine stability level achieved immediately prior to the sudden loading event.
270 citations
TL;DR: The nature and origins of the mechanical abnormalities associated with hypertonia are elucidate and a better understanding of its functional and clinical implications is provided.
Abstract: Mechanical changes underlying spastic hypertonia were explored using a parallel cascade system identification technique to evaluate the relative contributions of intrinsic and reflex mechanisms to dynamic ankle stiffness in healthy subjects (controls) and spastic, spinal cord injured (SCI) patients. We examined the modulation of the gain and dynamics of these components with ankle angle for both passive and active conditions. Four main findings emerged. First, intrinsic and reflex stiffness dynamics were qualitatively similar in SCI patients and controls. Intrinsic stiffness dynamics were well modeled by a linear second-order model relating intrinsic torque to joint position, while reflex stiffness dynamics were accurately described by a linear, third-order system relating half-wave rectified velocity to reflex torque. Differences between the two groups were evident in the values of four parameters, the elastic and viscous parameters for intrinsic stiffness and the gain and first-order cut-off frequency for reflex stiffness. Second, reflex stiffness was substantially increased in SCI patients, where it generated as much as 40% of the total torque variance, compared with controls, where reflex contributions never exceeded 7%. Third, differences between SCI patients and controls depended strongly on joint position, becoming larger as the ankle was dorsiflexed. At full plantarflexion, there was no difference between SCI and control subjects; in the mid-range, reflex stiffness was abnormally high in SCI patients; at full dorsiflexion, both reflex and intrinsic stiffness were larger than normal. Fourth, differences between SCI and control subjects were smaller during the active than the passive condition, because intrinsic stiffness increased more in controls than SCI subjects; nevertheless, reflex gain remained abnormally high in SCI patients. These results elucidate the nature and origins of the mechanical abnormalities associated with hypertonia and provide a better understanding of its functional and clinical implications.
226 citations
TL;DR: It is concluded that this new methodology to quantify reflexive feedback gains from the mechanical behavior of the human arm during posture maintenance can offer interesting insights into the ability of the central nervous system to modulate reflexive Feedback gains.
204 citations
References
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31 Jan 1986TL;DR: Numerical Recipes: The Art of Scientific Computing as discussed by the authors is a complete text and reference book on scientific computing with over 100 new routines (now well over 300 in all), plus upgraded versions of many of the original routines, with many new topics presented at the same accessible level.
Abstract: From the Publisher:
This is the revised and greatly expanded Second Edition of the hugely popular Numerical Recipes: The Art of Scientific Computing. The product of a unique collaboration among four leading scientists in academic research and industry, Numerical Recipes is a complete text and reference book on scientific computing. In a self-contained manner it proceeds from mathematical and theoretical considerations to actual practical computer routines. With over 100 new routines (now well over 300 in all), plus upgraded versions of many of the original routines, this book is more than ever the most practical, comprehensive handbook of scientific computing available today. The book retains the informal, easy-to-read style that made the first edition so popular, with many new topics presented at the same accessible level. In addition, some sections of more advanced material have been introduced, set off in small type from the main body of the text. Numerical Recipes is an ideal textbook for scientists and engineers and an indispensable reference for anyone who works in scientific computing. Highlights of the new material include a new chapter on integral equations and inverse methods; multigrid methods for solving partial differential equations; improved random number routines; wavelet transforms; the statistical bootstrap method; a new chapter on "less-numerical" algorithms including compression coding and arbitrary precision arithmetic; band diagonal linear systems; linear algebra on sparse matrices; Cholesky and QR decomposition; calculation of numerical derivatives; Pade approximants, and rational Chebyshev approximation; new special functions; Monte Carlo integration in high-dimensional spaces; globally convergent methods for sets of nonlinear equations; an expanded chapter on fast Fourier methods; spectral analysis on unevenly sampled data; Savitzky-Golay smoothing filters; and two-dimensional Kolmogorov-Smirnoff tests. All this is in addition to material on such basic top
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1,183 citations
"Identification of intrinsic and ref..." refers background or result in this paper
...High frequency, vibratory inputs are well known to inhibit stretch reflexes, both by occlusion of the firing of muscle spindle afferents and by presynaptic inhibition of transmission from the afferents to the motoneurons [ 10 ]....
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...Secondly, the dynamics estimated for the pathway are consistent with the known velocity sensitivity of peripheral reflex mechanisms (e.g., [ 10 ]) and the lowpass characteristics of muscle activation dynamics [21]‐[23]....
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658 citations
"Identification of intrinsic and ref..." refers background in this paper
..., [6]) and, to the extent that they contributed at all, the longer latency components were more significant (sometimes termed the functional stretch reflex [7])....
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TL;DR: The presence of a form of stretch reflex, previously described in the arm by other authors, has been confirmed in the gastrocnemius muscle of the human leg and is referred to in this article as the Functional Stretch Reflex (FSR).
Abstract: 1. The presence of a form of stretch reflex, previously described in the arm by other authors, has been confirmed in the gastrocnemius muscle of the human leg. The electromyographic (e.m.g.) manifestation of this reflex occurred 120 msec (S.E. of mean = 3.5 msec) following a sharply applied, and maintained, dorsiflexing force to the foot. This form of response is referred to in this article as the Functional Stretch Reflex (FSR).2. To determine the contribution of the FSR to the control of normal leg movement, the e.m.g. activity in the above muscle was monitored during single downward steps of 12.7, 25.4 and 38.1 cm and during repetitive, rhythmic, hopping movements on one foot.3. It was found that e.m.g. activity associated with steps to the ground began 141 msec (S.E. of mean = 8.5 msec) before contact with the ground and ended 131 msec (S.E. of mean = 7.6 msec) after contact, when the e.m.g. usually became temporarily inactive.4. It is inferred from these results that the muscular deceleration associated with landing was brought about by the release of a pre-programmed pattern of neuromuscular activity which was inaccessible to reflex activity resulting from the mechanical event of landing, rather than by a stretch reflex.5. It was found that subjects chose their preferred frequency of hopping with great accuracy and consistency. The mean value obtained was 2.06 Hz (S.E. of mean = 0.02 Hz).6. At the preferred frequency, e.m.g. activity began 84 msec (S.E. of mean = 9.6 msec) before and terminated 263 msec (S.E. of mean = 10 msec) after contact with the ground.7. It is inferred that in rhythmical hopping and perhaps also in running, each landing is effected, as in single steps, by a predetermined pattern of neuromuscular activity. However, when hopping at the preferred frequency, the take-off phase of muscular activity is timed to make maximal use of the FSR, i.e. between 120 and 260 msec after initial contact.8. The results emphasize the importance of pre-programming complex muscular contractions suitable for opposing sudden passive stretching forces, and of initiating them prior to the onset of these forces.
559 citations
"Identification of intrinsic and ref..." refers background in this paper
..., [6]) and, to the extent that they contributed at all, the longer latency components were more significant (sometimes termed the functional stretch reflex [7])....
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