Biomedical subjects
Publications and source records attributed to T Sinkjaer.
Stretching is a regular item in training programmes and it is employed to increase flexibility, to prevent muscle tenderness, overuse injuries and strains. Stretching is carried out by stretching muscles and connective tissue to the extreme positions of joints. A review of stretching exercises based on the literature is presented in relation to prevention of injuries. The various techniques employed are reviewed.
The EMG response and the mechanical response to 2 degree stretch of the human anterior tibial muscle was studied during contractions ranging from 0% to 80% of maximal voluntary contraction (MVC). The EMG response showed three distinct peaks M1, M2, and M3 with peak latencies of 59 ms, 86 ms, and 120 ms respectively. At low background torques M1 dominated while M2 and M3 were small or absent. M2 and M3 dominated above 40% of MVC and M2 in particular showed "automatic gain compensation", i.e. it constituted a - more or less - constant proportion of the background EMG for all contraction levels. The ratio between M1 amplitude and background EMG steadily decreased with contraction level. Even though the summed contributions of M1, M2, and M3 to some degree showed automatic gain compensation, this was not the case for the mechanical response to stretch. Between 0% and 30% of MVC the reflex mediated mechanical response increased approximately in proportion to the contraction level, but the reflex mediated mechanical response peaked at 40% of MVC and declined to zero at 80% of MVC. This discrepancy between EMG and mechanical response was explained by a simple model. The regression line between rectified and filtered tibialis anterior EMG and torque was used to predict the mechanical response from the EMG response. At increasing contraction levels the twitch elicited by supramaximal electrical stimulation decreases, and we reduced the predicted mechanical response by the same factor as the twitch.(ABSTRACT TRUNCATED AT 250 WORDS)
Viscous/plastic properties were investigated in the passive tissue opposing dorsiflexion of the human ankle. The foot was rotated (dorsiflexed) and fixed. Due to viscous/plastic properties, the passive torque declined with time. After 300 s, torque, relative to the initial torque at 0 s, had fallen by 22.7% +/- 2.6% (mean +/- 1S.D.), as an expression of the relative importance of the viscous/plastic tissue properties for the passive torque. By a peel-off technique the number of different tissue elements with viscous/plastic properties was found to be at least three. After 100 s, all but one of these elements had yielded completely. Viscous/plastic properties were unchanged by stretching when measured 90 min after a single stretching program and when measured 24 h after stretching procedures had been performed twice a day for three weeks.
In restoring the angular position after a displacement, the role of the muscle stretch reflex was investigated by comparing the restored angular torques and angular positions in the wrist under ischaemic and non-ischaemic conditions in normal human subjects. The wrist compliance (COM), defined as the dynamic relation between the angular position and the angular torque of the joint, was calculated to quantify the changes in the restoration of a displacement after abolishing the stretch reflex by ischaemia. The elasticity from the COM-function was found to be single most important factor controlled by the stretch reflex. The elasticity that equals the static stiffness of the system increased by more than 100%, from 0.21 Nm degree-1 with abolished reflex to 0.45 Nm degree-1 with intact reflex. Our results have shown that the stretch reflex assists in the rapid return of the limb to its original position after a mechanical displacement. When the reflex was blocked by ischaemia, the perturbation displaced the limb further away from the initial position.
The stretch reflex should ideally be quantitated for better clinical use by standardizing the muscle stretch and measuring the resulting muscle contraction. Quantitation of muscle contraction can be done by force measurements or electromyographic recordings. The electromyographic response to stretch consists of one component (short latency response) for short stretches (less than 15 ms) and of 2 or 3 components (short and long latency responses) for longer stretches (greater than 40-50 ms). The magnitude of the phasic stretch reflex is reflected by the short latency response, whereas the magnitude of the tonic stretch reflex is reflected by both the short and the long latency responses. In clinical studies of upper motor neuron syndromes, the knee jerk and the muscle tone correlated with the magnitude of the short latency response. In patients with paralysis agitans an increased long latency response, which correlated to the rigidity, was found.
The neural network excites when a stretch reflex is elicited during a physical examination. It is important to be aware that the stretch reflex depends on the contraction level of the muscle before stretch, and on the amplitude and velocity of the muscle stretch. A changed resistance from passive tissue or muscle fibre them self has to be taken into account when evaluating the size of the stretch reflex. A weak contraction compared with no contraction in the stretched muscle decrease the variation in the size of the stretch reflex. Influence from other types of receptors and synergistic and antagonistic muscles can also contribute to the stretch reflex.
1. The purpose of this study was to evaluate the mechanical response to stretch in normal human ankle dorsiflexors at different levels of voluntary contraction. In an active muscle, the total mechanical response is the sum of the intrinsic response from the contractile apparatus, the response from passive tissues, and the reflex mediated response. Each of these components was investigated. 2. The total incremental stiffness was defined as the ratio between the torque increment and the amplitude of the stretch. In 14 subjects the total stiffness increased from approximately 0.6 N.m/deg to approximately 2.5 N.m/deg at 50% of MVC and remained constant (+/- 10%) from 30 to 80% of MVC. 3. The contribution to incremental stiffness from intrinsic muscle properties was measured during electrical stimulation of the deep peroneal nerve at 7-50 Hz. Intrinsic stiffness increased linearly with torque from approximately 0.5 N.m/deg to approximately 2.5 N.m/deg at 80% of MVC. 4. The reflex component (total minus intrinsic stiffness) had a maximum of 0.5-1.5 N.m/deg at 30-50% of MVC and was approximately zero at no and maximal contraction. For intermediate levels of contraction the reflex increased the stiffness with 40-100% of the intrinsic stiffness in this flexor muscle. 5. The reflex contribution to total stiffness began approximately 50 ms after onset of stretch and peaked 150-300 ms after onset of stretch. 6. Total, intrinsic, and reflex mediated stiffness were all nearly independent of the amplitude of stretch in the range from 2 to 7 degrees. The higher stiffness observed for 1 degree stretches could be due to "short range stiffness" of the cross bridges. 7. Stretching of a contracting muscle generates large force increments even for moderate amplitudes of stretch. Approximately half of this force increment is due to the stretch reflex, which makes the muscle stiffer than predicted from the intrinsic stiffness. These findings in human flexor muscles are surprisingly similar to previous findings in extensor muscles of the decerebrate cat.
A computer-based system was developed to (1) train freely standing cats to match various target forces with the left hindlimb, (2) perturb the left ankle joint when the cat was maintaining a desired force and (3) compare reflex responses before and after decerebration. Cats quickly learned to stand unaided on 4 pedestals. During a training session, a range of target force windows was presented to the cat. A successful trial consisted of maintaining the force applied on the left rear pedestal within the target window for a preset time period. To assist the cat, a light was turned on whenever the force was within the target window. A food pellet reward was delivered by the computer after each successful trial. To test reflex responses, the position of the left hindlimb could be briefly perturbed by activating a servo-controlled printed motor configured to rotate the pedestal about the axis of the ankle joint. Perturbations that either flexed or extended the ankle joint were presented pseudo-randomly by the computer. This approach has been used to quantify the magnitude of muscle afferent volleys and the reflex EMG in ankle extensor muscles of normal and decerebrated cats, in response to similar mechanical perturbations. It has also been used to study dynamic features in the electroneurogram recorded from a cutaneous nerve by implanted nerve cuff electrodes, and the correlations among the electroneurogram, the vertical contact force applied on the pedestal and the force recorded from muscle tendons by implanted transducers. This approach may have general applications in the study of postural control, including the study of the discharge patterns of individual motor, sensory or spinal cord neurons in freely standing cats.
Fatigue of electrically activated paralysed muscles is a major factor limiting the duration of functional electrical stimulation (FES) supported paraplegic standing. Fatigue can be significantly delayed by changing the posture. Since paralysed individuals are deprived of proprio- and exteroception from the lower limbs they are not aware of the posture and loading of their paralysed legs. If suitable cognitive feedback (CF) information about posture in the sagittal and frontal planes is provided, they might be able to successfully exercise posture switching. A two-dimensional electrocutaneous CF system was developed. Relative limb loading and the location of the weighted centre of pressure were selected as informational variables. Discrete encoding schemes in the form of spatial and frequency codes were employed and the informational signals were divided into three sub-regions. The ability to correctly interpret the CF was investigated using one- and two-dimension tracking tests in three paralysed subjects, each of whom were studied over five consecutive days. All three subjects were able to use the CF in one-dimension tests. Two subjects could do the same also in two-dimension tests. The encoding scheme which was developed to communicate the selected biomechanical variables proved to be easily understood and thus appropriate for use in paraplegic standing.
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The passive tension resulting from dorsiflexion of the ankle was measured in relation to stretching in six handball players and six soccer players. Corresponding values of ankle angle and passive tension were measured by a strain gauge and a potentiometer connected to a pedal system. The passive tension versus ankle angle was measured before and 90 minutes after a single contract-relax stretching program of the plantar flexors. Stretching lowered the passive tension by up to 18%. Contract-relax stretching performed twice a day for 3 weeks lowered the passive tension in the plantar flexors by up to 36%. Before the last measurements, no stretching was performed for 20 hours or more. Stretching thus had both a short-term effect, matching the length of a training session, and a long-term effect, shown in a reduction of passive tension after 3 weeks. The relative decrease in passive tension after stretching exercises was constant from a neutral position of the ankle to maximal dorsiflexion. There was no correlation between 1) flexibility and the short-term effect of stretching, 2) flexibility and the long-term effect of stretching, or 3) the short-term and long-term effects of stretching. This indicates that passive tension was decreased in all subjects irrespective of their flexibility, and that subjects who had short-term effects after stretching did not necessarily demonstrate a long-term effect.
Recent studies indicate that the hamstrings are essential antagonists in ACL deficient patients. In order to evaluate muscle coordination under physiologic conditions, nine patients with arthroscopically verified total ACL ruptures were compared with nine control volunteers. This comparison was made from electromyograms of the thigh muscles and heel contact recordings made while walking on a treadmill. While walking on a horizontal level, the patient's muscle activity did not differ from controls, but when the knee load was increased by walking uphill, the hamstring muscles in patients were activated significantly earlier than among controls. It is proposed that an altered muscle coordination is probably essential in ACL deficient patients to secure knee stability. This study suggests a new approach to investigate the coordination of muscles under physiologic conditions in ACL deficient patients, and may guide rehabilitation programs in the future.
We present a novel mechanical apparatus, named Multi-purpose Rehabilitation Frame (MRF), and methods for balance training during standing of neurologically impaired individuals. The device has two degrees of freedom (DOF), which allow for constrained movement of both lower limbs and pelvis in the sagittal and frontal planes. The MRF aims at improving balancing in impaired individuals by providing a stiffness support and action of perturbations, which facilitate development of alternative balancing strategies. The level of stiffness support and strength of perturbations, which are generated by means of two hydraulic servo-controlled actuators, can be selected according to current balancing abilities of an impaired individual. We further present preliminary results of nine days of balance training in two paraplegic and two incomplete tetraplegic subjects standing in the MRF. All subjects improved their balancing abilities as measured from the level of needed supporting stiffness provided by the MRF.