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Biomedical subjects

U Proske

Publications and source records attributed to U Proske.

At least 37 records · Page 2Linked to original sources
PubMed2002

The role of the length-tension curve in the control of movement.

The length-tension curve of muscle is one of the important descriptors of mechanical performance, and also a direct reflection of the underlying structure, particularly the number of sarcomeres connected in series in muscle fibres. This number is one of the most plastic properties of muscle, changing within days after changes in activity patterns. We propose that this adaptation is to prevent eccentric contractions from occurring beyond the optimum length for tension generation, since this is the region of sarcomere instability and muscle damage. Evidence for this is presented for muscles from rats trained on a treadmill, and from motor units of the gastrocnemius muscle of the cat.

Adaptation, Physiological↗
PubMed2002

Damage to different motor units from active lengthening of the medial gastrocnemius muscle of the cat.

Slow-twitch motor units in the medial gastrocnemius muscle of the anesthetized cat were found to have an average optimum length for active tension that was 0.8 +/- 0.5 (SE) mm longer than the whole muscle optimum. For fast-twitch units (time to peak < 50 ms), the average optimum was 1.3 +/- 0.3 mm shorter than the whole muscle optimum. After the muscle had been subjected to 10 stretches while maximally activated, beginning at the whole muscle optimum length, the optimum lengths of the 27 fast-twitch motor units shifted significantly further in the direction of longer muscle lengths (mean 4.3 +/- 0.3 mm) than for the eight slow-twitch units (2.1 +/- 0.4 mm). A shift in the muscle's length-tension relation was interpreted as being due to sarcomere disruption. Statistical analysis showed that a motor unit's optimum length for a contraction, relative to the whole muscle optimum, was a better indicator of the unit's susceptibility to damage from active lengthenings than was motor unit type.

Animals↗
PubMed2001

Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications.

In eccentric exercise the contracting muscle is forcibly lengthened; in concentric exercise it shortens. While concentric contractions initiate movements, eccentric contractions slow or stop them. A unique feature of eccentric exercise is that untrained subjects become stiff and sore the day afterwards because of damage to muscle fibres. This review considers two possible initial events as responsible for the subsequent damage, damage to the excitation-contraction coupling system and disruption at the level of the sarcomeres. Other changes seen after eccentric exercise, a fall in active tension, shift in optimum length for active tension, and rise in passive tension, are seen, on balance, to favour sarcomere disruption as the starting point for the damage. As well as damage to muscle fibres there is evidence of disturbance of muscle sense organs and of proprioception. A second period of exercise, a week after the first, produces much less damage. This is the result of an adaptation process. One proposed mechanism for the adaptation is an increase in sarcomere number in muscle fibres. This leads to a secondary shift in the muscle's optimum length for active tension. The ability of muscle to rapidly adapt following the damage from eccentric exercise raises the possibility of clinical applications of mild eccentric exercise, such as for protecting a muscle against more major injuries.

Adaptation, Physiological↗
PubMed2001

Passive mechanical properties of the medial gastrocnemius muscle of the cat.

1. This is a report on the history dependence of the passive mechanical properties of the medial gastrocnemius muscle of the anaesthetised cat. 2. The muscle was conditioned with an isometric contraction at the test length, or at 3 mm longer than the test length and then returned to the test length, where the level of resting tension was measured, as well as tension changes during a slow stretch. 3. The level of resting tension depended on the form of conditioning and, at the optimum length for active tension, the history-dependent component was 9 % of the total passive tension. 4. During a slow stretch, tension initially rose steeply up to a yield point, beyond which it rose more gradually. The shape of the tension rise depended on the form of conditioning. The level of tension at the yield point consisted of a stretch-dependent component, the 'short-range tension' plus the resting tension for that length. 5. The short-range tension increased with muscle length to peak close to the optimum for active tension. The slope of the tension rise during a stretch, the short-range stiffness, peaked at 2 mm beyond the optimum. 6. The short-range tension was small immediately after a conditioning contraction but grew in size as the interval was increased up to 60 s, with a time constant of 9.9 +/- 0.6 s. After a series of conditioning movements, it recovered more rapidly, with a time constant of 6.6 +/- 0.5 s. 7. The history-dependent changes in passive tension and the response to stretch are interpreted in terms of the presence, in sarcomeres of resting muscle fibres, of crossbridges between actin and myosin which have very slow formation rates, both at rest and during movements.

Actins↗
PubMed2001

Changes in passive tension of muscle in humans and animals after eccentric exercise.

1. This is a report of experiments on ankle extensor muscles of human subjects and a parallel series on the medial gastrocnemius of the anaesthetised cat, investigating the origin of the rise in passive tension after a period of eccentric exercise. 2. Subjects exercised their triceps surae of one leg eccentrically by walking backwards on an inclined, forward-moving treadmill. Concentric exercise required walking forwards on a backwards-moving treadmill. For all subjects the other leg acted as a control. 3. Immediately after both eccentric and concentric exercise there was a significant drop in peak active torque, but only after eccentric exercise was this accompanied by a shift in optimum angle for torque generation and a rise in passive torque. In the eccentrically exercised group some swelling and soreness developed but not until 24 h post-exercise. 4. In the animal experiments the contracting muscle was stretched by 6 mm at 50 mm s(-1) over a length range symmetrical about the optimum length for tension generation. Measurements of passive tension were made before and after the eccentric contractions, using small stretches to a range of muscle lengths, or with large stretches covering the full physiological range. 5. After 150 eccentric contractions, passive tension was significantly elevated over most of the range of lengths. Measurements of work absorption during stretch-release cycles showed significant increases after the contractions. 6. It is suggested that the rise in passive tension in both human and animal muscles after eccentric contractions is the result of development of injury contractures in damaged muscle fibres.

Adult↗
PubMed2001

Summing responses of cat soleus muscle spindles to combined static and dynamic fusimotor stimulation(1).

This is a study of the summation of responses of primary endings of muscle spindles to combined static and dynamic fusimotor stimulation in the soleus muscle of the anaesthetised cat. Summation, expressed as a summation coefficient, K, was measured under a variety of conditions including (1) at several, fixed muscle lengths using steady rates of stimulation, (2) using ramp-shaped increases in stimulation rates, (3) during passive stretches after muscle conditioning, and (4) during combined stretch plus stimulation. The predominant effect observed was occlusion, that is, the combined response was less than the sum of the two individual responses. The calculated mean K value for responses at fixed length was 0.156 (+/-0.005 S.E.M.). It was hypothesised that summation arose from electrotonic spread of generator current between the afferent terminals, either directly, or as a result of mechanical interactions between the contracting intrafusal fibres. Summation for responses from pairs of static fusimotor fibres gave a larger mean K value, 0.340 (+/-0.020 S.E.M.). These findings were interpreted in terms of a model of the muscle spindle where responses to dynamic fusimotor stimulation arise at one impulse generating site, and static fusimotor responses arise at another.

Action Potentials↗
PubMed2001

On the branching of motoneurons.

Innervation ratios were estimated for motor units supplying superficial lumbrical muscles of the cat after assigning units to one of three types, FF/FI, FR, and S, and estimating their axonal conduction velocity, based on published data. Similarly, unit tensions were converted to muscle fiber number using published values for specific tension and fiber cross-sectional area. A relation was established between axonal area of cross-section and number of fibers innervated which was matched reasonably well by a model of a branching motoneuron with preterminal diameter of 4.28 microm and a daughter-to-parent ratio of 1.48 at each level of branching. It is proposed that the main features of the model are likely to apply to all motoneurons.

Animals↗
PubMed2001

An investigation into mechanisms of reflex reinforcement by the Jendrassik manoeuvre.

Tendon jerk and H-reflexes are both potentiated by the Jendrassik manoeuvre, but the mechanism of potentiation remains uncertain. We investigated several possibilities in human subjects. Evidence for fusimotor activation during the Jendrassik manoeuvre was sought by recording the tendon jerk reflex as surface EMG in triceps surae after the muscles had been conditioned to leave their spindles in a slack, insensitive state. Interposing a Jendrassik manoeuvre between conditioning and the test reflex should have increased reflex amplitude by restoring spindle sensitivity, but this was not the case. In humans, a close synergist of the triceps surae is the quadriceps. A possible presynaptic disinhibitory mechanism was investigated by testing the effect of a Jendrassik manoeuvre on facilitation of the soleus H-reflex produced by a quadriceps afferent volley. The Jendrassik manoeuvre failed to increase facilitation, contrary to what would be expected if it reduced the level of tonic presynaptic inhibition; the assumption being that the inhibition acts on both homonymous and synergist afferent terminals. The Jendrassik manoeuvre did not increase the level of ongoing EMG in the soleus during a weak voluntary contraction, indicating that it does not operate by direct facilitation of motoneurones. There was found to be less potentiation of soleus tendon jerk and H-reflexes by the Jendrassik manoeuvre under conditions when spindles in the soleus were likely to have a high resting discharge rate. A remaining possibility is discussed: that the Jendrassik manoeuvre operates by modulation of oligosynaptic pathways that may contribute to the largely monosynaptic reflex response. These experiments demonstrate, with new, more sensitive methods than previously used, that neither is the fusimotor system involved in reinforcement nor are direct excitatory or presynaptic disinhibitory effects on motoneurones. While this confirms the previously prevailing view, none of the lingering uncertainties associated with the methods used now remains.

Adult↗
PubMed2001

Large-fiber mechanoreceptors contribute to muscle soreness after eccentric exercise.

Muscles subjected to eccentric exercise, in which the contracting muscle is forcibly lengthened, become sore the next day (delayed onset muscle soreness). In subjects who had their triceps surae of 1 leg exercised eccentrically by walking backwards on an inclined moving treadmill, mapping the muscle 48 hours later with a calibrated probe showed sensitive areas were localized but not restricted to the muscle-tendon junction. Injection of 5% sodium chloride into a sensitive site in the exercised leg did not produce more pain than injections into the unexercised leg, suggesting that nociceptor sensitization was not responsible. Applying controlled indentations to a sensitive area showed that the pain could be exacerbated by 20-Hz or 80-Hz vibration. In an unexercised muscle, vibration had the opposite effect; it reduced pain. Pain thresholds were measured before, during, and after a pressure block of the sciatic nerve. The block affected only large-diameter nerve fibers, as evidenced by disappearance of the H reflex and a weakened voluntary contraction, leaving painful heat and cold sensations unaltered. Pain thresholds increased significantly during the block. It is concluded that muscle mechanoreceptors, including muscle spindles, contribute to the soreness after eccentric exercise.

Journal Article↗
PubMed2001

Human hamstring muscles adapt to eccentric exercise by changing optimum length.

PURPOSE: It is now established that unaccustomed eccentric exercise leads to muscle fiber damage and to delayed-onset muscle soreness (DOMS) in the days after exercise. However, a second bout of eccentric exercise, a week after the first, produces much less damage and soreness. The purpose of this study was to provide evidence from muscle mechanical properties of a proposed mechanism for this training effect in human hamstring muscles. METHODS: The eccentric exercise involved 12 sets of 6 repetition "hamstring lowers," performed on specially designed equipment. Hamstring angle-torque curves were constructed for each of 10 subjects (8 male and 2 female) while they performed maximum voluntary knee extension and flexion movements on an isokinetic dynamometer. Testing sessions were performed over the week before eccentric exercise, immediately post exercise, and daily, up to 8 d post exercise. Subject soreness ratings and leg girth measurements were also made post exercise. Six subjects performed a second bout of eccentric exercise, 8 d after the first, and measurements were continued up to 10 d beyond that. RESULTS: There was a significant shift in the optimum angle for torque generation (Lo), to longer muscle lengths immediately post exercise (7.7 degrees +/- 2.1 degrees, P < 0.01), indicating an increase in series compliance within some muscle fibers. Subsequent measurements showed increases in leg girth and some muscle soreness, suggesting muscle damage. The shift in Lo persisted, even after other injury parameters had returned to normal, consistent with a training effect. Subjects also showed fewer signs of muscle damage after the second exercise bout. CONCLUSION: This is the first study to show a sustained shift in optimum angle of human muscle as a protective strategy against injury from eccentric exercise. Implications of this work for athletes, particularly those prone to hamstring strains are discussed.

Adaptation, Physiological↗
PubMed2001

Fatigue in mammalian skeletal muscle stimulated under computer control.

Functional electrical stimulation (FES) is used to provide paralyzed human subjects with postural support and a limited range of movements. Problems encountered with FES include jerky movements from tension oscillations during stimulation and rapid muscle fatigue. In this paper, we report experiments on anesthetized cats that test a new, computer-controlled method of stimulation of the muscle nerve supply, distributed across several inputs, which reduces these problems. After 5 min of continuous, distributed stimulation of the medial gastrocnemius muscle at 6 pulses per second (pps) across 6 channels, tension fell to 55.9 +/- 3.9% (SE) of its original value. In comparison, after 5 min of synchronous stimulation of one muscle portion at 36 pps, tension fell to 11 +/- 3.7%. At higher stimulation rates, 10 pps per channel (distributed) and 60 pps (synchronous), the differences in fatigue were even greater. Similar results were obtained when an intermittent, rather than a continuous, stimulation protocol was used. These findings indicate that distributed stimulation has important advantages over other methods for applications such as FES.

Animals↗
PubMed2001

An investigation of the Jendrassik manoeuvre.

Several mechanisms were investigated by which the Jendrassik manoeuvre might potentiate tendon jerk and H-reflexes, recorded here as surface EMG from the triceps surae muscle group. If fusimotor activation is involved, then, after muscle conditioning designed to leave spindles in a slack state, interposing a Jendrassik manoeuvre before reflex testing should have increased reflex amplitude by restoring spindle sensitivity; but this was not the case. The Jendrassik manoeuvre failed to increase facilitation of the soleus H-reflex by a quadriceps volley, contrary to expectations if it operates by presynaptic disinhibition. There was no increase in the level of ongoing EMG during a Jendrassik manoeuvre, indicating that it does not operate by direct facilitation of motoneurones. The Jendrassik manoeuvre produced less reflex potentiation when spindles had a high rate of resting discharge. A remaining, untested mechanism is the modulation of oligosynaptic pathways that may contribute to the largely monosynaptic reflex response.

Electromyography↗
PubMed2000

Tension changes in the cat soleus muscle following slow stretch or shortening of the contracting muscle.

1. The permanent extra tension after a stretch and the deficit of tension after a shortening in the soleus muscle of the anaesthetised cat were measured using distributed nerve stimulation across five channels. At low rates of stimulation the optimum length for a contraction was several millimetres longer than that when higher rates of stimulation were used, so that movements applied over the same length range could be on the descending limb of the full activation curve but on the ascending limb of the submaximal activation curve. 2. The extra tension after stretch and the depression after shortening were present only near the peak and on the descending limb of the length-tension curve. Effects on final tension of changing the speed and amplitude of stretches or shortenings were found to be small. 3. Statistical analysis showed that variations in the tension excess or deficit due to changing stimulus rate could be entirely attributed to the effect of stimulus rate on the length-tension relation, as when length was expressed relative to optimum for each rate, stimulus rate was no longer a significant determinant of the tension excess or deficit. 4. The extra tension after stretch and the depression after shortening disappeared if stimulation was interrupted and tension briefly fell to zero. 5. These effects were explained in terms of a non-uniform distribution of sarcomere length changes at long muscle lengths. During stretch some sarcomeres are stretched to beyond overlap while others lengthen hardly at all. During shortening some sarcomeres shorten much further than others. 6. These mechanisms have important implications for exercise physiology and sports medicine.

Analysis of Variance↗
PubMed2000

Effects of body orientation, load and vibration on sensing position and movement at the human elbow joint.

Experiments were carried out to study the ability of human subjects to match the position of their forearms relative to the horizontal. The normal, arms-in-front position with the hands aligned and little forward flexion at the shoulder was called the reference position. When the arms were rotated to the side, one arm was raised, or both arms were raised, matching ability deteriorated compared with the reference position, when expressed as an increase in the standard deviation of matching errors. It was concluded that particular significance was assigned by the brain to the arms-in-front position, with the hands in their normal working space. Increases in errors were also observed when the reference arm was made weightless or its weight was increased by means of an adjustable load. This suggested that lifting the arm against gravity provided additional positional information. In a second experiment, dependence of the illusion of muscle lengthening evoked by vibration was tested after two different forms of muscle conditioning, a co-contraction of elbow muscles with the arm held flexed or with it held extended. The speed of the illusory extension of flexor muscles during their vibration increased three-fold after flexion conditioning compared with extension conditioning. Since after flexion conditioning, muscle spindles in flexor muscles are expected to be more sensitive to vibration than after extension conditioning, this observation provides additional support for the view that muscle spindles make an important contribution to kinaesthesia at the elbow joint.

Adult↗
PubMed2000

The role of muscle receptors in the detection of movements.

This review discusses the role of muscle receptors, in particular, that of muscle spindles, in the detection of movements, both passive and active. Emphasis is placed on the importance of conditioning the muscles acting at a joint before making measurements of thresholds to passive movements, to take into account muscle's thixotropic property. The detection threshold:movement velocity relation is discussed and described for a number of different joints. Implications for muscle spindles are considered from the generalisation that, when expressed in terms of proportion of muscle fascicle length change, detection thresholds are about the same at different joints. It is concluded that the available data supports the view that muscle spindles lie in parallel with only a portion of a muscle fascicle and not the whole fascicle. At the elbow joint, where it has been tested, movement detection threshold is lower during passive movements than during contraction of elbow muscles. Both peripheral mechanisms and mechanisms operating within the central nervous system may be responsible for the rise in threshold. The signalling of movements by spindles during a contraction raises the question of how the central nervous system is able to extract the length signal under such circumstances, given that there is likely to be co-activation of alpha and gamma motoneurones. The evidence for a central subtraction of fusimotor-evoked impulses and some recent experiments relevant to this idea are described. In conclusion, a number of points of uncertainly have been revealed in this area and these should be the subject of future experiments.

Animals↗
PubMed1999

The responses of muscle spindles to small, slow movements in passive muscle and during fusimotor activity.

We have previously shown that movement detection thresholds at the human elbow joint were less than a degree of joint rotation in the passive limb but were higher if they were measured while subjects co-contracted elbow muscles [A.K. Wise, J.E. Gregory, U. Proske, J. Physiol., 508 (1998) 325-330]. Here we report observations on the responses of muscle spindles of the soleus muscle of the anaesthetised cat to determine their ability to signal small length changes in the passive muscle and during a contraction, under conditions resembling those of the human experiments. After appropriate conditioning of the muscle to control for history effects, primary endings of muscle spindles showed thresholds to ramp stretch at 20 micrometers s-1 of between less than 5 micrometers and 15 micrometers, which translates to 0.05 degrees -0.15 degrees of human elbow joint rotation. Thresholds were much higher following conditioning to introduce slack in the muscle. Since during a voluntary contraction there is likely to be alpha:gamma co-activation, responses of spindles were also recorded during slow stretches (100 micrometers at 20 micrometers s-1) during static fusimotor stimulation, dynamic fusimotor stimulation, combined fusimotor stimulation and fusimotor plus skeletomotor stimulation. Invariably, responses to passive stretch were larger than during motor stimulation. It is concluded that spindles are sensitive enough to signal fractions of a degree of elbow joint rotation and that the rise in threshold observed during a voluntary contraction may be accounted for by the actions of fusimotor and skeletomotor axons on spindle stretch responses.

Animals↗
PubMed1999

Vibration sensitivity of cat muscle spindles at short muscle lengths.

Experiments are described in which the vibration sensitivity was tested for primary and secondary endings of soleus muscle spindles of the anaesthetised cat. The vibratory stimulus was applied longitudinally to the muscle tendon and, after locating the site of the spindle in the muscle, applied transversely to muscle fibres directly overlying the spindle. All measurements were made with the muscle slack, at 20 mm shorter than its maximum physiological length (Lm-20 mm). Spindles were separated into two groups: spontaneous spindles, which maintained background activity at this length, and silent spindles, which did not. Two forms of muscle conditioning were used: hold-long, in which the muscle was stretched by 5 mm, contracted and then returned to the test length, and hold-test, in which a conditioning contraction was given at the test length. After hold-test, most spindles responded to longitudinal vibration; after hold-long, most did not. This difference in responsiveness was attributed to the presence or absence of slack in intrafusal and extrafusal fibres, due to the muscle's thixotropic property. When the vibration was applied transversely, at a site directly overlying the spindle, responses of silent spindles continued to show thixotropic behaviour, whereas responses of spontaneous spindles were almost independent of the form of muscle conditioning. It is proposed that the ability of spontaneous spindles to maintain background activity at short muscle lengths is due to connective tissue or elastic fibre links between the sensory ending and the spindle capsule. The vibratory stimulus, applied transversely, reaches the sensory ending via these strands which, as non-muscle tissue, do not alter their mechanical state as a result of muscle conditioning.

Animals↗