U. Proske

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 muscles 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.

The Proprioceptive Senses: Their Roles in Signaling Body Shape, Body Position and Movement, and Muscle Force

This is a review of the proprioceptive senses generated as a result of our own actions. They include the senses of position and movement of our limbs and trunk, the sense of effort, the sense of force, and the sense of heaviness. Receptors involved in proprioception are located in skin, muscles, and joints. Information about limb position and movement is not generated by individual receptors, but by populations of afferents. Afferent signals generated during a movement are processed to code for endpoint position of a limb. The afferent input is referred to a central body map to determine the location of the limbs in space. Experimental phantom limbs, produced by blocking peripheral nerves, have shown that motor areas in the brain are able to generate conscious sensations of limb displacement and movement in the absence of any sensory input. In the normal limb tendon organs and possibly also muscle spindles contribute to the senses of force and heaviness. Exercise can disturb proprioception, and this has implications for musculoskeletal injuries. Proprioceptive senses, particularly of limb position and movement, deteriorate with age and are associated with an increased risk of falls in the elderly. The more recent information available on proprioception has given a better understanding of the mechanisms underlying these senses as well as providing new insight into a range of clinical conditions.

Damage to Skeletal Muscle from Eccentric Exercise

Evidence is provided for a mechanical event as the first step in the process leading to muscle damage after a series of eccentric contractions. Aspects discussed include the decline in active tension, increase in passive tension, shift in length–tension relation, soreness, swelling, and disturbed proprioception.

The kinaesthetic senses

This review of kinaesthesia, the senses of limb position and limb movement, has been prompted by recent new observations on the role of motor commands in position sense. They make it necessary to reassess the present‐day views of the underlying neural mechanisms. Peripheral receptors which contribute to kinaesthesia are muscle spindles and skin stretch receptors. Joint receptors do not appear to play a major role at most joints. The evidence supports the existence of two separate senses, the sense of limb position and the sense of limb movement. Receptors such as muscle spindle primary endings are able to contribute to both senses. While limb position and movement can be signalled by both skin and muscle receptors, new evidence has shown that if limb muscles are contracting, an additional cue is provided by centrally generated motor command signals. Observations using neuroimaging techniques indicate the involvement of both the cerebellum and parietal cortex in a multi‐sensory comparison, involving operation of a forward model between the feedback during a movement and its expected profile, based on past experience. Involvement of motor command signals in kinaesthesia has implications for interpretations of certain clinical conditions.

Predicting Hamstring Strain Injury in Elite Athletes

INTRODUCTION Eccentric exercise, where the contracting muscle is lengthened, produces microscopic damage in muscle fibers, and sensations of stiffness and soreness, the next day. These normally resolve within a week. A more major sports injury is the muscle strain. Because strain injuries are known to occur during eccentric contractions, it is hypothesized that the microscopic damage from eccentric exercise can, at times, progress to a muscle strain. As the amount of microscopic damage depends on the muscles optimum length for active tension, it is further proposed that optimum length is a measure of susceptibility for muscle strains. The athletes most at risk of a hamstring strain are those with a previous history of such injuries. Here the prediction is tested that optimum lengths of previously injured hamstrings are shorter and therefore more prone to eccentric damage than uninjured muscles. METHODS Mean optimum angle for peak torque in a previously injured muscle of nine athletes with a history of unilateral hamstring strains was compared with the uninjured muscle of the other leg and with muscles of 18 uninjured athletes. Optimum angle was determined with isokinetic dynamometry. RESULTS In previously injured muscles, torque peaked at significantly shorter lengths than for uninjured muscles. Peak torque and quadriceps:hamstrings torque ratios were not significantly different. CONCLUSIONS The shorter optimum of previously injured muscles makes them more prone to damage from eccentric exercise than uninjured muscles and this may account for the high reinjury rate. The shorter optimum may reflect the muscles preinjury state or be a consequence of the healing process. To reduce the incidence of strain injuries, it is recommended that a combined program of eccentric exercise and muscle testing be carried out.

Motor commands contribute to human position sense

The role of afferent inflow and efferent outflow (or command) signals in judgements of limb position has been debated for over a century. One way to assess this is to check for changes during complete paralysis, with the current view being that perceived movements or position changes do not usually accompany attempts to contract paralysed muscles. To re‐examine this, we asked six naïve subjects to carry out a simple position‐matching task at the wrist. In the absence of vision, subjects accurately perceived the position to which their right wrist had been moved by the experimenter by matching it with their left hand. There was no significant change in perception when position was matched during sustained flexion or extension efforts. Then we paralysed and anaesthetized the right arm with ischaemia in order to produce a ‘phantom’ hand. The perceived position of the wrist changed by more than 20 deg when subjects attempted to flex or extend their hand when it was paralysed and anaesthetized. Further studies showed that this illusion was not dependent on the way in which the paralysis was produced and that the size of the position illusion increased when the level of effort during paralysis increased. These results establish for the first time a definitive role for ‘outflow’ signals in position sense.

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 muscles 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.

Tendon stiffness: Methods of measurement and significance for the control of movement. A review

An appraisal of the role of tendons in transmitting muscle tension to skeletal parts during posture and movement requires accurate knowledge of the mechanical characteristics of the tendon. Here the most important property is tendon stiffness. While it is relatively easy to measure the stiffness of an isolated segment of tendon, more sophisticated methods must be sought to take into account the whole length of tendon, including its intramuscular portion. Two methods are currently available for measurement of whole tendon stiffness: each has a limited range of muscle tensions over which it appears to provide reliable values, one method being better at low tensions, the other at high tensions. Some controversy remains about the precise values obtained in the mid-tension range covered by both methods. Nevertheless it is now possible to achieve reasonable estimates of tendon stiffness over the whole working range of the muscle. An important consideration which has emerged from the discussion is that at low tensions the tendon is much less stiff than at higher tensions.

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

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. 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. 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. The extra tension after stretch and the depression after shortening disappeared if stimulation was interrupted and tension briefly fell to zero. 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. These mechanisms have important implications for exercise physiology and sports medicine.

Kinesthesia: the role of muscle receptors.

The kinesthetic sense, the sense of position and movement of our limbs, has been the subject of speculation for more than 400 years. The present‐day view is that it is signaled principally by muscle spindles, with a subsidiary role played by skin and joint receptors. The problem with muscle spindles as position sensors is that they are able to generate impulses in response to muscle length changes as well as from fusimotor activity. The central nervous system must be able to distinguish between activity from the two sources. Recent observations on position sense after fatigue and during load‐bearing suggest that an additional source of kinesthetic information comes from a centrally generated sensation, the sense of effort. This has consequences for kinesthesia in the presence of the force of gravity. A contribution from central feedback mechanisms to the sense of effort is relevant to certain clinical conditions. Muscle Nerve, 2006