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INVITED EDITORIAL
Discipline of Physiology and Research Centre for Human Movement Control,
School of Molecular and Biomedical Science,
The University of Adelaide,
Adelaide, Australia,
e-mail: john.semmler{at}adelaide.edu.au tim.miles{at}adelaide.edu.au
The importance of the sense of proprioception (literally, the perception of position) was made most forcefully in the book by Jonathon Cole about a patient who, overnight, lost his sense of where his limbs were in relation to his body (1). The title of this fascinating book, Pride and a Daily Marathon, reflects not only the fact that it was this patient's pride and determination that motivated him to undergo a grueling rehabilitation program to be able to carry out the normal activities of daily living but also that, in the absence of this important sense, carrying out these activities was indeed a marathon that requires enormous effort.
The paper in this issue of the Journal of Applied Physiology entitled "Effect of eccentric exercise on position sense at the human forearm in different postures" by L. D. Walsh, T. J. Allen, S. C. Gandevia, and U. Proske is the latest in a very long and revealing series of investigations on proprioception, many of which have been done by researchers at the Prince of Wales Medical Research Institute and Monash University in Australia. These investigations have continued to build on the seminal observation by Peter Matthews and his colleagues at Oxford University (4) that muscle spindles contribute to proprioception. Many of these studies have used muscle vibration that powerfully stimulates muscle spindles to induce interesting illusions of proprioception, some of which are quite bizarre. For example, vibrating a wrist flexor tendon can lead to the perception that the wrist joint is overextended in a way that is anatomically impossible (2).
Other studies have attempted to tease out the relative contributions of the putative receptors for proprioception. It transpires that, in different joint systems, different receptors play a greater or lesser role. However, proprioception does not arise solely from peripheral proprioceptors but also from centrally generated motor commands. These signals, which are associated with the sense of effort, are derived from the motor output required to perform voluntary contractions. Inputs from peripheral afferents are then compared with the sense of effort to give the perception of the relative positions of our limbs, trunk, and head (see Ref. 3 for a review).
Several recent studies have perturbed the sense of effort to highlight the role of this centrally generated command in limb position sense. For example, Walsh et al. (6) have shown that eccentric exercise that fatigued the elbow flexor muscles resulted in significant errors in a forearm position-matching task. They reported a significant correlation between the size of the matching errors and the decreased voluntary force induced by the exercise. These position-matching errors were thought to be due to muscle fatigue because similar errors occurred after concentric exercise in which muscle damage is not induced. Furthermore, when the reference arm was supported, subjects were less consistent at matching forearm position, which suggests that subjects were using a signal arising from the sense of effort required to maintain the forearm posture against the force of gravity.
The paper in the current issue by Walsh et al. (7) represents an important extension of these observations. The authors used several simple methods to alter the effort used to maintain forearm position. They first measured position sense in the usual way. The experimenter flexed and extended one elbow passively in the sagittal plane, and blindfolded subjects attempted to match the angle of this elbow by actively flexing and extending the other elbow. This maneuver was repeated with both arms counterweighted so that maintenance of limb position at each joint angle required minimal voluntary effort. Finally, subjects were asked to match elbow angle with their arms supported so that they were flexing and extending them in the horizontal plane, thus eliminating gravitational forces arising from the mass of the forearms at different elbow angles.
Subjects were less certain of limb position when gravitational cues were altered (Fig. 4 in Ref. 7). Although there was no significant difference in the absolute position-matching errors under the three matching conditions, the variability of position matching was greater in most subjects in both the counterweighted and in the horizontal-matching conditions. This suggests that, in the normal situation in which the arm is unsupported, subjects rely partly on the sense of effort exerted to hold the limb against gravity to estimate limb position.
Subjects then performed an eccentric exercise task: this induced prolonged muscle fatigue and damage so that, when tested 24 h later, their maximal elbow-flexion force was reduced by 15–50% (i.e., the normal relation between effort and force was altered). When the three tasks were then repeated, the matching errors increased in the unsupported condition but not in the counterweighted or horizontal conditions (i.e., those in which no effort is required to overcome gravity and that therefore result in position sense, independent of any gravity effects).
Pain was thought not to be a factor in the changed matching errors as subjects reported pain only on palpation of the muscle, and not during the matching procedure, and the matching errors cannot be attributed to altered peripheral receptor properties, because muscle spindle activity is not altered after a series of eccentric contractions (5).
These experiments confirm that the sense of limb position arises not only from signals in peripheral proprioceptors in muscles and skin but also from centrally generated signals related to the sense of effort used to move or support a limb. This may inter alia have implications for athletes who exercise when their muscles are fatigued. It is possible that the diminished proprioception that occurs after eccentric contractions may be a factor in, for example, hamstring injuries and ankle sprains.
REFERENCES
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