Errors in force estimation can be explained by tendon organ desensitization
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Here we report observations on the sense of muscle tension in human subjects and compare them with responses of tendon organs in cat hindlimb muscles. Human subjects learned under visual guidance to estimate a 4% maximum voluntary contraction (m.v.c.) of elbow flexors of one arm. When they were able to reproduce this force reliably without visual feedback, they repeated the estimation immediately after a 5 second m.v.c or a 5 second period of relaxation. In a second experiment the 4% m.v.c was generated under visual control with one arm, and matched with the other, test arm, without visual feedback. The matching task was then repeated after test arm conditioning. In both experiments subjects reported an accurate match using significantly more than the reference force (“overmatched”) after an m.v.c. The overmatching was greatest during the first 5 second period following the conditioning contraction, and during the subsequent 20 seconds it gradually declined to near reference levels. The size of the matching error was directly proportional to the duration of the conditioning contraction. In the first experiment extension of the arm immediately following conditioning increased the error, in the second it slightly decreased it, although tension continued to be overmatched. In a series of experiments on the soleus muscle of anaesthetised cats responses of tendon organs to 10% of maximum contraction were seen to drop sharply when preceded by a conditioning maximum contraction. The time course of recovery was comparable to the decline in matching error in the human experiments. In conclusion, one explanation for the error in force matching seen in human subjects after an m.v.c is that sensitivity of tendon organs has been lowered as a result of the activity generated during the conditioning contraction.
Key wordsTendon organ Tension Proprioception Contraction Afferent Human
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