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Effects of joint immobilization on firing rate modulation of human motor units
Author(s) -
Seki Kazuhiko,
Taniguchi Yuko,
Narusawa Mitsuo
Publication year - 2001
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2001.0507k.x
Subject(s) - isometric exercise , index finger , turnover , biomedical engineering , motor unit , chemistry , materials science , anatomy , mathematics , medicine , management , economics
1 We investigated the effects of 6 weeks of immobilization on firing rate modulation in motor units in the first dorsal interosseous (FDI) of human volunteers. The middle finger, index finger and thumb were immobilized for a period of 6 weeks in a fibre‐glass cast, which kept FDI in a shortened position. 2 During isometric contraction at 20, 40, 60 and 80 % of maximal voluntary contraction (MVC) (index finger abduction), motor unit action potentials were recorded from the FDI using a tungsten microelectrode, and the relationship between voluntary force and mean firing rate (MFR) was obtained by plotting the MFR of each motor unit action potential train as a function of voluntary force. Four recording sessions were held for each subject: before immobilization, after 3 and 6 weeks of immobilization, and after a 6 week recovery period. 3 As a result of immobilization, FDI volume (as measured by computerized tomography (CT) scanning) decreased, with an accompanying reduction in aggregate EMG activity per day ( P < 0.01 ). The force measured during MVC also decreased ( P < 0.05 ). 4 The slope of the relationship between voluntary force and MFR was significantly decreased after immobilization, as was the range of firing rate modulation ( P < 0.01 ). Maximal MFR, estimated from the relationship between voluntary force and MFR, was decreased ( P < 0.05 ). 5 MFR was also plotted against voluntary force without being normalized with respect to MVC, and the slope of the regression line was decreased ( P < 0.05 ). Voluntary force when the MFR was 15 Hz was estimated from regression equations for the absolute force‐MFR relationship, and it was increased after immobilization ( P < 0.05 ). 6 These results suggest that firing rate modulation shows two different adaptations to joint immobilization: a restriction of motoneurone firing to the lower rates and an enhancement of the voluntary force exerted when the MFR is relatively low.