Relaxation in distal and proximal arm muscles: a reaction time study.
OBJECTIVE
To investigate whether the same mechanisms underlie muscle relaxation in proximal and distal arm muscles of normal subjects.


METHODS
Fourteen healthy subjects were studied using a simple visual reaction time paradigm. Relaxation reaction time (R-RT) and contraction reaction time (C-RT) were compared across different tasks involving distal (first dorsal interosseus, FDI, flexor carpi radialis, FCR) and proximal (biceps brachii, BB, triceps brachii, TR) arm muscles. Changes of FCR H-reflex before and during voluntary relaxation were investigated in two subjects.


RESULTS
No significant difference was observed between R-RT and C-RT in the distal muscles. The R-RT was significantly shorter than C-RT in both the BB and TR muscles. The relaxation latency (R-RT) was significantly correlated to the subjects' age in all the muscles except the FDI. No inhibition of the FCR H-reflex could be observed in the 20 ms preceding muscle relaxation.


CONCLUSIONS
Our findings suggest that neural mechanisms contribute differently to the relaxation of muscles with a different functional role. Voluntary relaxation in distal arm muscles is mainly related to the reduction of motor cortical output, while in proximal muscles a spinal disfacilitation is also present and possibly sustained by the modulation of presynaptic inhibition.
Buccolieri
A
A
Department of Neurosciences, Ophthalmology and Genetics, Section of Neurology, University of Genoa, Italy.
Avanzino
L
L
Trompetto
C
C
Abbruzzese
G
G
D000328:Adult; D000367:Age Factors; D004576:Electromyography; D005260:Female; D006181:H-Reflex / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology; D009126:Muscle Relaxation / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D014836:Volition / Q000502:physiology