Dynamic changes in cortical and spinal activities with different representations of isometric motor actions and efforts.
BACKGROUND
Positioning the shoulder joint from 30 degrees adduction (anterior [ANT]) to 30 degrees abduction (posterior [POST]) in the horizontal plane modifies the corticospinal output to hand and forearm muscles in humans.


OBJECTIVE
We investigated the mechanisms by which the central nervous system (CNS) maintains force output under conditions of increased effort and reduced corticospinal activity.


METHODS
Ten healthy subjects were studied with the shoulder joint fully supported and passively kept either in ANT or POST. Changes in motor-evoked potentials (MEPs) to transcranial magnetic stimulation (TMS), intracortical inhibition (ICI), intracortical facilitation (ICF), H-reflex and F-waves were studied at force levels corresponding to 10% maximum voluntary contraction (MVC) of abductor digiti minimi (ADM) in ANT for both shoulder positions. In addition, premovement changes in ADM MEP size were assessed in a choice reaction time paradigm in the two shoulder positions.


RESULTS
ADM MEPs were larger in POST than in ANT either during or before ADM voluntary contraction, pointing to increased corticospinal excitability in both conditions. ICI and ICF were increased and decreased, respectively, indicating a general disfacilitation on primary motor cortical (M1) output to ADM in POST. F-waves and H-reflexes were increased and decreased, respectively, indicating postsynaptic facilitation and increased presynaptic inhibition at spinal cord level in POST.


CONCLUSIONS
A larger cortical output is produced in POST to maintain the same force levels as in ANT. A contributory role of premotor regions is hypothesized.
Mazzocchio
Riccardo
R
Sezione di Neurofisiologia Clinica, Dipartimento di Scienze Neurologiche e del Comportamento, Universita' di Siena, Policlinico Le Scotte, Siena, Italy. mazzocchio@unisi.it
Gelli
Francesca
F
Del Santo
Francesco
F
Popa
Traian
T
Rossi
Alessandro
A
D000328:Adult; D002540:Cerebral Cortex / Q000502:physiology*; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D005542:Forearm / Q000033:anatomy & histology / Q000502:physiology; D006181:H-Reflex / Q000502:physiology; D006225:Hand / Q000033:anatomy & histology / Q000502:physiology; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000502:physiology*; D009415:Nerve Net / Q000502:physiology; D011187:Posture; D012785:Shoulder Joint / Q000033:anatomy & histology / Q000502:physiology; D013116:Spinal Cord / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation