Effect of peripheral and cortical stimulation on the amount of associated activity

During strong voluntary contractions activity is not restricted to the target
muscles. Other ipsilateral and contralateral muscles also become active
(Gellhorn 1947; Dimitrijevic et al. 1992; Gandevia et al. 1993; Mayston et al.
1999; Zijdewind and Kernell, 2001, Zijdewind et al., 2006). This associated
activity is not always accompanied by overt movement and therefore is often not
noticed by the subject.
How this associated activity is produced is unknown. Previous experiments
showed that the amount of associated can be influenced by fatigue and by
peripheral stimualtion of the musce. The latter suggest that the ecitability of
cortical or spinal circuits is affected. However, it is unknown on what level
(spinal or supraspinal) the excitabilit is affected.

This experiment is a pilot experiment to detemine on what level (spinal or
supraspinal) fatigue affects the amount of associated activity.

Subjects are seated at an experimental table with their hands immobilized in a
vertical position. Subjects stay as immobile as possible, i.e. not leaning
forward or sideways with their upper body. Electromyographic recordings (EMG)
are made of both index finger muscles. After thorough cleaning of the skin with
alcohol, EMG recording electrodes are attached to the skin.
The subjects are asked to produce MVCs, with the dominant hand, non-dominant
hand and both hands simultaneously. After the maximal force recordings subjects
perform a fatigue test with their non-dominant (left) hand. The fatigue test
consists of 30-s contraction at 30% of their maximal force, followed by a
short-lasting maximal contraction and 4-s rest. The submaximal contraction,
maximal contraction and 4-s rest are repeated until subject can't maintain the
30% force any longer.

After the fatigue test subjects have to perform short-lasting maximall
contraction during with they are stimulated. Stimulation is evoked by
electrical stimulation of the muscle, and/or corticospinal tract and/or
magnetic stimulation of the motor cortex. The amount of associated activity
that is evoked by this stimualtion is measured for an half hour.

Electrical stimulation of the musce or cortiospinal tract. Magnetic stimualtion
of the motor cortex.

The stimulation is sometimes unpleasant for the subjects.