First dorsal interosseous muscle activation

Voluntary muscle activation is a measure of central nervous system output that
a muscle receives during a contraction. Muscle activation in an important
measure to monitor muscle performance in healthy individuals, and in patient
populations. During fatiguing muscle contractions, a reduction in muscle
activation is indicative of deficits in central nervous system output to
produce a maximal effort (Gandevia 2001).

One method to measure muscle activation is using the superimposed twitch
technique (Merton 1954). Electrical stimulation can be applied to a peripheral
nerve (peripheral nerve stimulation, PNS) or directly to a muscle (motor point
stimulation, MPS) during a voluntary muscle contraction. If the muscle fibers
are not maximally activated by the central nervous system output, the
electrical stimulus produces a force increment called a superimposed twitch.
The magnitude of the evoked twitch corresponds to the level of voluntary muscle
activation. A limitation of these methods is that it is only possible to
determine whether a deficit in central nervous system output exists, but not
whether this originates at the spinal or supraspinal level.

Another non-invasive method to measure voluntary muscle activation exists. This
method uses transcranial magnetic stimulation (TMS) (Todd et al. 2003, Todd et
al. 2016). In contrast to the methods described above where stimulation is
applied in the periphery, TMS is applied to the motor cortex. This stimulation
activates spinal motoneurons which are not maximally activated by the voluntary
drive (i.e. supraspinal output). This provides useful additional information
with regard to the mechanism underlying reduced muscle activation. This method
has not been explored for the first dorsal interosseous muscle.

1) To determine the effect of the level of voluntary force on the magnitude of
superimposed twitches evoked using peripheral nerve stimulation (PNS), motor
point stimulation (MPS), and transcranial magnetic stimulation (TMS) in first
dorsal interosseous muscle.

2) To assess the change in the magnitude of twitches during a sustained maximal
voluntary contraction of the first dorsal interosseous muscle, for twitches
evoked using PNS, MPS, and TMS.

(Secondary Objective):
3) To assess the activation of the antagonist muscle (first palmar
interosseous) during stimulation of the first dorsal interosseous at different
TMS stimulation intensities (in a subgroup of participants).

4) 4. To assess the effect of fatiguing the antagonist muscle (first palmar
interosseous) by eccentric exercise on the magnitude of superimposed twitches
evoked using PNS, MPS, and TMS.

This is an intervention study during which participants perform multiple motor
tasks with the right index finger (FDI muscle). Voluntary muscle activation is
assessed by evoking superimposed twitches using electrical stimulation (MPS and
PNS) and transcranial magnetic stimulation during the tasks.

There are no known risks of TMS or electrical stimulation. Electrical
stimulation causes a brief, painful sensation. Insertion of the needle for the
intramuscular EMG is painful while placing the electrodes. The time investment
is 1.5 hours in total.