Motor unit activity during voluntary contractions.

When producing large amounts of force or during a sustained submaximal
contraction except from contractions in the target-muscle also non-target
muscles are activated. There are even muscles activated who are contralateral
to the 'target' muscle (CAC, contralateraal associated contraction) (Gellhorn
1947; Dimitrijevic et al. 1992; Gandevia et al. 1993; Mayston et al. 1999;
Zijdewind and Kernell, 2001). A recent study proved the contralateral motor
cortex activated both the associated contractions and the voluntary
contractions (Zijdewind, 2006).
We want to research if the activation from this motor cortex to the
motor-neuron pool is develops equally for CAC and voluntary contractions.

In this research we want to study the possible differences in discharge rate,
recruitement treshold and de-recruitement treshold of individual motor units
during voluntary contractions and CAC.

Our protocol will consist of two measure sessions, which will be executed on
two different days. In the first session we will use the index finger for the
measurements, in the second session we will use the upper arms.
During the experiments subjects will be fixed with both their index fingers or
arms in a set up. Force will be measured of the index fingers or the lower arms
with help of a force transducer. In the muscle that abduces the index finger
(first dorsal interroseus; FDI) or in the muscle that flexes the upper arm
(Biceps Brachii; BB) wire electrodes will be placed on both sides (Hence, both
index finger abductors and both upper arm flexors). On the FDIs or the BBs and
Triceps Brachii (upper arm extensor) EMG electrodes will be placed. During the
experiments subjects are asked to perform the following tasks with their index
fingers or upper arms:
- Maximal voluntary contraction (MVC) with the target muscle while no
instruction is given to the non-target muscle or while the non-target muscle is
contracted a little bit (5%MVC). After when subjetcs are to make a voluntary
submaximal contraction with the non-target index muscle.
- A fatigue task where subjects have to maintain as long as possible a force of
30% MVC with their target index finger. The test is finished when subject is no
longer able to reach the 30% of is original maximal force.
The first task will be executed with the FDI and BB on both sides as target
muscle. During the fatigue task only one of the muscles will be used as a
target muscle.

There is a very small risk of bleeding or infection during the insertion of the
needle. Furthermore, there is a small risk that during the extraction of the
wires from the muscle small pieces are left behind. However, the material of
the wires are biological inert, which means that if there are pieces left
behind this has no serious consequence.
The needles we use to implant the wire electrodes in the muscle are sterilised
as are the wire electrodes themselves.