Age-related modulation of cortical and spinal excitability is associated with postural sway in standing*

Over 1/3 of individuals age 65 and older falls each year. A better
understanding of neural, especially cortical and corticospinal, mechanisms of
balance control would help to design more accurate balance assessments and
treatments. With the combination of H-reflex and transcranial magnectic
stimulataion (TMS) measures it is now possible to determine the role of
cortical and spinal control of balance. We are interested in how age affects
cortical and spinal control mechanisms of balance.

Primary Objective: To determine if spinal and cortical balance control
mechanisms mediate the age-associated decline in balance skills. Secondary
Objective: To determine the reliability of TMS and H-reflex measures in the
soleus muscle in different standing postures.

The primary objective will be investigated using a cross-sectional design,
comparing young versus older adults. Reliability will be investigated with a
test-retest design.

During the experiment participants undergo TMS and peripheral nerve
stimulation, while they are standing in 5 different postures:

Condition 1: Standing with a wide base stance (Stand WB): Standing with feet
shoulder width apart.
Condition 2: Standing with a narrow base stance (Stand NB): Standing with
medial foot borders touching.
Condition 3: Standing with a straddle stance (Stand Step): Non-dominant left
foot on a solid wooden block in front of the body and the dominant right foot
on the force plate.
Condition 4: Standing with a straddle stance (Stand Spring): Non-dominant left
foot on a spring in front of the body and the dominant right foot on the force
plate.
Condition 5: Standing on the right-dominant leg: (One leg stance): Standing on
dominant foot with the non-dominant leg raised and flexed at the knee. The
trial is stopped if the non-dominant left foot touches the floor.

Old adults will visit the Center for Human Movement Sciences one time. Young
adults will visit the Center for Human Movement Sciences two times within a
week. One testing session will last for maximal two and half hours. Subject
will stand for a total of about one hours of the two and the half hours.
Resting periods of 2-3 minutes are given as needed between trials and
conditions. A longer resting period of 10 minutes is built in between the TMS
and H-reflex data collection blocks.

Participation in this study includes electrical stimulation of the tibial nerve
and magnetic stimulation of the motor cortex during standing. The TMS may cause
slight discomfort lasting less than a second on the scalp near the coil. It may
also cause some twitching of the muscles, the face and jaw, which may be
unpleasant and surprising but not painful. Peripheral nerve stimulation causes
the muscles to twitch that can be more surprising than painful. It can cause
some momentary burning and tingling sensation. There are no known long-term
risks of peripheral nerve or magnetic brain stimulation. Subjects wear a
harness that is attached to the ceiling to minimize any risk of falling.
Electromyography (EMG) of the soleus and tibialis anterior muscles will be
recorded. Therefore the skin underneath the three electrodes will be shaved and
cleaned. This may cause some light irritation of the skin.