Acute effects of unilateral lower limb electrical stimulation induced muscle activation on peripheral vascular blood flow in SCI

Following a spinal cord injury (SCI) there are significant changes in
peripheral vascular structure and function. A decrease in conduit artery
diameter (de Groot et al., 2004; de Groot et al., 2006a; de Groot et al.,
2006b), increased vascular resistance (Hopman et al., 2002), reduced
capillarization (Chilibeck et al., 1999) and impaired cutaneous
microcirculation (Nicotra et al., 2004; Van Duijnhoven et al., 2009) are
typically observed in the paralysed, inactive limbs. Collectively, such
vascular changes are associated with increased risk of cardiovascular related
mortality and likely contribute to frequently reported pathologies such as skin
breakdown lesions (Deitrick et al., 2007) and poor wound healing. Furthermore,
individuals with a SCI experience changes in body composition including an
increase in adipose tissue and a significant loss in muscle mass. These changes
importantly contribute to reductions in daily energy expenditure and increase
the risk of obesity among individuals with SCI.

Regular muscular contractions act as a pumping mechanism to facilitate the
movement of blood through the vascular network and prevent venous pooling in
the lower limbs. Due to motor paralysis below the lesion, this blood pumping
mechanism is no longer active. Electro-stimulation (ES) induced muscle
activation is an alternative method to overcome the loss of regular voluntary
muscular contractions and improve vascular health through increased regional
blood flow. Indeed, various methods of ES in SCI have resulted in improved
transcutaneous oxygen levels (Smit et al., 2013b; Wu et al., 2013), increase
femoral artery diameters (Thijssen et al., 2006) and decreased peripheral
vascular resistance (Hopman et al., 2002). Additionally, the use of ES
increases circulating blood volume to the normally inactive limbs, thus
increasing oxygen demand of the working muscles and potentially increasing
energy expenditure. However, a majority of ES methods currently used require
specialist facilities and trained staff, are labour intensive and impractical.

Specially fabricated clothing with built in electrodes may be an alternative
way to administer ES without the expense and practicality issues associated
with other methods of ES. For example, Smit et al have previously used
partially elasticated lycra shorts with embedded surface electrodes as a way of
activating the gluteal and hamstring muscles in persons with SCI. They found
that an acute bout of ES lead to significant pressure relief in areas at risk
of skin breakdown lesions (Smit et al., 2013a). Furthermore, the participants
involved in the study stated that they would be satisfied wearing the shorts on
a daily basis providing they helped ameliorate the risk factors associated with
pressure ulcers. Considering that ES induced gluteal and hamstring activation
has previously been shown to increase (sub)cutaneous blood flow and oxygenation
(Smit et al., 2013b), other areas of the deeper vasculature may also be subject
to increases in blood flow. To our knowledge, no study has measured conduit
artery blood flow during an acute bout of ES in SCI individuals.

The purpose of this study is to investigate the acute effects of gluteal and
hamstring ES on femoral artery blood flow, skin vascular function and energy
expenditure using ES shorts as a simplistic non-invasive method of ES.

Ten individuals with a spinal cord injury will be recruited to take part in
this study. The ES intervention consists of a portable functional electrical
stimulation box (Neuropro, BerkelBikes, Sint-Michielsgestel, The Netherlands)
connected to a wearable garment (shorts), which will safely house the wires
from the stimulator. Two built-in surface electrodes will be placed over the
upper part of the gluteal muscles and 1 over the hamstring muscles of 1 leg.
The surface electrodes (with conductive gel) are connected to elastic
conductors, guided through the side of the shorts to the front, ensuring the
participant does not sit on the wires. Stimulation will be delivered
biphasically at 50Hz to induce a (visible) tetanic contraction. The current
amplitude will be adjusted for each subject by increasing the current amplitude
in steps of 5mA to a point that doesn*t cause discomfort or excessive muscle
contractions. The stimulation protocol will consist of 3-minute blocks of
gluteal and hamstring activation for a total of 2 hours. A duty cycle of 1s
stimulation and 4s off will be used during the 3 minutes followed by a rest
period of 16 minutes. The average current amplitude using this stimulation
protocol has been 94±13 mA, ranging from 70 to 115 mA, in our previous studies.
The individuals will remain in a supine position for the duration of the
protocol.

The ES intervention consists of a portable functional electrical stimulation
box (Neuropro, BerkelBikes, Sint-Michielsgestel, The Netherlands) connected to
a wearable garment (shorts), which will safely house the wires from the
stimulator. Two built-in surface electrodes will be placed over the upper part
of the gluteal muscles and 1 over the hamstring muscles of 1 leg. The surface
electrodes (with conductive gel) are connected to elastic conductors, guided
through the side of the shorts to the front, ensuring the participant does not
sit on the wires. Stimulation will be delivered biphasically at 50Hz to induce
a (visible) tetanic contraction. The current amplitude will be adjusted for
each subject by increasing the current amplitude in steps of 5mA to a point
that doesn*t cause discomfort or excessive muscle contractions. The stimulation
protocol will consist of 3-minute blocks of gluteal and hamstring activation
for a total of 2 hours. A duty cycle of 1s stimulation and 4s off will be used
during the 3 minutes followed by a rest period of 16 minutes. The average
current amplitude using this stimulation protocol has been 94±13 mA, ranging
from 70 to 115 mA, in our previous studies. The individuals will remain in a
supine position for the duration of the protocol.

The paralyzed buttock and leg muscles of the participants will be activated,
which does not evoke a marked burden or any discomfort. A minor risk is that
the skin under the electrodes may be lightly irritated, but this will disappear
quickly. The experiments will take 3 hours in total.