Approximately 700,000 strokes occur annually in the United
States; 50 percent of the 550,000 survivors experience
residual
hemiparesis
affecting one side of the body
and approximately 165,000 of
those individuals have mobility deficits requiring
assistance with walking [1-3]In this population, hemiparetic
gait is a major problem that limits mobility, increases risk
of falls, and imposes higher energy demands for basic daily
activities [4-5]. Gait deviations due to hemiparesis are
well documented, in terms of both clinical manifestation and
biomechanical analyses [6-7]. Classic models of stroke
recovery indicate that improvements in both upper- and
lower-limb motor function plateau between 3 and 6 months
poststroke [8]. Recent studies have challenged this
assumption by suggesting that specific training
interventions that target use of the hemiparetic limbs can
improve motor control and neural plasticity. The research
community now widely accepts that the central nervous system
comprises inherently plastic neural networks that are
continuously amenable to reorganization in the service of
functional behaviors [9]. As a consequence, new therapeutic
approaches seek to exploit experience-based CNS plasticity
to mediate functional improvements. A common thread among
most of these interventions is an adherence to the
principles of motor learning, as defined by incorporating
high volumes of task-oriented practice along with the added
dimensions of goal setting and performance feedback [10].
Studies of therapies that improve function and induce
neuroplasticity in hemiparetic upper limbs in human
survivors of stroke have supported an emerging focus on
developing new learning-based strategies for improving gait
and balance function in individuals with lower-limb
hemiparesis after stroke [11-17]. Here we review evidence
that one particular mode of exercise, treadmill (TM)
training as applied in a number of approaches, can be
employed to improve gait function in survivors of stroke
with residual hemiparesis. We will suggest that basic motor
learning strategies can alter underlying neural mechanisms
to improve hemiparetic function of the lower limb and may
also be effective in recovery of walking ability after
stroke. Following a brief overview of the rationale and
early results from studies using TM training with stroke, we
provide examples that illustrate the role of the CNS in
lower-limb motor control and gait. Our focus then shifts to
an overview of how the neurophysiologyof lower-limb motor control is sensitive to
short-term adaptations and rapid plasticity. Finally, we
review the early evidence of central neuroplasticity
underlying lower-limb function and gait using long-term TM
training protocols.
RATIONALE FOR TREADMILL
LOCOMOTOR
LEARNING AFTER STROKE
Findings from spinalized animal models demonstrate that
walking without supraspinal inputs can occur when the animal
is placed on a moving TM [18]. Thus, several investigations
have studied TM training as a means to improve locomotor
function in subjects with incomplete
spinal cord injury and stroke. Visintin et al. first
adapted the findings from spinalized animals to human
experiments, reasoning that activation of
subcortical neural
structures by TM walking could provide a physiological
stimulus for recovery of gait function [19-20]. These
studies support the rationale that TM-generated stepping
patterns in neurologically injured humans can help deliver
repetitive sensory inputs to the spinal cord, which in turn
could mediate locomotor learning and neural plasticity
through a process of sensory motor integration [21].
Additional feasibility for this idea was shown in a study of
the immediate effects of the TM stimulus on hemiparetic gait
patterns in naive subjects with chronic stroke [22]. While
controlling for walking speed,
paretic
paretic /pa·ret·ic/ (pah-ret´ik)
pertaining to or affected with paresis. limb
stance-swing parameters and loading impulse immediately
became more symmetrical on the TM compared with usual
overground
TREADMILL-BASED
EXERCISE TRAINING IMPROVES GAIT FUNCTION
The initial studies with human SCI and subacute stroke
subjects used TM training in conjunction with partial
body-weight suspension (PBWS
PBWS Performance Based Work Statement
(contracting mechanism) ).
In a randomized study of more severely impaired subjects
with subacute stroke, Barbeau and Visintin found TM with
PBWS to be more effective than TM without PBWS for improving
selected mobility outcomes in those subjects with more
severe motor deficits (i.e., <0.2 m/s walking velocity and
Berg Balance scores <15) [24]. By week 6 of training, 79
percent of subjects were able to train at 0 percent PBWS. In
a noncontrolled 3-week study, 25 PBWS TM training sessions
improved mobility scores and gait temporal-distance
parameters in nine nonambulatory stroke subjects [25]. PBWS
was not required after day 6 of training in seven of these
nine cases. Similar results were reported in a follow-up
study using the same approach in an A-B-A design [26]. These
studies indicate an important role for PBWS as a bridge to
full-weight-bearing TM exercise, particularly in subjects
more severely affected.










