Of the many debilitating symptoms present in Parkinson’s disease (PD), hesitation associated with freezing of gait (FOG) is a common manifestation of the disease. Approximately one third of individuals with PD experience transient breaks in voluntary motor activity that interfere with executing complex movements or switching between different movements .
During gait initiation, an anticipatory postural adjustment (APA) phase precedes stepping [2–6]. For forward stepping, these APAs involve muscle-activated changes in ground reaction forces that shift the center of pressure backward and toward the initial swing limb, propelling the body center of mass forward and towards the single-stance limb prior to stepping. Compared to healthy subjects, the medio-lateral (M-L) and antero-posterior (A-P) ground forces and center of foot pressure changes characterizing APAs in PD patients are longer in duration and reduced in amplitude with prolonged delays between APA onset and step onset [7–10]. Moreover, while APAs are normally present during voluntary step initiation, they are often absent in PD patients experiencing hesitation delays [4, 9].
A longstanding clinical observation indicates that difficulties with initiating locomotor movements such as gait with PD may be transiently overcome if the normally automatic APAs that precede and accompany such movements (I.e. lateral weight shift during step initiation) receive modest manual assistance from a clinician . This observation suggests a possible disruption in the normal coupling between posture and locomotion. In this regard, neurophysiological studies have indicated that the control of posture and locomotion are normally interdependent at many levels of the central nervous system (CNS) involving supraspinal and spinal networks [12–15]. What remains to be determined, however, are the ways by which locomotion may be affected by initial postural conditions. This issue appears to have important implications for current rehabilitative interventions in PD which mainly focus on separate aspects of the problems such as posture and balance training  or gait training [17–19].
We have previously shown that the APA-stepping sequence in PD subjects could be improved through neuromechanical assistance with lateral weight transfer prior to step onset . In these experiments we introduced a novel self-triggering stimulus loosely based on the aforementioned manual assistance during step initiation from a clinician . In the case of manual assistance, the clinician provides mechanical assistance that enhances the deficient lateral weight shift observed in PD prior to stepping. In the self-triggering paradigm the stimulus is activated by the subject instead of a clinician. When a PD subject attempts step initiation, deficiencies can be overcome by a computer-controlled device that responds to their attempt at lateral weight shift by measuring changes in ground reaction forces and providing mechanical enhancement while they are shifting weight. The self-triggering paradigm requires the subject to initiate the movement, but provides real-time assistance if movement deficiencies exist, improving the coupling between postural control and stepping control during step initiation .
The responsiveness of hesitation-prone PD subjects to a small-magnitude, posture assistance stimulus (lateral displacement of the pelvis using a computer-controlled, motor-driven device incorporating cables attached to a waist belt) was demonstrated following a 50-trial training intervention. Subjects were tested without the stimulus before and immediately after training showing decreases in APA duration, earlier step onset times, and faster first step speeds. Furthermore, step duration was retained one-week post-training . To more directly facilitate the changes in APA forces affected indirectly by the waist-pull stimulus, a drop (or elevation) of support surface beneath the stance foot was substituted for the lateral waist-pull stimulus . PD subjects responded favorably to the posture assisted locomotion (PAL) drop stimulus that reinforced the intended APA action by showing reduced APA time durations, increased peak APA amplitudes, and earlier step onset times.
Because gait and balance problems often respond poorly to treatment with anti-parkinsonian medications, and to other interventions such as deep brain stimulation [22, 23], physical therapy interventions are an important clinical treatment for individuals with PD [19, 24, 25]. For acute applications, the PAL stimulus has been useful for improving the linkage between posture and locomotion during gait initiation, but its effectiveness for long-term applications remains unknown. The purpose of this study was to assess the feasibility of a training program incorporating the PAL stimulus that could ultimately be applied clinically in order to help PD patients overcome difficulties associated with start hesitation and possibly freezing of gait. With the application of this intervention we expected to see improved APA and stepping performance consistent with reduced start hesitation.