Changes in the biomechanics of gait may alter the energy requirements of walking in Parkinson's Disease (PD). This study investigated economy of gait during submaximal treadmill walking in 79 subjects with mild to moderate PD and the relationship between gait economy and 6-minute walk distance (6 MW). Oxygen consumption (VO2) at the self-selected treadmill walking speed averaged 64% of peak oxygen consumption (VO2 peak). Submaximal VO2 levels exceeded 70% of VO2 peak in 30% of the subjects. Overall the mean submaximal VO2 was 51% higher than VO2 levels expected for the speed and grade consistent with severe impairment in economy of gait. There was an inverse relationship between economy of gait and 6MW (
Walking capacity is central to the performance of many activities of daily living. Difficulty with walking is one of the cardinal symptoms of Parkinson’s Disease (PD). Alterations in the biomechanics of gait, such as decreased stride length, increased stride length variability, and reduced gait speed, are common even in early stages of PD [
Participants for this study were recruited from the University of Maryland Parkinson’s Disease Center and the Baltimore VA Medical Center neurology clinics as part of an exercise intervention trial in PD [
All physical performance measures, rating scales, and functional tests were performed while the subjects were “on” or within 3 hours of medication intake. Subjects used an additional dose of medication to maintain the “on” state when necessary.
The UPDRS was administered by a neurologist with expertise in movement disorders (LS). The Total UPDRS includes three subscales: Mentation, Behavior, and Mood (Part I), Activities of Daily Living (Part II), and the Motor Examination (Part III). Short distance ambulatory function was assessed with three-timed 10 meter walks. The self-selected walking speed was defined as the average velocity of the three tests. This short-distance test is widely recognized as a valid index of mobility recovery and simulates the distance required for many home-based daily functions. The 6 MW is a distance that is more representative of community-based daily activities. Participants were instructed to cover as much distance as possible in 6 minutes, turning every 100 feet, as prompted by orange traffic cones set apart across a flat, clear space.
A screening graded-treadmill test to voluntary exhaustion without measurement of the rate of oxygen consumption (VO2) was performed using a manual protocol as previously described [
The initial target speed for treadmill testing was the subject’s self-selected over ground walking velocity, with the incline set at 0%. The first stage was conducted for 2 minutes at 0% grade, the next stage was conducted for 2 minutes at 4% grade, and then the grade was subsequently advanced by 2% every minute until voluntary exhaustion. In frailer subjects, the second stage was conducted at 2% instead of 4% for a more gradual increase in workload. Once the grade reached 10%, subjects were asked if the speed of the treadmill could be simultaneously advanced with grade (generally by 0.2 mph). The electrocardiogram (ECG) was monitored continuously, and blood pressure was measured during the first 3 stages of the tests and every 2 minutes during recovery.
At the next study visit one week later, subjects underwent a progressive-graded exercise treadmill test to voluntary exhaustion as described above with measurement of peak oxygen consumption (VO2 peak using a Quark Cardio Pulmonary Exercise Testing metabolic analyzer (Cosmed, Rome, Italy)). In some subjects, the initial treadmill speed was adjusted slightly based on the results of the screening treadmill test and feedback from the research subjects. As a result, the average self-selected walking speed on the treadmill was 94% of their self-selected over ground speed (2.31 ± 0.59 miles per hour (mph) versus 2.46 ± 0.53 mph). The first stage was conducted for 2 minutes at 0% grade (first submaximal treadmill stage), and then advanced as described above. O2 consumption, CO2 production, and minute ventilation were measured breath-by-breath, and values averaged for 20 second intervals. Subjects were instructed not to talk during the test as this is known to affect the depth of breathing and gas exchange. Based on our pilot study [
We used the average O2 consumption values obtained over the final 40 seconds of the first sub-maximal treadmill stage to measure economy of gait. The 2-minute duration of this stage is similar to the time spent on many activities of daily living. Economy of gait was calculated as the measured VO2 during the first treadmill stage divided by the predicted VO2 for non-PD age-matched subjects based on commonly accepted American College of Sports Medicines equations for subjects walking accounting for treadmill speed and grade [ VO2 = horizontal component + vertical component + resting component, VO2 (mL/kg/min) = 0.1 (speed) + 1.8 (speed) (fractional grade) + 3.5, Speed = speed in meter/minute, to convert to mph, 1 mph = 26.8 meter/minute.
Higher oxygen consumption levels for any given speed and treadmill grade imply increased energy expenditure and impaired economy of gait.
SAS version 9.2 (SAS Institute, Inc, Cary, NC, USA) was used for the statistical analyses. Descriptive statistics are expressed as mean ± standard deviation (SD). Pearson's correlation coefficients were used to calculate strength of relationship between variables. All statistical tests were two sided and performed at a significance level of 0.05.
Seventy-nine subjects (57 men and 22 women) completed this cross-sectional study. Physical characteristics and PD severity scores are summarized in Table
Subject characteristics, disease severity, and physical performance measures.
Parameter ( | Mean ± SD | Range |
---|---|---|
Age (years) | 65.1 ± 10.7 | 42 to 86 |
UPDRS total | 47.2 ± 14 | 15 to 96 |
UPDRS motor | 32.4 ± 10.2 | 11 to 66 |
Hoehn and Yahr stage | 2.2 ± 0.4 | 1.5 to 3.0 |
Hoehn and Yahr Stage 1.5 | — | |
Hoehn and Yahr Stage 2.0 | — | |
Hoehn and Yahr Stage 2.5 | — | |
Hoehn and Yahr Stage 3.0 | — | |
Body mass index (kg/m2) | 28.1 ± 4.9 | 18.0 to 41.6 |
VO2 peak (mL/kg/min) | 22.4 ± 4.8 | 12.6 to 37.4 |
Submaximal VO2 (mL/kg/min) | 13.0 + 3.3 | 5.1 to 21.6 |
Walking speed (mph) | 2.31 ± 0.59 | 1.0 to 3.8 |
6 min walk distance (meters) | 424 ± 106 | 122 to 695 |
*6-min walk performed in 75 subjects.
The VO2 at the self-selected treadmill walking speed averaged 64% of their VO2 peak. There were, however, a wide range of values (31% to 89% of VO2 peak). Interestingly, 24 of 79 subjects had submaximal VO2 levels that exceeded 70% of their VO2 peak, indicating severe reduction in economy of gait, with 3 subjects approaching 90% of their VO2 peak. Overall the subjects had mean submaximal, self-selected walking speed VO2 values that were 51% higher than the VO2 levels expected for the same speed and grade for non-PD subjects (13.0 ± 3.3 mL/kg/min versus 9.7 ± 1.6 m/kg/min). This observation provides clear evidence of the large decreases in economy caused by parkinsonian gait patterns (Figure
Submaximal VO2 measured at self-selected walking speed during the last 40 seconds of the first 2-minute stage of treadmill test versus walking speed in mph. Diamonds show measured values, where solid line shows expected value (VO2 mL/kg/min predicted = 0.1 × 26.8 speed in mph + 3.5). The vast majority of subjects had measured values higher than the predicted values indicative of poor economy of gait.
We examined whether PD severity was associated with economy of gait (the ratio of measured VO2 and predicted VO2). There was a significant correlation of HY stage with economy of gait (Figure
Relationship between the Hoehn and Yahr stage and economy of gait (ratio of measured VO2 to predicted VO2). Higher values of the ratio of measured VO2 to predicted VO2 are indicative of impaired economy of gait.
There was an inverse relationship between the distance covered during the 6 min walk (6 MW) and the amount of oxygen subjects consumed at their self-selected walking speed during the first stage of the treadmill expressed as a percentage of their VO2 peak.
Our results demonstrate that economy of gait is markedly impaired in people with mild to moderate PD that increases the energy demands of physical activity. Our subjects walking at their self-selected pace on the treadmill required on average 64% of their VO2 peak. Indeed, 30% of our subjects used over 70% of their VO2 peak during their self-selected treadmill speed, and several subjects approached 90% of their VO2 peak. By contrast in healthy younger and older individuals, most activities require a small percentage of the maximal or peak working capacity as indexed by their VO2 peak [
There was a relationship between HY stage and economy of gait, such that individuals with more severe PD had poorer economy of gait. Impairments in gait and mobility impact on the ability of subjects with PD to perform a number of gait-dependent daily activities including housework, dressing, and transferring in and out of bed [
Few studies have directly measured walking economy in PD. Christiansen et al. examined walking economy at a number of walking speeds in subjects with PD compared to healthy subjects without PD [
There is growing interest in the effects of aging and medical comorbidities on bioenergetics and their impact on mobility and other measures of physical performance [
This study has limitations that may result in an underestimation of the severity of the impairment of economy of gait. (1) The submaximal O2 utilization was measured by using O2 utilization during the last 40 seconds of the first stage of the treadmill test, when subjects walked at their self-selected speed and 0% grade. We chose this time as representative of the time period in which our subjects typically walked. A number of investigators have advocated measuring submaximal O2 for longer periods of time [
There is substantial interest in whether the abnormalities in gait and functional performance in PD can be improved by treadmill exercise training [
In summary, this study reinforces prior evidence showing impaired economy of gait in PD that is associated with impairment of ambulation at both short and long distance. Reduced economy of gait combined with the reduced VO2 peak results in lower physiologic reserve where even comfortable gait is performed at a high percentage of VO2 peak. Future research should examine the biomechanical and neuromuscular factors that contribute to impaired walking economy in PD. A better understanding of these factors may lead to new approaches to improve functional performance and quality of life in PD.
This work was supported by the Michael J. Fox Foundation for Parkinson’s Research, The National Institute on Aging (NIA) Claude D. Pepper Older Americans Independence Center NIH Grant P30-AG02874, VA Rehabilitation Research & Development Maryland Exercise and Robotics Center of Excellence, and the Baltimore VA Medical Center GRECC. The authers also wish to acknowledge the hard work and efforts of Terra Hill, Jessica Hammers, Kate Fisk, Brad Hennessie and other members of the study team.