Physiological Cost Index of Walking Performance Over Treadmill and Synthetic Track in Post-COVID Patients

Introduction

The global pandemic of Coronavirus disease (COVID-19) was caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Many people around the world have become victims of this disease.¹ Around 241 million Coronavirus cases have been reported worldwide and among them, 34 million cases were reported in India. About 218 million people recovered from the disease while four million deaths have been reported worldwide.² The main impact of the SARS-CoV-2 virus is on the respiratory system; nevertheless, it also affects various other organ systems. Initial cases reported symptoms associated with lower respiratory tract infections, such as fever, dry cough, and difficulty in breathing. Furthermore, individuals exhibited additional symptoms like headache, dizziness, generalized weakness, vomiting, and diarrhoea.³ Symptoms may begin one to fourteen days after exposure to the virus. In our experience, the most common symptoms reported among patients post-COVID-19 were, respiratory and cardiovascular impairment with breathlessness, continued need for oxygen support, excessive fatigue and activity intolerance, musculoskeletal impairment with deconditioning, loss of weight, muscle and joint pains, neurocognitive dysfunction with impaired memory, lack of concentration and loss of sleep, psychological disturbances due to social isolation, fear of illness and insecurity with an impaired post-COVID-19 functional status.⁴

Post the disease, the energy levels of the individual reduces and results in various complications. Physiotherapy can play a crucial role in post COVID-19 patients to restore their functional abilities and endurance.⁵ Considering this, we analysed the post COVID-19 patient’s energy levels in walking by using a standardised measurement method called ‘Physiological Cost Index’ (PCI). The PCI uses heart rate to indicate the energy cost of a person in terms of walking.

The PCI was developed by McGregor as a simple and practical method to assess gait efficiency and energy expenditure.⁶ The PCI relies on the research conducted by Astrand and Rodahl, indicating a linear relationship between heart rate, walking speed, and oxygen consumption at submaximal levels.⁶ The PCI has been used to assess energy expenditure in children and adults with functional impairments.^7-9 Research demonstrated a consistent correlation between energy expenditure, PCI, and level of functional disability.^10,11

The PCI is calculated as below:

PCI = Working Heart Rate - Resting Heart Rate (beats/ min) / Walking Speed (meters/min)

Earlier research reported the average PCI in healthy adults as 0.35 beats/meter, with a variability spanning from 0.11 to 0.66 beats/meter.^12,13 A decrease in PCI reflects increased walking efficiency. To adhere to 6 Thomsun T et al., RJPT 2024;4(2):5-11 testing guidelines, three specific conditions must be satisfied.^14-16 Participants walk at a pace of their own selected speed, and the identification of a genuine resting heart rate, along with the application of a steady-state walking heart rate in the formula, has been recognized as the heart rate at minute 3 during submaximal workloads in healthy individuals.⁶

Various studies revealed a significant reduction in the energy levels of individuals after COVID-19, and occurrence of various complications such as respiratory and cardiovascular impairment with breathlessness, continued need for oxygen support, excessive fatigue, activity intolerance, musculoskeletal impairment with deconditioning, loss of weight, muscle and joint pains.¹⁷ Due to this, the day to day activities of an individual are affected. This study was conducted to estimate the energy efficiency of walking by measuring PCI among post-Covid patients.

Materials and Methods

The objective of this study was investigated using a single group repeated measure design. The study was conducted in Alva’s Fitmaxx, Fitness performance Centre, Moodbidri, Dakshina Kannada Taluk, Mangalore, Karnataka. Covid positive individuals discharged from the Covid unit of Alva’s Health Center, Moodbidri following isolation were included in this study. Convenient sampling method was used to select the participants. All the participants were made to perform walking on synthetic track for 50 meters at a comfortable pace. After an interval of one day, the participants were instructed to perform treadmill walking for 50 meters at a pace comfortable for them. A total of 11 participants were included in this study based on the inclusion criteria. Subjects above 18 years of age, belonging to both the genders, subjects in mild symptomatic stage of Covid in which the individual does not require medical supervision, subjects with no symptoms following the isolation, with ability to walk independently, stable medical condition, willing to participate in the study were included. The study excluded individuals with any musculoskeletal or neurological comorbidities, history of recent surgeries, and individuals dependent on assistive devices. To estimate physiological cost index, the parameters such as walking speed, heart rate and time taken were used. Vitals such as blood pressure, pulse rate and saturation of oxygen were recorded before, during and after the completion of study procedures.

Ethical Approval

Ethical approval for the study was obtained from Institutional ethical committee review board of Alva’s College of Physiotherapy, Moodbidri, Mangalore, Dakshina Kannada Taluk, Karnataka. Following the ethical committee approval, the study was conducted from August 2021 to October 2021. The study was carried out as per the standards set by the Ethical committee review board of Alva’s College of Physiotherapy, Moodbidri, Mangalore, Dakshina Kannada, Karnataka.

Procedure

The subjects were explained about the study and written informed consent was obtained. All the subjects were taken to Alva’s fitness center in their comfortable clothing and were made to sit for at least three minutes to record resting heart rate and vitals (Figure 1).

Procedures to perform the test were explained and demonstrated to all the participants. Each subject had to walk at their comfortable speed on a synthetic track for a distance of 50 meters with a pulse oximeter probe on their fingertips. The time taken for completing the target distance was measured using a stop watch. After completing the walk on synthetic track (Figure 2), each participant’s working heart rate and time taken for completing 50 meters were recorded.

After walking on synthetic track, the participants were rested for about 5 to 10 minutes until their heart rate reached a steady resting state which was similar to the measurement taken before initiating walking on synthetic track. Once the resting heart rate was achieved, and if there were no signs of physical exertion, subjects were asked to perform walking on a treadmill (Figure 3) with digital readings until they reached 50 meters, following which the vitals and time taken for completion were recorded.

From the working heart rate, resting heart rate and walking speed, the PCI was estimated for each individual using the formula, PCI= (Working HR - Rest HR) / Walking speed. For this, resting heart rate was measured first in beats per minute. To estimate walking speed, the participant was made to walk on a level, indoor surface and the meters covered per minute was measured.^18-20

Working heart rate was also measured in beats per minute, when walking heart rate reached a steady state. The resting heart of the individual was subtracted from working heart rate and was divided with the walking speed to obtain PCI. Lower PCI indicates efficient energy expenditure, whereas higher PCI indicates lower energy efficiency.²¹

Statistical Analysis

Statistical Package for Social Sciences (SPSS IBM 22) was used for statistical analysis. Paired t test was performed and level of significance was set at 5%. Kolmogorov Smirnov technique was used for confirming normality of data.

Results

A total of 11 participants (10 males, and one female) performed and completed the walk performance test. The mean age of subjects in this study was 22.90 years. Overall, velocity among participants was higher on the synthetic track compared to the treadmill, and the heart rate and time consumption were more on the treadmill than synthetic track (Table 1). However, the PCI of participants increased on treadmill compared to the synthetic track. The Kolmogorov Smirnov test revealed that the statistics of PCI on synthetic track was 0.26 and the treadmill was 0.17. The significance was higher on the treadmill (0.20) than synthetic track (0.03). The skewness was more on the synthetic track (0.70) compared to the treadmill (0.33) (Table 2). A paired t-test found that the mean and standard deviation on the treadmill were more than the synthetic track.

The t-value was 3.892 and the significance was 0.003 (Table 3).

Discussion

The present study tested the hypothesis that there exists a significant difference between energy expenditure for walking on synthetic track and treadmill, following discharge from COVID isolation ward. This hypothesis was accepted as the study demonstrated a significant difference in energy expenditure for walking on synthetic track and treadmill. The synthetic track has economical energy expenditure compared to treadmill. The difference in energy expenditure observed could be attributed to the shock absorption and cushioning effect characteristics of the synthetic track surfaces. This could be a contributing factor for the better energy expenditure on synthetic track than for walking on the treadmill. Though the platform in the treadmill is movable, the physiological demand to perform walking on a treadmill is higher than that of ground/level surfaces.²² The reason for this increase in physiological demand could be that the gait mechanics of level surface differ from that of treadmill surface and it is also influenced by the type of gait, the physiological condition of the patient, and footwear used.

Generally, the energy expenditure is lower at a freely chosen stride frequency compared to gait performed with the target stride frequency. This stride frequency could be a possible mechanism for the participants of this study to spend less energy on synthetic track compared to treadmill.²³

Also surface stiffness has a significant role in determining energy expenditure. It is reported that stiffer surfaces need more physiological demand than soft surfaces. Due to the cushioning effect of the synthetic track and the surface stiffness of the treadmill, there exists a difference in energy expenditure for walking.²⁴

Treadmill walking is performed within a limited and controlled space and the target distance speed shall be modified according to the potential of the participants. Even though it simulates the level surface of walking, most of the individuals require time to perceive their limitations to modify their target, distance/speed, failing to do so will exhaust them physically. This could be the reason that our participants failed to achieve a higher physiological cost index on the treadmill.²⁵

The aerodynamic drag force is reported to be more on the level surface than treadmill unless the treadmill is inclined and hence more calories are burned while walking outside the treadmill compared to calories burned on the treadmill.²⁶ Post discharge from Covid units, symptoms tend to continue beyond three months after being infected. Those symptoms are termed as post Covid conditions or long Covid or long-haul COVID or post-acute COVID-19 or chronic COVID. Post-COVID fatigue is one of the most commonly reported symptoms for which several physical therapy interventions are prescribed by identifying the most economical energy expenditure surface so that the exercise programs are performed without much difficulty and the individual can cope with rehabilitation.

The result of this study demonstrated that prescribing exercises on level surfaces is effective in terms of improving physical activity through the progressive nature of adaptation in physiological demand exhibited by the individuals while performing those activities.

Conclusion

It can be concluded that the energy expenditure for walking on level surfaces such as synthetic tracks was better compared to treadmills among individuals following discharge from Covid isolation units. The velocity of walking and the time taken to complete 50 meters of level surface walking were significantly better than treadmill walking. Most of the participants in this study were below the age of 25 years, had no preexisting comorbidities. Hence to validate the results of this study, future studies with different age groups and genders are necessary. This study suggests that level walking on synthetic track is comparatively better than the treadmill, in rehabilitating post Covid individuals.

Conflict of Interest

Nil