INTRODUCTION
Coronavirus disease 2019 (COVID-19) is an infectious state characterized by rapid person-to-person transmission. It leads to clinical symptoms such as fever, acute respiratory failure, and pneumonia [1]. In addition, COVID-19 causes musculoskeletal disorders including fatigue, myalgia, arthralgia, muscle weakness, and muscle soreness [2-4]. These outcomes occur due to the COVID-19 virus-induced spectrum of myopathic changes via severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) binding to the angiotensin-converting enzyme 2 (ACE2) receptor on the surface of the skeletal muscle cells [5,6]. Furthermore, SARS-CoV-2 can also cause inflammatory processes on the skeletal muscle tissue [6]. Therefore, COVID-19 patients could present with myalgia and fatigue due to these inflammatory processes [7]. A previous study has reported that muscle atrophy and myofiber necrosis were found in severe acute respiratory syndrome patients [8].
Additionally, these symptoms could present after the end of a COVID-19 infection, known as post-COVID-19 [9]. Fatigue, dyspnea, anosmia, and headache are the most common symptoms found in post-COVID-19 [10-12]. Fatigue is found with high frequency among musculoskeletal symptoms in individuals with post-COVID-19, resulting in intolerance to physical activities and a difficult return to normal daily life [13-15].
Several studies showed that female sex, high body mass index (BMI), presenting underlying diseases, decreased physical activity and functional status, and age is one of the most important risk factors for post-COVID-19 [3,16-19]. A previous study found that hospitalized patients recovering from COVID-19 pneumonia who had no locomotor disability before the infection showed decreased skeletal muscle strength and physical performance [20]. Interestingly, older adults had a higher risk of musculoskeletal disorders during post-COVID-19. These results may be due to the combined effect of SARS-CoV-2 infection and previous age-related reductions in muscle function and mass [21]. In addition, muscle strength and cardiorespiratory parameters in older adults with recovery from mild COVID-19 have not been reported.
Therefore, this study aims to evaluate muscle strength using the handgrip strength test and to the sit-to-stand test (STS10) and assess cardiorespiratory parameters using the 1-minute sit-to-stand (1-min STS) test and 6-minute walk test (6MWT) in older adults with recovery from mild COVID-19. These results may help to determine muscle strength and cardiorespiratory parameters in these populations.
METHODS
Study design and participants
A cross-sectional and prospective study was conducted to evaluate muscle strength using the handgrip strength test and STS10 and to assess cardiorespiratory parameters using the 1-min STS test and 6MWT in older adults with and without recovery from mild COVID-19. All participants were recruited based on the inclusion criteria, and the physical assessments, including hand grip strength, the 6MWT, and the 1-min STS, were conducted after the participants had been included in the study. These assessments were part of the data collection process and were not based on pre-existing data. The present study was approved by the Clinical Research Ethics Committee of the University of Phayao, Phayao, Thailand. The IRB code was 1.3/032/65. All participants gave written informed consent after being fully informed about the purpose and methods of the study. A total of 80 older adults with and without recovery from mild COVID-19 were recruited, and divided into two groups (n=40/group): those who have recovered from mild COVID-19 (referred to as the “recovered group”) and those who never had COVID-19 (referred to as the “control group”). The sample size was calculated using a power of 0.80, power analysis with an alpha of 0.05, and the effect size d of 0.32 [22]. Inclusion criteria were as follows: subjects who were 60 years old with recovery from mild COVID-19 and had polymerase chain reaction or antigen test kit confirmed infection with SARS-CoV-2 at least 3 months before the evaluation procedure, had mild symptoms of COVID-19 such as fever, cough, nausea, headaches, coryza, malaise, sore throat, and/or muscle aches, and subjects who were 60 years old without COVID-19 infection, All subjects had normal BMI values (18.5–24.9 kg/m2) [23], a respiratory rate at rest of fewer than 22 beats per minute, pulse oximetry at rest of more than 94%, and were able to stand and walk without assistive walking devices. Exclusion criteria were as follows: subjects who had problems with vision, communication, hearing, and musculoskeletal impairment limiting the ability to perform the activities of daily life.
Procedures
The control group and recovered group were evaluated via the handgrip strength test, STS10 test, 1-min STS test, and 6MWT. There was a 30-minute interval between each test.
In the handgrip strength test protocol, subjects were instructed to position themselves standing with the elbow extended at the moment of the test. Three consecutive repetitions of 3 seconds, with 15 seconds of rest between the repetitions, were performed with the dominant arm. The highest peak force was recorded [24].
In the STS10 test protocol, subjects were instructed to stand up until the hips and knees were fully extended and then to sit down as the arms were folded across the chest. Subsequently, subjects were asked to perform this move as fast as possible ten times. The time required to complete the test was recorded [24].
In the 1-min STS test protocol, subjects were instructed to stand up until the hips and knees were fully extended, and then to sit down while the arms were folded across the chest. After that, subjects were instructed to perform this move repeatedly as fast as possible for 1 minute. The number of completed actions during the test was recorded [24]. Each subject was investigated and the cardiorespiratory parameters of systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), pulse oxygen saturation (SpO2), rate of perceived exertion (RPE) (Borg rating of perceived exertion scale; 6–20 scales), and leg fatigue (Borg CR10 scale) were recorded before performing the test.
In the 6MWT protocol, subjects were asked to wear comfortable clothing and shoes. Before the test, each subject was instructed to sit on a chair and breathe normally for 30 minutes. Each subject performed the 6MWT. The subjects walked as fast as possible without running for the six minutes of the test duration, continuing at the same pace without stopping. The distance completed in each 6MWT was recorded [25]. Each subject was investigated and the cardiorespiratory parameters of SBP, DBP, HR, SpO2, RPE (Borg rating of perceived exertion scale; 6–20 scales), and leg fatigue (Borg CR10 scale) were recorded before performing the test. Cardiorespiratory parameters were recorded after the 1-min STS test and the 6MWT, respectively.
Statistical analysis
Descriptive statistics were used to present demographic data. An independent sample t-test was used to compare the parameters of handgrip strength, STS10, 1-min STS, and 6MWT between older adults with and without recovery from mild COVID-19. IBM SPSS Statistics software version 22.0 (IBM Corp.), was used in this study, with a p-value of less than 0.05 set to denote significance.
RESULTS
The characteristics of the subjects are shown in Table 1. The results showed that the control group and recovered group had normal values of BMI (18.5–24.9 kg/m2), blood pressure, HR, and SpO2.
All subjects completed the handgrip strength test (kg) and STS10 test (seconds). The recovered group showed significantly decreased performance in the handgrip strength test (24.73±6.99 vs. 22.03±4.36, p=0.041) and increased a duration for the STS10 test (25.15±6.11 vs. 33.40±7.56, p<0.001) when compared to control group (Table 2).
After performing the 1-min STS, the recovered group had significantly decreased the repetitions of a 1-min STS (number) (31.38±4.89 vs. 21.25±3.64, p<0.001) and increased the RPE (7.43±1.20 vs. 8.95±1.01, p=0.01) and leg fatigue (1.49±1.13 vs. 3.00±1.04, p=0.03) when compared with the control group. However, blood pressure, HR, and SpO2 were not different between groups (Table 3).
In post-performance of the 6MWT, the recovered group had significantly decreased distance for the 6MWT (m) (421.68±8.28 vs. 384.35±6.17, p<0.001) and increased the RPE (7.63±1.37 vs. 12.05±1.63, p<0.001) and leg fatigue (1.71±0.88 vs. 5.28±0.91, p<0.001) when compared to control group. The results are shown in Table 3.
DISCUSSION
This study points out that recovered group showed decreased upper limb and lower limb muscle strength and also increased perceived exertion and leg fatigue at post-performance of the 1-min STS and 6MWT when compared to control group.
Our results found that recovered group exhibited decreased handgrip strength and a longer duration for the STS10 test when compared to control group. The handgrip strength test was used to investigate the upper limb muscle strength [24], whereas, the STS10 test was used to evaluate the lower limb muscle strength [26]. Furthermore, these tests could indicate physical performance, especially in older adults [24,26]. The decrease in muscle strength in the upper and lower limbs of older adults post-COVID-19 may be attributed to SARS-CoV-2 damaging skeletal muscle cells by attaching to ACE2 receptors on their surfaces. This interaction disrupts cellular functions, leading to tissue dysfunction and cell death [27-29]. Additionally, this mechanism could contribute to the muscle damage observed in post-COVID-19 patients [30]. In addition, a previous study found that patients with type 2 diabetes who had mild to moderate COVID-19 had lower muscle strength and higher fatigue than those who did not have COVID-19 infection [31]. These results occur due to patients with type 2 diabetes who had mild to moderate COVID-19 and had receded serum albumin levels [31]. A previous study reported that post-COVID-19 without prior musculoskeletal problems had a high prevalence of low physical performance and muscle weakness [32]. Furthermore, older adults with post-COVID-19 increased risk of musculoskeletal problems due to the combined effect of viral infection and preexisting age-related declines in muscle mass and function [21,33]. Our results are consistent with previous studies, in finding that adults with post-COVID-19 showed decreased handgrip and quadriceps muscle strength [20,22]. According, the present study indicated that recovered group had reduced upper and lower limb muscle strength. To provide a broader context for our findings, it is valuable to integrate existing research on recovery from COVID-19 and other infectious diseases. Comparative studies can offer insights into whether the observed muscle strength reductions and changes in physical performance are specific to COVID-19 or if they are common across various infectious diseases. For instance, research on recovery from diseases like influenza or severe acute respiratory syndrome (SARS) may reveal similar patterns of muscle weakness and functional decline. A study by Ong et al. [34] demonstrated that patients recovering from SARS experienced prolonged muscle weakness, which was attributed to both the direct effects of the virus and the prolonged immobilization during illness. Similarly, research on post-viral fatigue syndrome following influenza has shown that muscle strength and physical performance can remain impaired long after the acute phase of the disease [35]. By comparing these findings with those from COVID-19 recovery studies, we can assess whether the muscle strength reductions observed in our study are unique to COVID-19 or part of a broader trend seen in post-infectious recovery.
The control group and recovered group showed increased blood pressure, HR, and reduced SpO2 at post-performance of the 1-min STS test and 6MWT. In addition, our results found differences in repetitions of the 1-min STS test and the distance of the 6MWT between the control group and recovered group. Moreover, the recovered group had significantly increased RPE and leg fatigue when compared to control group. These findings suggested the recovered group experiences reductions in some parameters of cardiorespiratory performance. Several studies reported that fatigue and exercise intolerance were found in adults with post-COVID-19 [36]. These responses occurred due to post-COVID-19, associated with muscular impairment, resulting in decreased fitness [37]. Moreover, the mechanism of exercise intolerance in post-COVID-19 may be involved in the cardiorespiratory pathway [38]. Additionally, previous studies reported a decreased repetitions of post-1-min STS as indicated by RPE and leg fatigue in post-COVID-19 [20]. These results are consistent with our own finding; we found recovered group had decreased repetitions of a-1-min STS and increased RPE and leg fatigue when compared with the control group. Our study has shown that transient increases in blood pressure following physical activity are common and often within normal physiological ranges. A previous study found that while blood pressure may rise post-exercise, it typically returns to baseline levels within a short period [39]. Additionally, comparisons with studies involving other populations or disease groups can provide further insights. For instance, individuals with cardiovascular conditions may experience different blood pressure responses to exercise compared to those without such conditions [40]. To enhance our understanding, future research could focus on the implications of these blood pressure changes for post-COVID-19 patients. Investigating whether these changes are associated with increased cardiovascular risk or if they are simply a transient response to physical exertion will be crucial. This approach will help determine whether specific recommendations or interventions are necessary to manage blood pressure in this population effectively.
Conclusions
This study indicated that the recovered group had reduced muscle strength and exercise tolerance when compared with the control group. These findings highlight the urgent need for targeted rehabilitation programs tailored to post-COVID-19 patients. Such programs should incorporate resistance training, aerobic exercises, and respiratory therapy to address both muscle weakness and cardiorespiratory limitations.
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