The objective of this systematic review was to summarize the physical and cardiorespiratory impairments observed in individuals suffering from long COVID. Our results do show physical impairments in people with long COVID, mainly a lower score on the SPPB (582 out of 1079 adults had a score of 10 or lower). It is important to point out that a score of 10 or lower provided the most accurate discrimination of physical limitations [48], and is associated with greater and progressively increasing odds of all-cause mortality [49]. Similar results have been reported in 41 adults recovering from pneumonia following COVID-19 even though they had no mobility deficits before the infection [50]. Their results showed a decrease of performance in 25% of included adults expressed by a high level of disability (scores < 5) and 22% of included adults expressed by a low-moderate disability (scores = 5–8). It has been reported that health-related quality of life is altered in adults with persistent symptoms when retrospectively comparing data before a COVID-19 diagnosis and 5 to 12 months after the original infection [41].
We identified a lower performance in STS in only one study out of the three studies included in this systematic review. Two studies assessed 5STS and the third assessed 1min STS in adults of the same age group. Abnormal performance (5STS : 14.3 ± 9.2s) was only reported in a study that included hospitalized long COVID patients [34] (21% of the adults of this study were hospitalized vs 0% in the others studies), which might be a factor contributing to the performance differences. Other findings show that people who were hospitalized and more severely ill during their hospitalization appear to have more severe persistent symptoms [51]. Caution should be taken about this result given the small number of studies assessing STS performance.
Furthermore, when focusing on 6MWT results, long COVID adults seem to have an altered physical capacity. A reduced walking distance in the 6MWT was reported in six out of eight studies. Long COVID adults walked a shorter distance than expected for their age-adjusted predicted value. A reduced distance in the 6MWT is associated with a decreased functional capacity and is a predictor for mortality [52]. Similar reductions have been shown in other populations, such as adults with chronic obstructive pulmonary disease, cancer or that have received an organ transplant [53–55]. Alternatively, SpO2 levels remained normal in long COVID adults when compared to the age-matched reference values, which means that some aspects of physical function were impaired without any significant change in SpO2 levels. In line with those results, a prospective cross-sectional study showed that 69% of hospitalized long COVID patients (n = 60) experienced a decrease in physical function while identifying reduced SpO2 in only 10% of the individuals [56]. Knowing that body mass index (BMI) is often correlated with walking distance [57], it is important to note that the mean BMI in the adults of included studies that assessed 6MWT was between 26–30, which is considered as overweight [58]. In this regard, a high BMI is associated with a greater likelihood of functional limitations and decline [59], which suggests that a high BMI may be a factor in the development of long COVID. A study identified obesity as a strong risk factors for the development of long COVID [60]. Furthermore, evidence shows how hormone and nutrient dysregulation in obese people can alter the response to infection [61], given that obesity is associated with many underlying risk factors for COVID-19, including hypertension, dyslipidemia, type 2 diabetes and chronic kidney or liver disease [47].
For oxygen consumption, a lower VO2 in the long COVID group was observed compared to control groups in some studies [31, 37, 45], but VO2 remained normal in all studies when compared to reference values [26]. In line with our results, Johnsen et al. (2021) reported a lower VO2 in only 2 out of 57 adults who had at least one respiratory symptom post COVID-19 diagnosis [62].
Except VO2 and SpO2 which do not seem to be altered in long COVID adults, physical capacities of these adults seem to be reduced when compared to reference values.
Clinical implications
Although there is some evidence of physical impairments in adults with long COVID such as a lower SPPB score and lower STS and 6MWT performances, the underlying mechanisms remain unknown. These should be monitored in adults experiencing persistent symptoms following COVID infection since those impairments can lead to decreased participation and quality of life as well as potentially increase mortality risks. Clinicians should ensure appropriate rehabilitation management is put in place for these patients to promote return to pre-infection function levels, or at the very least, ensure that impairments stemming from long COVID have as a little an impact on function and quality of life as possible.
Strengths and limitations
This systematic review was conducted following the rigorous methodological process suggested by the PRISMA guidelines with an extensive search strategy.
However, we also acknowledge some of the limitations of this review. Most of the included studies did not use strategies to deal with confounding factors which may impact results since reduced physical function can be explained by factors such as comorbidities, BMI, age and other relevant factors. This is especially true since long COVID has been shown to be more prevalent in populations with pre-existing comorbidities. This must thus be taken into account when interpreting the present results. The variability in symptoms duration for the included long COVID population can also impact results since some participants have had symptoms for four weeks while for others it has been many months. Also, even though there has been an exponential surge of publications on the subject, long COVID remains a novel condition that we do not totally understand yet. There are no clear diagnosis criteria besides persistent symptoms which limits our certainty regarding adults’ inclusion in the various studies. Finally, the most affected adults, those who would not be able to perform tests like the 6MWT, might not be part of studies included in this review, which might lead to an underestimation of the true burden of long COVID on physical and cardiorespiratory function.