An increasing number of patients are being diagnosed with SIER. While there have been systematic reviews evaluating other types of ER42, 43, there has been no systematic review of SIER to date. In our systematic review, we describe the clinical characteristics and outcomes of 97 patients with SIER. We also report several significant findings that might have potential implications in the regulation and conduct of spinning classes.
In this systematic review, we elucidated several key findings. First, data from our systematic review suggests that SIER is a relatively new clinical phenomenon. The majority of articles (19 out of 21) relevant to SIER were published within the last decade. The recent surge in the number of patients diagnosed with SIER can be correlated to the increasing popularity of spinning classes. Ever since spinning classes first started in the 1980s, the number of spinning venues and participants has surged exponentially. According to the UK Group Exercise National Survey Report, 745,000 individuals participated in spinning classes in 2018. Similarly, survey data from the ukactive Research Institute revealed that spinning classes were the most popular amongst various group workouts.44 A similar trend was reflected in data from the American College of Sports Medicine, where group training activities such as spinning classes was the third-ranked worldwide fitness trend of 2020. The reasons why spinning classes have soared in popularity over the last decade can be attributed to several reasons, such as easy accessibility and a low barrier of entry. The number of dedicated spinning studios or fitness facilities that offer spinning classes is increasing, possibly fueled by lucrative margins and good participation rates.45 Approximately 7000 businesses in the United States offer spinning classes, according to the latest market research reports.46 Similarly, more than 70% of fitness centers tracked by the International Health, Racquet and Sportsclub Association (IHRSA) provide spinning classes.47 Moreover, in a 400 spinning participants and instructors survey, 50% of respondents report a commute time of fewer than 10 minutes to their spinning class.48 Undoubtedly, the commonality, easy accessibility, and convenience of participating in a spinning session have contributed to spinning classes' popularity. Spinning classes also generally have a low barrier of entry, and individuals of varying fitness levels can sign up and participate.
Second, we found that the predominant demographic profile of patients diagnosed with SIER were young adult females with no significant comorbidities. This preponderance can be explained by the fact that the same young female demographic profile generally attends group exercises such as spinning classes. Data from the UK Group Exercise National Survey Report revealed that almost 80% of group exercise participants were female. This trend is also corroborated by survey data from Australia, where 76% of individuals who were willing to spend money on exercises classes such as spinning were female.49 A plausible explanation for the stark female preponderance in spinning class participation may explain the difference in exercise motivators between males and females. Several studies have demonstrated that females were more likely to engage in exercise conducted in a group setting for social reasons such as meeting friends or creating social bonds.50–52
Third, we found that most patients who develop SIER are participating in their first spinning session. This is unsurprising as several studies have shown that ER tends to affect individuals who are untrained or unconditioned to the causative activity.53 However, several factors are inherent to spinning classes that may further predispose individuals to develop ER. Spinning classes are usually high-intensity in nature and conducted in an indoor group setting with rhythmic music accompaniment. The Köhler effect is a well-documented phenomenon that describes increased motivational gain in individuals when executing a task in a group setting.54 Hence, spinning in a group setting may spur weaker participants to be more motivated and push themselves harder, increasing the risk of developing SIER. The Köhler effect has been demonstrated in several studies involving different physical activities, such as stationary bike exercise.55 Next, dissociative attentional stimuli such as music has also been shown to reduce perceived exertion during strenuous physical activity. A clinical trial showed that listening to music resulted in lower perceived exertion in adults undergoing high-intensity exercises using a cycling ergometer.56 A recent meta-analysis also found that listening to music during physical activity improved performance and reduced perceived exertion.57 These positive effects were further enhanced by listening to fast tempo music. Therefore, it is highly plausible that the environment and setting of spinning classes may reduce perceived exertion in first-time participants, putting them at increased risk of developing SIER.
The classical triad of symptoms associated with ER is myalgia, weakness and dark urine.58 However, only a handful of patients present with the classical triad of symptoms in our study.59 We found that 69.1% of patients diagnosed with SIER had reported presenting symptoms of myalgia, whilst 56.7% reported dark urine and only 16.5% reported weakness. More importantly, we also found that patients with SIER typically present in a delayed fashion, with a mean time to presentation to a healthcare facility of 3.1 ± 1.5 days, despite developing symptoms much earlier. Healthcare professionals should be cognizant of the variability of symptoms and the potential for delayed presentation in patients with SIER. If in doubt, patients who present with suspicious clinical history or symptoms should be referred to the emergency department for diagnostic work-up.59 The absence of dark-colored urine or myoglobinuria also does not preclude the diagnosis of ER, or more specifically, SIER.60
During the clinical examination of patients with SIER, the most commonly reported signs were muscle tenderness, swelling, reduced power and reduced range of motion. In 97.7% of patients, the signs and symptoms reported during the clinical presentation were predominantly in the thigh. This preponderance can be attributed to the muscle groups that are involved in indoor cycling. In general, the cycling pedal stroke can be divided into the downstroke and the upstroke. During the downstroke, the quadriceps, hamstrings, and gluteus maximus work together to generate the power required to push the pedal downwards.61 Interesting, a study has found that the use of the gluteus maximum is significantly lower in indoor cycling than outdoor cycling.62 This phenomenon may be due to steeper seat tube angles in indoor cycling, leading to more quadriceps and hamstring activation.
The diagnosis of ER is usually based on the clinical history and is supported by biochemical investigations. There must be a preceding or inciting exercise activity, followed by the onset of muscle-related symptoms. In terms of biochemical investigations, the diagnosis of ER is typically based on serum CK levels. Serum CK is a reliable diagnostic marker of ER as it gradually rises to its peak 24-36 hours after the inciting exercise activity and then slowly declines back to baseline over the next few days.63 There, however, has not been a consensus regarding the diagnostic cut-off level of serum CK for ER. Some authors have suggested a level of more than five times the upper limit of normal, while others have proposed using a level of more than fifty times the upper limit of normal.64–66 In our systematic review, we found that most studies had performed and reported CK levels on presentation. On the other hand, serum myoglobin levels were only reported in five studies. This paucity of serum myoglobin data likely reflects real-world clinical practice where myoglobin may not be performed routinely in the diagnosis of ER. Serum myoglobin, however, has been shown to have some predictive value in the development of acute kidney injury and should be performed if available in the laboratory.67 In our systematic review, we also found that liver enzymes were commonly performed and reported. Abnormal liver enzymes are frequently observed in patients with severe rhabdomyolysis and they seem to have a predictive role in mortality for critically ill patients with rhabdomyolysis.68 However, more specific to ER and SIER, the role of abnormal liver enzymes has not been well established.
The main goal of management for patients with ER is to prevent complications such as acute renal failure. In our systematic review, all patients received intravenous hydration, which is the primary supportive treatment of ER. Patients with ER can be severely hypovolemia due to water sequestration by the injured muscle and must be rehydrated aggressively.69 In additional to intravenous hydration, one-third of patients with SIER received intravenous sodium bicarbonate. The administration of sodium bicarbonate for alkalinization of the urine is routinely performed. For instance, some authors propose a fluid regimen consisting of half isotonic saline and sodium bicarbonate.70 Urine alkalinization with sodium bicarbonate may reduce the precipitation of Tamm-Horsfall protein complexes and the formation of brown granular casts.71 Moreover, urine alkalization has been demonstrated to help diminish redox cycling and lipid peroxidation, thus preventing oxidative stress, tubular damage, renal vasoconstriction and eventual renal impairment.72 However, there is limited high-quality evidence that alkaline diuresis has a proven clinical benefit over standard saline resuscitation.73, 74 More trials are thus needed to ascertain if bicarbonate therapy should be part of the mainstay treatment for SIER.
While we did not observe any long-term sequelae of SIER, a small proportion of patients developed acute kidney injury.7, 23, 25, 29, 35, 38 Although two out of the seven patients who developed acute kidney injury required temporary inpatient hemodialysis, no patients developed long-term renal sequelae.35, 38 This finding is consistent with current literature, where a generally favorable renal prognosis is observed following acute kidney injury related to exertional rhabdomyolysis. Several studies have reported the risk of renal failure in healthy young individuals with ER to be from 0–8%.75, 76 In our systematic review, aside from transient renal dysfunction, four patients with SIER developed compartment syndrome of the thigh requiring fasciotomies. Compartment syndrome in the setting of ER is a rare complication. As most patients with SIER may present with typical symptoms of muscle pain, tenderness and swelling, we need to have a high index of suspicion for compartment syndrome if the pain does not improve or subside with medical management.77 As the clinical diagnosis of compartment syndrome in patients with SIER can be challenging due to overlapping signs and symptoms; we should consider using a handheld pressure monitor to evaluate the intercompartmental pressures of the lower extremities if compartment syndrome is suspected.78 An intercompartmental pressure of more than 30 mmHg is suggestive of compartment syndrome.79 If in doubt, consider an urgent orthopedic surgery or vascular surgery consult. Expedient diagnosis and treatment are vital to prevent sequelae such as irreversible muscle damage, nerve injury or limb ischemia.
With the increasing popularity of spinning, there is an urgent need to exercise greater caution to prevent SIER. Guidelines that recommend safe ways to start indoor spinning should be established for the public. First and foremost, participants of spinning classes, especially first-timers, should be adequately informed about the risks of SIER and taught how to prevent over-exertion. Second, spinning classes should take precautions by reducing the intensity of exercise for first-time spinners by prescribing a graded program. Third, spinning instructors should be taught how to identify participants at higher risk of developing SIER. Next, there should be greater public awareness of the signs and symptoms of SIER such that patients can seek medical attention earlier. Lastly, medical professionals should be aware of the clinical signs and symptoms of SIER and possess a high index of suspicion to diagnose SIER. Early intervention is vital to prevent short and long-term sequelae of SIER.
The findings of this review should be interpreted in the context of known limitations. Due to our included studies' small sample sizes and methodology, we were unable to conduct formal weighted statistical analysis. Thus, we could not draw any statistical associations for the various outcomes of interest in our article. The majority of articles included in our systematic review were case reports or case series that may be associated with significant publication bias, in which only positive or interesting findings are published.