Cardiac cycle-synchronized electrical muscle stimulator for lower limb training with the potential to reduce the heart's pumping workload

Association between skeletal muscle and left ventricular mass in patients with hyperthyroidism

Objective

This study aims to investigate the relationship between skeletal muscle and left ventricular mass (LVM) in patients with hyperthyroidism, providing theoretical and data-based foundations for further research on the interaction between secondary muscle atrophy and cardiac remodeling.

Methods

A retrospective data collection was conducted, including 136 patients with hyperthyroidism (Study group) and 50 healthy participants (control group). The Study group was further divided into Group A (high LVM) and Group B (low LVM) based on LVM size. Multiple linear regression analysis was performed to examine the correlation between skeletal muscle and LVM, with model evaluation. Based on the results, further nonlinear regression analysis was conducted to explore the detailed relationship between skeletal muscle and LVM.

Results

Compared to the control group, the Study group exhibited significantly lower LVM, skeletal muscle mass index (SMI), and skeletal muscle mass (SMM) (P<0.05). Within the subgroups, Group A had significantly higher SMI, SMM, and hand grip strength compared to Group B (P<0.05). The results of the multiple linear regression showed a certain correlation between SMI (β=0.60, P=0.042, 95% CI=0.02~1.17) and hand grip strength (β=0.34, P=0.045, 95% CI=0.01~0.67) with LVM. However, the residuals of the multiple regression did not follow a normal distribution (K-S=2.50, P<0.01). Further results from a generalized linear model and structural equation modeling regression also demonstrated a correlation between SMI (β=0.60, P=0.040, 95% CI=0.03~1.17) (β=0.60, P=0.042, 95% CI=0.02~1.17) and hand grip strength (β=0.34, P=0.043, 95% CI=0.01~0.67) (β=0.34, P=0.045, 95% CI=0.01~0.67) with LVM.

Conclusion

Patients with hyperthyroidism may exhibit simultaneous decreases in LVM, SMM, and SMI. The LVM in patients is correlated with SMM and hand grip strength, highlighting the need for further exploration of the causal relationship and underlying mechanisms. These findings provide a basis for the prevention and treatment of secondary sarcopenia and cardiac pathology in patients with hyperthyroidism.

Sarcopenia as a comorbidity of cardiovascular disease

Sarcopenia, the lowered skeletal muscle mass, weakened skeletal muscle strength, and reduced physical performance with aging, is a component of frailty and high-risk factor for falls, resulting in an increase in mortality. In cardiovascular disease (CVD) patients, systemic inflammation, oxidative stress, overactivation of ubiquitin-proteasome system, endothelial dysfunction, lowering muscle blood flow, impaired glucose tolerance, hormonal changes, and physical inactivity possibly contribute to CVD-related sarcopenia. Prevalence of sarcopenia and osteosarcopenia, which is osteopenia and sarcopenia coexisting together, seems to be higher in CVD patients than in community-dwelling adults, suggesting the necessity of early diagnosis and prevention of CVD-related sarcopenia. Atrial stiffness, coronary artery calcification score, and serum vitamin D levels may be of help as the biomarkers to suspect sarcopenia, and renin-angiotensin-aldosterone system inhibitors may play a role in the medical prevention and treatment of CVD-related sarcopenia. There are few reports to convince the efficacies of dietary and antioxidant supplementation on sarcopenia at present, whereas aerobic and resistance training exercises have been recognized as an effective strategy to prevent and treat sarcopenia.

Electrical muscle stimulation: Application and potential role in aging society

Neurodegenerative diseases and sarcopenia become more prevalent as individuals age and, therefore, represent a serious issue for the healthcare system. Several studies have reported the relationship between physical activity and reduced incidence of dementia or cognitive deterioration. Thus, exercise and strength training are most recommended treatments, but it is proving difficult to engage individuals to initiate exercise and strength training. Electrical muscle stimulation (EMS) may provide an alternative and more efficient solution. Although EMS has undergone a decline in use, mainly because of stimulation discomfort, new technologies allow painless application of strong contractions. Such activation can be applied in higher exercise dosages and more efficiently than people are likely to achieve with exercise. Unlike orderly recruitment of motor units (MUs) during low intensity voluntary exercise, EMS activates large fast-twitch MUs with glycolytic fibers preferentially and this could have benefit for prevention and treatment of diabetes and chronic diseases associated with muscle atrophy that ultimately lead to bed-ridden conditions. Recent evidence highlights the potential for EMS to make a major impact on these and other lifestyle related diseases and its role as a useful modality for orthopedic and cardiac rehabilitation. This paper will discuss the potential for EMS to break new ground in effective interventions in these frontiers of medical science.

Timed synchronization of muscle contraction to heartbeat enhances muscle hyperemia

Blood flow (BF) to exercising muscles is susceptible to variations of intensity, and duration of skeletal muscle contractions, cardiac cycle, blood velocity, and vessel dilation. During cyclic muscle activity, these elements may change proportionally with or without direct optimal temporal alignment, likely influencing BF to active muscle. Ideally, the pulsed delivery of blood to active muscle timed with the inactive phase of muscle duty-cycle would enhance the peak and average BF. To investigate the phenomenon of muscle contraction and pulse synchronicity, electrically evoked muscle contractions (trains of 20 Hz, 200-ms duration) were synchronized with each systolic phase of the anterograde blood velocity spectrum (aBVS). Specifically, unilateral quadriceps contractions matched in-phase (IP) with the aBVS were compared with contractions matched out-of-phase (OP) with the aBVS in 10 healthy participants (26 ± 3 yr). During each trial, femoral BF of the contracting limb and central hemodynamics were recorded for 5 min with an ultrasound Doppler, a plethysmograph, and a cardioimpedance device. At steady state (5th min) IP BF (454 ± 30 mL/min) and vascular conductance (4.3 ± 0.2 mL·min^-1·mmHg^-1), and OP MAP (108 ± 2 mmHg) were significantly lower (P < 0.001) in comparison to OP BF (784 ± 25 mL/min) and vascular conductance (6.7 ± 0.2 mL·min^-1·mmHg^-1), and IP MAP (113 ± 3 mmHg). On the contrary, no significant difference (all, P > 0.05) was observed between IP and OP central hemodynamics (HR: 79 ± 10 vs. 76 ± 11 bpm, CO: 8.0 ± 1.6 vs. 7.3 ± 1.6 L/min), and ventilatory patterns (V̇e:14 ± 2 vs. 14 ± 1 L/min, V̇o₂:421 ± 70 vs. 397 ± 34 mL/min). The results suggest that muscle contractions occurring during OP that do not interfere with aBVS elicit a maximization of muscle functional hyperemia.NEW & NOTEWORTHY When muscle contraction is synchronized with the pulsed delivery of blood flow to active muscle, muscle functional hyperemia can be either maximized or minimized. This suggests a possibility to couple different strategies to enhance the acute and chronic effects of exercise on the cardiovascular system.