BACKGROUND: The heart coordinates with skeletal muscles to facilitate movement, maintain cardiovascular homeostasis, and adapt to exercise. However, the precise mechanisms through which autonomic control of heart rate variability facilitates coordination with distinct muscles have yet to be elucidated, especially in the context of aging. Here we investigate the temporal evolution of cardio-muscular coordination during a maximal squat test, with specific focus on comparing young and older adults. METHODS: Thirty young and ten older adults performed one maximal body weight squat test until exhaustion. During the protocol, simultaneous recordings were taken of a 3-lead electrocardiogram (EKG Lead II) along with electromyography (EMG) signals from the following leg and back muscles: left and right vastus lateralis, and left and right erector spinae. We first obtained instantaneous heart rate (IHR, representing heart rate variability) derived from the EKG signal (Pan-Tomkins QRS detection), and decompose the EMG recordings in ten frequency bands [F1-F10], representing distinct muscle fiber types. We next quantified pair-wise coupling (cross-correlation C; amplitude-amplitude coupling) between the time series for IHR and all EMG spectral power frequency bands in each leg/back muscle. RESULTS: Young adults showed a stronger level of cardio-muscular coordination between the heart and all leg and back muscles. Specifically, low [F1-F5] EMG frequency bands, associated with type-I slow muscle fibers, exhibited stronger coupling with IHR (CMEAN = 0.35: SD = 0.03) compared to intermediate/fast frequency [F6-F10] EMG bands (CMEAN = 0.20: SD = 0.02). In contrast, older adults showed an overall significant decline in coupling strength between IHR and all EMG frequency bands (CMEAN = 0.05: SD = 0.02) compared to young adults (p < 0.001). CONCLUSION: The pronounced break-down in cardio-muscular coordination in older adults may be attributed to the age-related deterioration of heart rate variability, resulting from impaired autonomic nervous system function and reduced cardiac flexibility. Understanding the mechanisms underlying the decline in cardio-muscular coordination during exercise is crucial for developing strategies to counteract the effects of aging. This dynamic network approach can lead to the development of novel network-based markers to assess the impact of aging on cardiac and neuro-muscular function.

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