The LF/HF ratio does not accurately measure cardiac sympatho-vagal balance. This study challenges the long-standing assumption that the LF/HF ratio reflects the balance between sympathetic and parasympathetic nervous system activities. The LF component of heart rate variability (HRV) is not solely due to sympathetic activity but is influenced by parasympathetic and other factors. Similarly, the HF component is not exclusively due to parasympathetic activity, as sympathetic activity can also modulate it. The LF/HF ratio is further complicated by non-linear interactions between sympathetic and parasympathetic activities, as well as influences from respiratory parameters and mechanical events. Additionally, the LF/HF ratio is affected by the average heart rate, making it difficult to interpret changes in HRV solely based on this ratio. The study also highlights that the LF/HF ratio can be distorted by mathematical manipulations and that respiratory parameters significantly influence HRV. Therefore, the LF/HF ratio is not a reliable indicator of sympatho-vagal balance and should not be used to assess cardiovascular autonomic regulation. The study concludes that the physiological basis for the LF/HF ratio is not clearly discernible, and the ratio may provide misleading information about the balance between sympathetic and parasympathetic activities.The LF/HF ratio does not accurately measure cardiac sympatho-vagal balance. This study challenges the long-standing assumption that the LF/HF ratio reflects the balance between sympathetic and parasympathetic nervous system activities. The LF component of heart rate variability (HRV) is not solely due to sympathetic activity but is influenced by parasympathetic and other factors. Similarly, the HF component is not exclusively due to parasympathetic activity, as sympathetic activity can also modulate it. The LF/HF ratio is further complicated by non-linear interactions between sympathetic and parasympathetic activities, as well as influences from respiratory parameters and mechanical events. Additionally, the LF/HF ratio is affected by the average heart rate, making it difficult to interpret changes in HRV solely based on this ratio. The study also highlights that the LF/HF ratio can be distorted by mathematical manipulations and that respiratory parameters significantly influence HRV. Therefore, the LF/HF ratio is not a reliable indicator of sympatho-vagal balance and should not be used to assess cardiovascular autonomic regulation. The study concludes that the physiological basis for the LF/HF ratio is not clearly discernible, and the ratio may provide misleading information about the balance between sympathetic and parasympathetic activities.