Question: Please explain in HRV / physiological terms what changing the challenge level means to the program? Does it require greater frequency power in the LF zone? Does a session that produced higher levels of power (close to 0.1HZ) reflect increased coherence?
Answer: Increasing the challenge level raises the threshold in the coherence algorithm for determining Low, Medium, and High coherence levels. The coherence scoring algorithm uses power spectrum analyses, a measure of frequency and frequency magnitude, of the heart rhythm to determine the amount of physiological coherence in the HRV waveform. By tracking the ratio of power in the coherence peak relative to the rest of the frequency spectrum a coherence ratio is calculated. The challenge level sets the level at which the algorithm scores the coherence ratio as low, medium or high. The higher the challenge level, the higher the coherence ratio score must be in order to accumulate coherence points. The more stable the frequency in both amplitude and frequency the HRV waveform becomes (visually this appears as a smooth regular sine wave like pattern in the HRV waveform) the more power is concentrated in the coherence frequency peak and thus the ratio of power in this peak relative to the rest of the spectrum.
Coherence is not strictly bound to the fixed boundaries of the LF region; most physiological processes vary over a range of frequencies. The coherence range spans from the top of the VLF through the lower portion of the HF range of the HRV power spectrum (approximately 0.03-0.24 Hz). It is not necessary for the coherence frequency to be at 0.1 Hz to be coherent, although 0.1 Hz is the center of the range around which the coherence or resonant frequency typically occurs when looking at large samples of the population. Each person tends to naturally find their own resonate frequency which can vary some from day to day or with different states.