For what it's worth I did a fair bit of research on using accelerometers for heart rate and respiratory rate determination (https://ieeexplore.ieee.org/abstract

For what it's worth I did a fair bit of research on using accelerometers for heart rate and respiratory rate determination (https://ieeexplore.ieee.org/abstract/document/5504743 https://ieeexplore.ieee.org/abstract/document/5504743) and while placement on the abdomen/chest is preferable, if the subject has a high excursion and low adipose tissue I was in some instances able to get a good signal from a device worn in a trouser or jacket pocket (especially when sitting). Never published that because it wasn't useful in a medical setting but could be of interest if you're recording it opportunistically.