I started window cleaning thirteen years ago using wfd pole system.
After a short time I had tennis elbow in both arms and lost the strength in both hands due to tennis elbow,my neck also used to hurt from constantly looking up. Both problems sorted themselves out after a few months I was 47 at that time.
Thirteen years on,no tennis elbow,no neck problems,just a dodgeyhip,and I hate the winter and summer more now than I did then,and more customers appear to be arxxholxs,or maybe it's just my age.
Good luck I hope it works out for you