This video shows a new sort of exercise for me: assisted heel slides from a sitting position. Position makes a big difference in muscle response. That's why it is so important to vary exercises while standing, sitting, or lying down. That's also why it is important (I think) to have as much active physical therapy as possible. Your brain and body adjust to static situations, and your nervous system easily adapts to doing less work. Hence therapists say "use it or lose it." If the muscles are working, if the nerves are trying to trace new pathways, then it's time to find innovative ways to encourage more improvement.
Easier said than done. It isn't easy for anyone with a spinal cord injury to go through this process alone. Not everyone has returns to encourage, and certainly not everyone has a way to follow up with therapy or exercise. Thank you to all those who are helping me take advantage of the good changes!