I suffered an achilles tendon injury when I was 24 on the left side.I never thought much of it until I had a neuro exam by my PCP about 5 months ago. I can't walk tiptoe because of weakness in my left calf. Neuro workup was negative (MRI, etc). I imagine that might be the explanation for why I had difficulty mounting a horse from the ground. Admittedly, I never tried it from the off-side.
Anyhow, I just tried standing on one foot. On the left, I was able to do about 7 seconds. On the right, I quit after 25 because, like @bachophile I got bored.