I once spent three months on crutches because I thought it was absolutely necessary to my happiness and well-being to run five miles a day. I also went to the gym regularly to use step-machines, bicycles, weights and attend aerobics, kick-boxing and body pump classes. All I got in return was a knee problem that left me in sever pain. I then decided to be kinder to myself.
Legs are important
We usually don't pay any attention to our legs until muscles are overworked and hurt, veins bulge, ankles, knees and hips cause pain, feet swell up or toes disfigure. Western sports, with an exception of swimming perhaps, are not body friendly at all, hence the unfortunate popularity of knee operations, hip replacements, physiotherapy and pain killers.
Yoga understands the aim of fitness better
In yoga, after strengthening a muscle it is stretched out, so that its shape elongates and the tissue becomes flexible. This is how the body grows strong - by stretching and strengthening it equal amount.
In addition, yoga postures are not designed to put any extra pressure or weight on the joints. You will not see anyone wearing a knee-support bandage in a class or hopping out on one leg after twisting the ankle. Our bodies are not designed to take all the kicking, pumping, jumping and bashing about that western sports are.
People can successfully practise yoga at the age of 70 and over (Iyengar is over 90), but to see anyone over the age of 35 kicking their legs about in fitness classes or running a marathon is less so. This is because western sports are actually not leg friendly. It is wise to pay more attention to your legs - they will in return serve you well in years to come.