Ah, the Biomechanics of Locomotion!
This is how I think of it.
Muscles play an important role. They contract, expand when you need them too.
Have you ever worked out? In early days of work-outs when you worked heavily on biceps, you may have felt pain in the triceps. Why? Thats because of muscle contraction.
Basically, two types: Eccentric Contraction and Concentric Contraction.
When someone pose for Biceps (like, a 90o angle between shoulder and the biceps, and the same between the biceps and the rest of the arm), what contraction he/she had? Both! Concentric Contraction at the Bicep and Eccentric Contraction at the Triceps.
Why Triceps pained more? If you can imagine, Concentric Contraction contracts you muscle and Eccentric Contraction expands your muscles (in length mostly).
I am more concerned about center of gravity (balance, speaking vaguely), When I am descending than when I am ascending. The center of gravity shifts on foot when I am descending since I don't want to let my body tip-off due to pace. Internally, muscles of the shin contracts eccentrically to control the descent of the toes after I place my heel down. If that doesn't happen, I will no longer have control on how fast my leg goes down and I would hurt myself awfully with every step downwards.
Its all gravity, when I walk downwards, descending a step, my knees are always always always straight. Thats because of the gravity. But as I step on surface, because the quadriceps contract eccentrically to control my knees from bending too much, there a spring action at the knees.. That helps to deal with impact. Quadriceps Muscles are group of 4 muscles around thigh. The reason I mentioned about Quadriceps is to hint you that its not just the Calf you are worried about. Its the whole mechanism!
If you think, its just the calf, try heavy running on a flat hard surface for a long time. You'd complain more about Shins than Calf and Knees.
If it goes beyond what you can and should deal with, you might want to consult your physiotherapist. And, you might want to read about DOMS: Delayed Onset Muscle Soreness.
Advice on how to walk on descents:
Well, consider taking a less sharper route way down the mountain, specifically if you are hauling a heavy backpack.
The other option can be effective use of walking sticks.
Some people do prefer one-step-down, then the other exactly besides it, one-step-down and so on...
I never paid too much attention to that ever, so..
The way to avoid it and/or partially live with it is:
- Stretching exercises should be a routine before you start the day
- Muscle strengthening
Hope this helps!