Resistance Stretching exercises. These generally need the help of another person, but some can be performed alone depending on the muscle you are working on etc.. (i.e. you could provide resistance against an object or use a towel etc...) In the end having a helper is the best method.
I take no responsibility for any injury caused by trying these exercise. If in doubt please consult a qualified person/doctor before trying these etc..
These should be done as a gently stretching and not a challenge to show off how strong you are as this may cause injury. Probalby start off really gently, then as you become more experienced, you may be able to increase the pressure, but these exercises should not be painful (unless thsi is post injury where I would be very cautious about attempting any such exercise without proper training).
Background:
These stretches are based on taking advantage the way the muscles work in unison. i.e. for every muscle in the body there is at least one apposing muscle (think of Bicep and Tricep).
There are fours exercises in total.
As the OP was about the Calf muscles, I'll write this based on that, but can be adapted to any muscle.
All calf exercises can be done either on the floor, or on a massage couch etc.. with the patient lying down or seated with legs horizontal. The helper is located facing the soles of the patients feet.
The Helper should explain each exercise and control the duration (count outloud).
1/ The first exercise is designed to work the opposite muscle which in turn should cause the calf muscle to relax.
With the foot roughly in the central point (in-between point and fully lifted) the foot should be held on top by the helper and the patient should work against the pressure but whilst the foot remains static. This is NOT a test of streangth, the idea is the cause the calf muscles to relax. This is held for a count of 10, then the patient relaxes. As they relax the helper should gently move the foot up, taking the calf muscle to it's full stretch. Note this position for reference. Hold this full streatch for a few seconds then release.
Repeat 3-4 times.

2/ Muscles work by continually contracting an relaxing, even when you hold a static position against a pressure, individual muscles sections are contracting and relaxing continually. This execrcise extends this further by stretching whilst working against a pressure.
Again with the foot at about the mid-point the helper applies pressure against the patient pushing down. Then the patient needs to ease the pressure so that the helper is able to slowly push the foot to it's full extension then hold this for a few seconds before relexing. Again this is done GENTLY. It may be easier to cup one hand under the heal to give better control.
3/ By working the Calf muscle then relaxing it, it is possible to increase the stretch.
Gently push the foot so that the calf is fully extended (Patient MUST relax). This is your Starting position. Now HOLD this position against the patients pressure for a count of 10, then the patient relaxes. As the patient does so gently stretch the calf further. This is your new starting position. Again have the patient apply pressure whilst the position is held for a count of 10, then stretch again to a new position. This should be repeated 3-4 times in total.
4/ Reverse of 2.
Pressure is applied to extend the foot. The patient lets the helper 'win' slowly. Once the foot is fully extended, the patient relaxes and helper stretches the calf to the full extent. Repeat 3-4 times.

Note:
It will also help if some massage if given pre & post these stretching excercises. After concentrate on aiding the removal of toxins towards the heart. Next installment maybe I can give some techniques here.
This was done from memory (from many yers ago), but somewhere I have notes on this and will try and dig these out when I have a minute so I can update this post.