Strength
Strength affects a soldier's carrying capacity/encumbrance as well as how far objects may be thrown. Also see item weights.
Starting Values
New recruits will always begin with a value between 20 and 40.
Improvement
Strength points are awarded (at end of mission) any time you perform at least one primary action. Just one action is enough - additional ones do not cause more Strength points to be awarded.
The possible points earned are dependent on your current Strength. At recruit minimum (20 Strength), an average of 6.0 points are earned. Thus, while your soldiers may be weak initially, they usually reach a reasonable level within a few combat missions. Points awarded after missions slope evenly down to where, at cap minus one (69), an average of 1.0 points are earned (range 0-2).
Unlike real life, carrying heavy weights throughout a mission has no effect on Strength improvement (sigh). Only primary actions cause Strength to increase.
Maximum Caps
A soldier's Strength is capped at 70. It is possible to 'overflow' the cap by one point on the last combat when you reach it. On average, you have a 25% chance of getting capped at 71 instead of 70. Once at 70+, all Strength improvements cease.