As my last post explained, in doing tai-chi, muscles acting on opposing directions will be activated at the same time. And such directions are actually multiple or unlimited in scope, because we are talking about 3D! In the lingo of I-Quan/Da ChengQuan (意拳/大成拳) of Master Wang XiangZhai (王薌齋), it is called "Eight direction tuck of force" 八面爭力). Tai-chi is more circular whereas I-Quan is more angular (for executing punches). The logic behind each is the same.
In the limiting case of positive exactly equal negative, there will be no motion. This is the situation of Zhan Zhuang. One interesting point to note is that the practitioner will be motionless not withstanding the actual amount of balanced positive and negative force. If, say, the positive and negative force being +/- 10 force units or being +/- 1,000 force units, a person will remain motionless!
How then can motion be actualized? It is by tipping the positive force (defined as the direction towards which one wants to move) slightly over the negative force. If, say, using the numerical example above, a positive force of 11 and a negative force of 10 is equivalent to a positive force of 1,001 and a negative force of 1,000! I mean, equivalent in the sense of seeing the actualized movement by a casual observer!
This is tai-chi. And as the above analysis shows, it doesn't necessarily have to be done like a weak-old man (of course, it also doesn't mean it is not an ideal exercise for older folks!)