Anything can be the thing we choose to ruin our posture […]
Anything can be the thing we choose to ruin our posture. So yes a computer chair could be that object. Equally sitting incorrectly even on the most ergonomically suitable chair may also ruin/affect your posture.
Posture isn't something that comes about because of factors outside of us; bad posture is a way of thinking as well as a way of standing, sitting, walking and generally using our bodies.
What may be particularly unhelpful is to persevere in using a computer chair that proports to be ideal for good posture when in fact it isn't suitable for us and we ignore the signs.
Our posture and our body in general represents the life we lead and the thoughts we think. We get to choose both these and therefore we get to choose our posture.