As he neared the bottom of the stairs, his knees almost locked.

The left knee has locked causing a lifting action.

His knees locked, his hold broke down, and he collapsed.

Grasp the handles and slowly push the plate out until your knees are straight but not locked.

He tried to stop himself but his knees wouldn't lock.

"Even if your knees lock," she said, "you cannot free your voice."

Maybe- He tried to get on his feet but his knees wouldn't lock.

My knees locked up, keeping me from bolting, but I didn't know if that was good or bad.

They were sleeping standing up, heads bowed, knees locked against their weight.

But he didn't sway or fall; his knees locked and he stood rigid.