there doesn't seem to be and in between human model
There would have been a gradual transition along the lineage leading to modern humans. There would be a reward in terms of physical resources by not growing a skeleton and muscles that were larger than necessary to cope with changing environments and social circumstances.
One of the transitions in that respect that may have opened the possibility for our brains to expand was covered in this thread...
#9 Less Chewing More Thinking
A broken gene for a type of muscle fibre we no longer have tells a story about our evolutionary past.