The body needs all that energy either for speaking or for running - and at this point in time - maybe the required energy amount is so great it can only be used for one aspect at a time. After the running is under control - the language will zoom ahead. It hasn't stopped developing - it is still undergoing the process - it's just that we aren't seeing it - or hearing it right now.
Can you put yourself in the place of a young child and listen to the language surrounds that child lives in? One researcher [Friedlander, 1971] gives a great description:
Careful analysis of home language interactions, well recorded on quality audio equipment, ends to breed an enhanced respect for the awesome intricacy of the language learning process in its natural habitat. As one listens to such recordings, it seems altogether inexplicable how so delicate a fabric as language can be woven from such crude, chaotically tangled, poorly organized, and seemingly random raw materials as the natural sounds that surround an infant in a bustling household...Speech articulation in adult and sibling conversation is almost uniformly poor...Speech messages and language signals are deeply embedded in background noise; sound intensity levels are often inaudibly low or assaultively high; the speech stream flows with great rapidity; two or more people are often speaking at one time; and grammatical structures are often incomplete or very distorted. In such a mess, what is the stimulus?
Think about the non-human background noise we live with – pets, appliances, TV’s, radios, traffic, airplanes, etc. Yet in every culture children do learn their own language – and sometimes more than one language - and they learn in spite of all else that is going their lives :- )
