I think we should not underestimate the physical power O-sensei still had later in life. Here's a perspective from Kanai sensei:

for full interview : http://www.neaikikai.com/do_k_interview1.htm

Interview questio : From seeing films and videos, people talk about O Sensei practicing differently at various stages of his life. Is one stage more important than another, or do you take in the 'whole' concept and sometimes practice 'hard' and sometimes practice 'soft'?

Kanai: I never knew O Sensei since the young times. But O Sensei said young people (train) 'hard'. (They) need to do that daily, (they) need to hold 'tight', to make sure (if) the technique works or not...that is what O Sensei told me, told us. But when I met O Sensei (he) was old, his technique was very soft. There is a problem (with that)...people think that's 'soft' (and) that's 'easy'. People think that. (Thinking) that way, those people don't know what O Sensei was doing...because I never felt O Sensei (as) 'soft' or 'weak'...always strong. (He would) bounce me off (the mat) when I took ukemi. People don't know that. Those (uninformed) people (are) bringing Aikido down, (they are) destroying it.