The Dons recorded their second SPFL Premiership 0-0 in a match that followed away games in Slovenia, Perth, and a 360-mile round trip to face Ayr in the League Cup.
And the gaffer suspects his players' fourth match in just 10 days was a step too far for some of his men.
He said: "I always felt getting a goal in the first half was going to be crucial in this game and getting ourselves ahead, as there was a clear lack of energy in that second half,.
"That was without question, there was a lack of quality at times because there are players who are clearly out on their feet. (Graeme) Shinnie and (Ryan) Jack are real indications for me as they are as fit as any player going.
"You see them plodding through the game in the last half hour and that's just a consequence of playing four games in 10 days for the majority of them.
"Sometimes that lack of quality isn't there and I fully understand that, I felt for my players as they tried to give more than was physically possible.
"We deserved to get in at the break ahead but Hearts had enough moments in the second half to deserve a point. It was two teams who looked a bit tired but it was two teams who were doing their best to win the game."
Source: STV