Playing with a strong dog is a matter of leverage, not strength. I can usually play with a dog that is up to half of my weight - more than that and I will suffer. The trick is one of technique....the attached video will illustrate a few points.
This is Cisu; she's about 55 pounds. When she plays tug she uses her entire body. She tries to pull me backwards and she thrashes her head back and forth. She's powerful for her size - I'm grateful that she's no bigger. In this video she's 9.5 years old; imagine what playing with her was like in her prime!
Here's what you may notice when I play with Cisu:
My body is absolutely straight. Do not let the dog pull you over or you'll end up with a back injury.
My arms are close to my body and my hands move rather little. My feet do most of the moving and my arms act as shock absorbers.
I keep her front feet off the ground and I discourage her from putting her front feet up on me by moving when she attempts to do this.
With this dog, I either use a jute ball on a stiff rope or a two handled tug. As a rule I do not like two handled tugs, because they take your arms too far from your body. It works here because the toy is short, so keep that in mind if you prefer two handled toys.
When I ask for an out, I put my hands on the toy in order to hold it still. If I held it by the handles when I asked for an out, she would be able to move the toy, and therefore would be unlikely to release cleanly.
Tug toys are presented horizontally so she won't target my hands. When you hold a tug toy vertically, you are asking for trouble because dogs have trouble turning their heads and targeting a vertical toy unless you've specifically trained for this skill.
Balls on ropes can be presented vertically, but it takes some practice to learn how to do this so the dog targets the ball and not the rope or your hands. The trick is keeping the ball portion over their heads, which is why I like toys with a stiff handle when I play tug with a ball. In this video, you'll notice that she misses the toy. This is not intentional; I simply forgot that Cisu has a hard time using her rear end to propel her upwards. As I said earlier, she's getting older. When I realized she was missing the toy, I adjusted my expectations.
There will be plenty of trial and error, and likely a few bruises as you figure it out. Try videotaping and compare what you are doing to what I am doing.
Go practice. Good luck!