Of course if the trip happens when you start the treadmill, then you can be pretty sure that it is the problem. There are probable reasons for the treadmill to cause the trip. It's likely to be an overloaded circuit where the treadmill is trying to draw too much power in its usual function. There could be a few reasons it's doing this.
The problem is usually the motor is the part drawing more power than normal. This is more likely in older treadmills. Not because they're older as such but due to other issues making the motor try harder than normal. When there is too much weight on the treadmill then it has to try harder to turn the belt. The power cord needs to be checked to see if there's a problem there.
When the breaker trips is important. Does it happen when you switch on the treadmill? When the belt starts or when you stand on the belt? Further diagnoses can be found on this tripped circuit breaker page. An old belt can mean the motor needs to try harder to power itself and overloads the circuit and trips the breaker.
What can also overload a circuit is many appliances working at once with a high amperage. This is inconvenient and you may just need to turn the highest drawers on your circuit off. If there is anything you're not sure about then a professional is best called. Especially where live electrical wires are concerned. Better to be safe than sorry.
I'm not a qualified electrician but with a bit a bit of research you may be able to save a lot of time and money when it comes to diagnosing a fault. When looking for an electrician, look for one who is qualified in your state and or comes recommended by others. There are around 700,000 electricians in the US so you've a lot of choice.
Electric treadmills usually last around 10 to 12 years, so if yours is old you may want to consider getting a new one. Constantly repairing one can start increasing in cost and may make a new treadmill viable. If you do have an old treadmill you'll be surprised at the new technology and what they can do now.