It is important that the dog be set up properly at the start of a pull. The dog needs to be leaning into the harness with his head down as shown. When pulling the weight cart the hardest part of the pull is getting the cart in motion. The sledge is a different situation, as the level of resistance for the dog remains consistent for the distance of the pull. Working the pulling track.
Repetition is what builds and conditions the dog.
Our target objective is to work the dog through 3 sets of 5 pulls with the dog pulling 3 to 4 times a week. As the weight increases and the dog is starting to have a hard time with the pull, reduce the weight considerably and increase the number of repetitions. Start building the weight up once again. The dog needs go be given a few minutes rest between pulling sets.