Like I previously stated in a topic, I feel that these advice topics for new riders are ineffective because they are too vague and broad. Plus they also lack things that seasoned riders think are important topics that should be covered. This will be one of several future posts that I hope to clarify things.
New prospects to the motorcycle world tend to have little clue on what they are getting themselves into. Unlike a bicycle, you can't just get on these machines and exploit them. It's a lot more complex than the twist of the throttle and work on the clutch. People don't understand how physical riding a motorcycle can be.
The riding position alone can put fatigue on a person's body. Rider geometry plays a huge part on safety. If the bars are too low and a bit forward it might create fatigue within the wrists, shoulders, and the upper back. Riding long distances on sport bikes will tire out the body, in which a tired body will make mistakes thus putting themselves in danger.
Another concept on how physical riding a bike can be is how the body is used in different ways to manipulate the bike. The way the rider shifts his body makes huge differences in front end to back end weight. In essense, the rider's weight will shift the overall weight distribution and can do several things with it. One example: the rear wheel begins to spin when launching from a stop. You have two options 1) letting off the throttle a bit 2) move your butt to the back of the seat. Doing both at the same time will bring the best results. This also applies to rolling on the throttle when exiting off a turn. Second example: the front end of the bike beings to lift as you accelerate. An inexperienced rider will probably ease of the throttle a bit to bring the front down. The best way to keep the drive yet alleviate the front end lift is using the legs to push the body forward into the tank which puts more weight onto the front end. This works under braking too. If the rear ends begins to lift shifting the body rearward will help bring the bike down.
Turning the bike at speeds over 25 miles per hour will require some physical force when turning. The bike really wants to go straight. You can move the bars in what direction you want to go but the rear wheel will want to go straight. So the body is used to coax the whole bike to travel into and through turns. This is where things get complex. Leaning the bike over isn't hard but shifting the body around with your arms and legs takes getting use to. If the rear end starts to feel vague while leaning, moving the body to the rear may cure feeling of nothing. Leg's can cramp and get worn out when riding at a fierce pace.
Another part of the body that will get tiresome is the wrists. When under braking whole body wants to move forward thus putting a lot of pressure onto them. When you accelerate harder, your wrists will get a workout as you hold tightly unto the bars.
Aerodynamics will play a part on fatigue. If a sport bike has poor wind protection, accelerating and keeping speeds become a task. The feeling is best described as "being pried off the handle bars". If a rider does a long haul on the highway on a bike with little wind-protection might have no feeling in the upper body since his hands were just holding on so tightly.
If your body is extremely tired don't go any further than you need to because it becomes dangerous. All that I talked about, I doubt a person will get tired riding around town doing tasks like braking and using the body to manipulate the bike. But if you are doing longer hauls you may want to plan on where you take break-stops.