Many clients don not understand the term "bonking" when I first use it with them. It was originally a term coined by runners to describe what happens to them after a long run where the body runs out of fuel and is no longer capable of movement. Much like a car that runs out of gasoline, the body has depleted its ready sources of glucose and is unable to adequately create energy from fats or glucose at that point and begins to shut down.
This phenomenon normally happens to marathon runners but it most commonly happens to 90% of the population in a much more mild form when they begin any new exercise regimen. That shakiness you get 5, 10 or 15 minutes into your new exercise program that makes you feel unstable on your feet and unable to finish your routine comes from a lack of glucose or more properly in this case an inability to convert glucose or fat into energy fast enough to fuel the demand being made on the muscle tissue.
Sadly, this is a result of a severe deconditioned state and indeed affects as much as 90% of the population if we were to get them all started in an exercise program today. The reason for this is why we have such an increase in Type II diabetes today. We use our muscles so little in our sedentary lifestyles, that they have no reason to store glucose for ready energy in the muscle tissues (commonly called glycogen storage). The tissue becomes insulin resistant meaning it will no longer allow insulin to feed glucose up to the tissue since it does not need it. As such all the energy you eat (fats and carbs) are then just stored as fat in your body rather than being stored in the muscle tissue for energy. In addition the body becomes very inefficient in converting these fat stores back to readily usable energy and therefor the body is incapable of completing its physical task.
In a marathon runner, they totally deplete the body of this energy even though they are highly trained and have very efficient storage and burning mechanisms. The end result is the same, bonking. They are unable to create movement due to a lack of fuel in the body. In the case of the rest of the population, they are cut short in the early stages of their exercise routines as they don't have efficient storage mechanisms and they are unable to convert fat to energy fast enough to fuel their bodies. The result again is bonking. This is what causes many people to try an exercise program once and then just stop because they believe it is too hard.
The reality is if you just accept your short-term limitations and don't push past that point initially you will quickly begin to overcome this problem starting with your very next workout. After the first workout your muscle tissue immediately begins adaptation by taking in more fuel (glucose) than they normally would for the new increased challenge demanded of them. Although slight at first, the muscle tissue rapidly becomes better at storing energy and at converting fat to fuel its movements within even a few short workouts.
In our gym, this problem normally happens in the first few days while performing our warm-ups. The client quickly runs out of fuel and is unable to complete the warm-up routines but this is designed to be this way. The initial days of new training are meant to reeducate the muscle tissue to begin to store more glucose, burn fats better and develop neuromuscular pathways from the brain to improve form and technique. So don't let what is normal distract you from your goals. Take it one step at a time as you quickly begin to move through your entire workout routines with no problem. When you reach this point you will be storing more energy in the muscles and less as fat causing you to be leaner and more energetic.
See Pre-Workout Nutrition
See Post-Workout Nutrition