Weight Training Improve Your Muscular Fitness
The term "muscular fitness" can be applied scientifically which means muscular endurance or it can mean improving muscle tone a density that has very little to do with endurance. We will very briefly cover these two different points hoping to get to what is meant by muscular fitness.
If you are training with weights on a regular basis then doing any set that takes less than 10 seconds to reach complete failure will be using ATP taken directly from the muscle glycogen stored in your muscles. Movements like 5 sets of 5 reps will improve muscle strength as well as muscle fitness, over an extended period of time.
This is called anaerobic exercise because no oxygen is used to perform the movement. You may be out of breath after the set but that is recuperation as the air you are sucking in is not used for acquiring the energy needed to complete the set. A 30-70-yard sprint run in under 10 seconds would be the same.
Scientifically speaking Phosphate-Creatine (PC) is what your body uses to restore the ATP levels used when doing the set or the sprint. This will continue until the entire PC in the muscle has been depleted. Sports science tell us that the average athlete will get around 10 seconds of ATP usage in total before depletion.
If you are training with weights and doing 12, 15 or even 20 reps that can take up to 60 seconds to complete then you are still not using oxygen to do the set (anaerobic) as the ATP and all the remaining PC have been depleted. This is called the "Lactic Acid" or Glycolytic pathway.
Without getting too complicated it is all because of glycosis which is the carbs getting broken down WITHOUT the use of oxygen in order to restore the ATP lost. The by-product of this fast and rather complicated process is Hydrogen ions, which is the pain you feel when lifting a weight for a lot of reps and NOT the lactic acid build-up as most people think.
The aerobic system is used when any movement is done for more than 60 seconds because it needs a consistent supply of oxygen to continue preventing glycosis and creating lactic acid build-up. All aerobic movements reduce lactic acid build-up compared to anaerobic movements with the disadvantage being that it will prevent strength and speed gains when trained in the long term.