You are on the leg press machine, your goal is to complete 20 reps before you rest, but, at rep number 10, your legs are starting to tire. By rep 15, you are worried you won’t be able to complete the last 5 reps and at rep 17 you have to take a short break because your muscles are too fatigued to keep proper form. After resting for just a few seconds, you are back at it to complete your last 3 reps. But why did you feel yourself fatigue? You recover quickly, why does it even happen in the first place? Now, let me forewarn you, this can get complicated fast, so I will try to break it down as much as possible.
When you are lifting weights you are placing excess stress on your body. In order for you to complete an exercise, your brain must set off a chain reaction of events to allow you to complete the task to the best of your ability while expending as little energy/resources as possible. When exercising, you will notice your breathing increases, to bring in more oxygen, and your heart rate will increase to quickly deliver the oxygen rich blood to the muscles at work.
Muscles use an energy source called ATP (adenosine triphosphate) for contraction. There is a minimal amount of ATP stored in the muscles (enough for about 3 seconds of work), so once muscle contraction begins, so does the process of making additional ATP. There are several biochemical systems that work together in phases for ATP production. In order, they are: creatine phosphate system (used for work up to 15 seconds), glycolysis system (used for work up to 60 seconds and is an anaerobic process, requiring no additional oxygen) and aerobic respiration (used for several hours or more with proper fuel and oxygen supply) (1).
So, back to the leg press. As you get to the 10th rep, the fatigue starts to kick in and you are now starting to use a greater amount of energy (ATP) than what is readily available. Continued muscle contraction is dependent on the supply of ATP. The creatine phosphate system has also used its ATP supply, so the glycolysis system is taking over. As ATP is depleted, and without enough oxygen being delivered to support the work demand, the muscle environment will turn acidic. Lactate is released to help neutralize this acidic environment (and break down glucose for energy), but this acidic environment is why you start to feel burning in the muscle (2). The muscle burn will make it hard to continue (especially with good form) so resting will give time for oxygen to be delivered and ATP levels restore; levels can be replenished quickly, so you will be able to finish your full rep range of 20 without issue. However, you will need a longer rest in between sets in order to allow for a full recovery.
Muscle fatigue during exercise cannot be discussed without addressing why muscle soreness occurs after exercise. This is known as DOMS or Delayed Onset of Muscles Soreness. DOMS occurs post-exercise anywhere from 8 hours to 72 hours after a strenuous session or when new exercises are introduced. DOMS occurs the inflammatory response to exercise-induced micro-tears in the muscles. This response will lead to swelling, soreness and limited range of motion for a day or two after exercise and will resolve within a few days. It is important to perform a warm up to help the joints/muscles prepare for the exercise at hand. It is also important to stay lightly active in between exercise sessions as resting will cause muscles to get tighter and take longer to recover (3).
Follow these steps to help improve exercise sessions (4):
- Lean Muscle Mass: by increasing your lean muscle mass you will have more muscle fibers to help handle the resistance load. It is recommended to exercise a muscle group for 10 to 20 reps, performing 3 to 5 sets.
- Form: All exercises should be performed with proper form. If you are unable to maintain good form, speak with a trainer for guidance or lighten your weight or change the exercise. Improper body mechanics will deplete energy stores more rapidly, so, it is important to learn what your muscle imbalances and incorrect movement patters are so you can correct them with a proper exercise program.
- Avoid Over training: It is important to give your muscles rest, do not exercise the same muscle on group back to back days.
- Nutrition: try to have your daily intake of carbohydrates be between 40% – 60% of your total daily intake. Eat a light carbohydrate based snack/meal 30 minutes to 2 hours before exercise to fuel the workout, and a protein rich meal within an hour of exercise completion to help repair and refuel muscles.
- Hydration: The body is made up of about 60% water, so staying properly hydrated will reduce muscle fatigue and cramps. Also be sure to replenish electrolytes during or after exercise.
- Sleep: getting 8 hours of sleep will help your body to recover and rebuild.
