Week 9 - Exercise & Muscles: Explain 3

 Week 9 - Exercise & Muscles: Explain 3
Longer Term Structural Changes

As you likely know from personal experience, the more you exercise the easier that exercise becomes.  But why is that?  What changes about your muscle cells that allows them to adjust to new demands?
Angiogenesis and addition of mitochondriaStructural Changes in Aerobic Exercise

Recall that, if you are practicing an exercise that lasts more than a few minutes at a time, your muscle cells are probably relying on aerobic respiration to generate ATP.  Initially, it might be difficult to sustain your efforts during that exercise.  Your muscles will become fatigued as they struggle to produce enough ATP to sustain the activity and you will feel out of breath.  Over time, though, your muscles will develop new features to allow them to handle the exercise.  Quite sensibly, they will gain more of exactly the things they need to more effectively perform aerobic respiration!  Angiogenesis will occur to bring more blood, including its oxygen supply, to your muscles.  In addition, mitochondria within your muscle cells will reproduce to provide more sites for aerobic respiration.
Structural Changes in Anaerobic Exercise

Addition of sarcomere proteinsOn the other hand, if you are performing an exercise with very heavy weights, your muscles are probably not relying heavily on aerobic respiration for ATP.  Those exercises probably don't last more than a minute or so before you take a rest, so your cells can rely entirely on CP and glycolysis to generate ATP.  In that situation, gaining more blood vessels and mitochondria wouldn't really make sense.  What your muscles need is more power, not more oxygen and mitochondrial activity.

It is thought that myofibrils develop small tears when muscles are asked to lift very heavy weights.  Those tears and the inflammation that accompanies them like accounts for the stiffness and soreness that comes a day or so following the start of a new weight lifting program.  In response, your muscles once again attempt to add the required material to handle the exercise - in this case, more contractile proteins!  It's the pulling of actin by myosin that creates force in your muscles, so adding more acting and myosin will add more force.  It will also make your muscles thicker, which accounts for your more muscular appearance if you continue to lift heavier and heavier weights.  Remember that muscle cells don't divide a whole bunch after we're born, so muscles don't grow bigger due to muscle cell division.  Rather, the existing cells pack themselves fuller and fuller of sarcomeres, which makes the cells (and therefore your muscles) thicker.
A Time to Think & Put the Pieces Together

Before moving on, take a moment to think about the above information in relation to our hypothetical friend's question.  Our friend started a new workout program where "she cycles quickly through various dumbbell exercises for a few minutes before resting and repeating later."

    What type of exercise do you suppose your friend is performing?  Aerobic?  Anaerobic?  Could you make the case that it could be either?
    Based on the above question, what changes could your friend expect to see in her muscles over the coming months?  Why would those changes occur