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I'm getting into a better routine of training regularly and I know I'm getting more fit. I can go for longer at the same rate of exertion that used to blow me out after a couple of minutes. I haven't tested myself recently, but I know that my HR at AT (implied by HR at VT) is higher than it was when I started training a year ago.

(AT is the Anaerobic Threshold - more modernly termed the Lactate Threshold - the point at which lactate starts to build up in the blood; VT is the Ventilatory Threshold - the point at which you start breathing much faster than you were at only a slightly lower level of exertion. VT is much more easily measured than AT or LT, and it serves as a proxy measure for them.)

Here's my question - what is different in my body from a year ago? Do I have more mitochondria? Are they more efficient? Is the chemical balance in my body different? I know the effects of training - I want to know what is actually changing as I get more fit.

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What is AT and VT? –  user3085 Apr 18 '12 at 0:35
    
@Sancho - I edited my question to include an explanation. I hope this helps. –  D Mac Apr 18 '12 at 1:32
    
I don't think I know the answer, but you might find the beginnings of one in this discussion of energy systems in this discussion of the Prowler (PDF). –  Dave Liepmann Apr 18 '12 at 3:04
    
The book Body By Science provides some discussion on this topic. Traveling without access to the book, or I would post a synopsis of some relevant parts as an answer... (anyone else with a copy of the book feel free to jump in!) –  Greg Apr 22 '12 at 3:20

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The short answer to your question, “What is different in my body from a year ago?" Just about everything!

  • More mitochondria? - Yes, esp. if your training includes aerobic or endurance exercise. According to Dudley's research,

    "an increase in the intensity of training brings about the greatest adaptive response in the mitochondria."

  • Are they more efficient? - Yes. According to this reference

    “Endurance exercise enhances mitochondrial function across the life span.” “Eight weeks of treadmill training (80% peak O2 uptake, 5 d/wk) augmented mitochondrial function, maximal rate of ATP synthesis in isolated mitochondria, and whole-body maximal O2 uptake."

  • Chemical Balance different? - Yes, AceFitness has a nice summary of some of the changes exercise causes in the glands of the endocrine system and hormone secreations affecting bone and muscle growth, blood pressure, insulin levels, and mood elevation.

    • Exercise affects: thyroid hormones that regulate "general metabolism, growth, and tissue differentiation as well as gene expression" and fatty acid oxidation.

      The results of this study show that exercise performed at the anaerobic threshold (70% of maximum heart rate, lactate level 4.59 ± 1.75 mmol/l) caused the most prominent changes in the amount of any hormone values.

    • Increases in concentrations of HDL (High-density lipoprotein cholesterol)

Your cardio-vascular system adapts and two important changes include:

  1. Increase in stroke volume - how much blood the heart pumps with each contraction making it more efficient with each heart beat. The heart muscle increases to meet that need.

  2. Lower resting heart rate so it beats less frequently.

There is a start to some of the changes taking place, but the full answer includes changes in muscle fibers, respiration, blood vessel elasticity, body composition etc.

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