Chapter 7 Metabolic Responses and Adaptations to Training

Chapter 7

Metabolic Responses and

Adaptations to Training

Copyright © 2012 American College of Sports Medicine

Introduction

• Definitions

– Metabolism: sum of all chemical reactions in the human body to sustain life

– Exergonic reactions: result in energy release

– Endergonic reactions: result in stored or absorbed energy

– Bioenergetics: flow of energy change within human body

– Energy

• Ability to perform work

• Changes in proportion to magnitude of work performed

• Chemical energy needed for several metabolic processes


Adenosine Triphosphate (ATP) and

Metabolic Systems

• Overview

– Body requires continuous chemical energy for life & exercise

– Potential energy transferred from storage or food to fuel muscle

– ATP

• High-energy compound used to fuel body

• Composed of adenine & ribose (adenosine) + 3 phosphates

• Hydrolysis: cleavage of phosphate bond releases energy

• ATP + H

2

O  ADP + P i

+ energy


Adenosine Triphosphate (ATP)



Metabolic Systems (cont’d)

• Three Ways Energy Can Be Used Quickly

1.

Skeletal muscle ATP stores

• Capacity: a few seconds of exercise

2.

Phosphocreatine (PC) system

• Capacity: 5-10 seconds of high-intensity exercise

• PC stored in skeletal muscle ( × 4 > than ATP)

• ADP + phosphocreatine  ATP + creatine

3.

Production of ATP from multiple ADP sources

• Capacity: >10 seconds of exercise

• 2 ADP  ATP + AMP




• Phosphagen Repletion

– ATP-PC resynthesis is critical to explosive exercise performance

– High-intensity exercise depletes PC by:

• 60-80% in first 30 seconds

• 70% in first 12 seconds

– Longer-duration high-intensity exercise reduces PC by 89%

– Greater the PC degradation, the longer the time to recover PC

– Biphasic response: faster + slower components

– Factors: intensity, volume, muscle pH, ADP level, O

2 availability




• Anaerobic Training Adaptations

– Positive adaptations in ATP-PC & adenylate kinase metabolic systems

– Occur in three ways:

• Greater substrate storage at rest

• Altered enzyme activity

• Limited accumulations of fatiguing metabolites




• Glycolysis

– IS—the breakdown of CHOs to resynthesize ATP in cytoplasm

– Anaerobic metabolic system

– Capacity: 2 min of high-intensity exercise

– Rate of ATP resynthesis not as rapid as that of PC

– Larger glycogen than PC supply in body

– Gluconeogenesis: reforming of glucose in opposite direction of glycolysis


• Fast glycolysis is also known as anaerobic glycolysis and slow glycolysis is commonly called aerobic glycolysis. These are dictated by the energy demands of the cells. If there is a rapid or high rate of type II muscle fibers being utilized then fast glycolysis is utilized. If there is a demand for primarily type I muscle fibers and oxygen present then slow glycolysis is utilized.




• Control of Glycolysis

– Inhibited by:

• Sufficient oxygen levels (steady-state exercise & rest)

• Reductions in pH

• Increased ATP, PC, citrate, & free fatty acids

– Stimulated by:

• High concentrations of ADP, P i

, & ammonia

• Slight decreases in pH & AMP

– Regulated by enzyme control & negative feedback systems




• Glycogen Metabolism

– Muscle glycogen = quick source of glucose

– > glycogen availability preexercise  endurance performance

– Glycogen use:

• Most rapid at beginning of exercise

• Increases exponentially as intensity increases

– Muscle & liver glycogen repletion:

• Critical to recovery after exercise

• Factors: hormonal action, glucose uptake, blood flow,

Amount of CHOs consumed!!




• Training Adaptations

– Changes in substrate storage & enzyme activity

– Aerobic training (AT):  muscle glycogen in FT & ST fibers

– Steady-state AT & high-intensity interval training:  muscle glycogen storage

– Sprint training: may not change or increase glycogen content

– RT: increases resting glycogen content by up to 112%




• Lactate

(p-111)

– Negative impact on performance

– Lactate production from pyruvate contributes to muscle fatigue

– Lactate is produced by your body in response to aerobic exercise and serves as a fuel for the muscles, delays fatigue and prevents injury. Lactic acid contains one additional proton and is not produced by the body at all during exercise.

– The technical difference between lactate and lactic acid is chemical. Lactate is lactic acid, missing one proton.


• To be an acid, a substance must be able to donate a hydrogen ion; when lactic acid donates its proton, it becomes its conjugate base, or lactate. When you're talking about body's lactate production and lactate or lactic acid threshold, the difference is largely a matter of semantics. But, the body produces and uses lactate -not lactic acid.




• Metabolic Acidosis and Buffer Capacity

– Blood & muscle pH decrease during & after anaerobic exercise

– Acidosis:

• Adversely affects energy metabolism & force production

• Causes onset of fatigue to be rapid

– Buffering capacity:

• Ability to resist changes in pH

• Increased after 7-8 weeks of sprint training

• Greater in trained than in untrained people


• Increasing muscle buffer capacity enables greater tolerance for acidosis. Higher lactate concentrations are seen following anaerobic training with concomitant improvements in performance. Enhanced buffer capacity prolongs high-intensity exercise. Trained individuals have a greater buffer capacity than untrained individuals. Muscle buffer capacity may increase by 16%-44% after 7-8 weeks of sprint training. RT may lesson H+ accumulation but may not increase buffer capacity.




• Aerobic Metabolism

– Occurs when adequate oxygen is available

– Is primary source of ATP:

• At rest

• During low to moderate steady-state exercise

– Majority of energy comes from oxidation of CHOs & fats

– Krebs cycle

(where the aerobic system begins)

• Continues oxidation of acetyl CoA

• Produces 2 ATP indirectly


Krebs Cycle


The Electron Transport Chain




• Energy Yield From Carbohydrates

– 3 ATP produced per molecule of NADH

– 2 ATP produced from FADH

2

– Glucose oxidation: total of 38 or 39 ATP produced

• 2 ATP from blood glucose glycolysis OR 3 ATP from stored glycogen glycolysis

• 2 ATP from Krebs cycle

• 12 ATP from 4 NADH produced from glycolysis & pyruvate conversion to acetyl CoA

• 22 ATP from electron transport chain




• Energy Yield From Fats

– Fat metabolism predominates at rest & in low/moderate exercise

– Lipolysis: breakdown of fats by hormone-sensitive lipase into:

• Glycerol

• 3 free fatty acids

– Fatty acids enter circulation or are oxidized from muscle stores via beta oxidation

– Beta oxidation: splitting of 2-carbon acyl fragments from a long chain of fatty acids

– People with high aerobic capacity can oxidize fats at a large rate




• Aerobic Training Adaptations

  # of capillaries surrounding each muscle fiber

  Capillary density: # of capillaries relative to muscle CSA

  Nutrient & oxygen exchange during exercise

  Reliance on fat metabolism

  # of mitochondria & mitochondrial density in muscle

  Myoglobin content

  Enzyme activity

  Muscle glycogen stores at rest




• Anaerobic Training Adaptations

  # of capillaries surrounding each muscle fiber

– No change in capillary density (&  with hypertrophy)

  Mitochondrial density in muscle

– No change in myoglobin content

– No change or  enzyme activity




• Energy System Contribution and Athletics

– All energy systems are engaged at all times

– One may predominate based on exercise:

• Intensity

• Volume/duration

• Recovery intervals

– Training systems can be designed to target each system


Interval Training: Work-to-Rest Ratio

• The work-to-rest ratio can target energy systems:

• 1:12 to 1:20 ratios target ATP-PC

• 1:3 to 1:5 target fast glycolysis

• 1:3 to 1:4 target fast glycolysis and aerobic oxidation

• 1:1 to 1:3 target aerobic oxidation


Metabolic Demands and Exercise

• Indirect Calorimetry

– Measurement of O

2 consumption via open-circuit spirometry

– Changes in O

2

& CO inspired ambient air

2

%’s in expired air compared with normal,

– Components: flow meter, computer interface

– Measure of energy expenditure

– Respiratory quotient (RQ): measure of CO

2 of O

2 produced per unit


Difference between RQ and RER

• Similar to RQ, is the respiratory exchange ratio (RER)

• Although RQ assumes that gas exchange results from nutrient breakdown ONLY, RER reflects more accurately the role of anaerobic exercise metabolism during exhaustive exercise. It is calculated the same way as RQ.


Metabolic Demands and Exercise (cont’d)

• Basal Metabolic Rate (BMR)

– Minimal level of energy needed to sustain bodily functions

– Factors affecting BMR:

• Body mass

(the larger the body size, the higher the BMR—lean body mass is a strong component of BMR)

• Regular exercise

(enhances BMR by 8-10%—based on intensity, volume/duration and muscle mass involvement)

• Diet-induced thermogenesis

(Increase in BMR with digestions, absorption, and assimilation of nutrients and activation of the sympathetic nervous system in response to meal consumption) [protein—P. 119].

• Environment

(warm or cold environments can produce higher BMRs)

• Low calorie intake lowers BMR


• Low calorie diets have the opposite effect. Decreasing calories can decrease RMR and lead to weight can.


BMR vs. RMR

• RMR is the energy required to perform normal bodily functions while the body is at rest and is somewhat similar to BMR although it is measured under less stringent conditions (does not require a 12-hour fast or 8 hours of sleep).



• Estimating Resting Energy Expenditure

– Important for weight loss/gain programs

– Several population-specific equations developed

– Predictor variables: body mass or LBM, height, age

– Equations

• Harris & Benedict [1 st and still commonly used]

• Mifflin-St Jeor

• Cunningham



• Estimating Energy Expenditure During Exercise

– Average energy expenditure at rest:

• 0.20-0.35 L of O

2 min -1

• 1.0-1.8 kcal min -1 (about 1.2 calories per min @ rest on average—72 per hour)

– In metabolic equivalents (METs):

• Men: 250 mL min -1

• Women: 200 mL min -1

– Exercise increases energy expenditure based on intensity, volume, muscle mass involvement, rest intervals



• Oxygen Consumption and Acute Training Variables

– O

2 consumption

• Increases during exercise in proportion to intensity

• Increases exponentially as exercise approaches steady state

• Remains elevated during recovery after exercise

– O

2 deficit

• Difference between O

2 supply & demand

• Larger during anaerobic than aerobic exercise

• Smaller in aerobically trained athletes than in untrained & strength/power athletes


Oxygen Consumption During Exercise and

Excess Postexercise Oxygen Consumption



• Resistance Exercise and Oxygen Consumption

– Resistance exercise increases VO

2 during & after a workout

– VO

2

:

• Greater during large muscle-group exercises than smaller

• Varies based on lifting velocity

• Greater when exercises are performed with high intensity

• Greater when exercises are performed for high rep #

• Greater when exercises are performed with short rest intervals

• Not affected by exercise order



• Body Fat Reductions

– Require proper diet & exercise

– Energy expenditure must exceed energy intake for net kilocalorie deficit

– Dietary recommendations:

• Well-balanced diet from major food groups

• High water intake (this is now disputed!!)

• 55-60% of kcal from CHOs

• 15% of kcal from protein

• <25% of kcal fats


Body Fat Loss

• Aerobic exercise helps by increasing fat oxidation during exercise and EPOC.

• Strength training helps by increasing LBM—an increase in LBM stimulates metabolism (BMR) and energy expenditure throughout the day.


Chapter 7 Metabolic Responses and Adaptations to Training

• Definitions

• Overview

Interval Training: Work-to-Rest Ratio

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Chapter 7 Metabolic Responses and Adaptations to Training

• Definitions

• Overview

Interval Training: Work-to-Rest Ratio

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