Lactic Acid & The Cori Cycle
(Text Pg 88 & 130)
Lactic Acid
 A by-product due to anaerobic metabolic processes in muscles (glycolysis).
 Produced when the energy demands of exercise can no longer be met by only the aerobic
metabolic systems.
 Blood lactate levels remain relatively low during exercise yet will increase if the intensity of exercise
rises too high.
Lactate/Anaerobic Threshold (LT):
 The point where blood lactate levels begin to rise exponentially
 OBLA (Onset of Blood Lactate Accumulation)
 Blood Lactate levels begin to accumulate dramatically above resting levels.
 Untrained individuals: Lactate Threshold occurs at ~ 50 – 60% of their VO2 max
 Trained individual: Lactate Threshold occurs at ~ 70 – 85% of their VO2 max
LT & OBLA for Trained and Untrained Individuals
Hypothesized Causes of Blood Lactate Accumulation:
 Increased reliance on anaerobic metabolism
 Decreased oxygen delivery to working muscle
 Decreased blood flow to working muscle
The Cori Cycle:
 The process by which Lactic Acid is converted back into glucose in the liver.
 Gluconeogenesis: Glucose
Glycogen (stored glucose)
 Essentially then, Lactate is not all bad, it is not an end-product, rather it can ultimately be seen as
an energy source.
Let’s Draw the Cori Cycle:
Fibre Type & Athletic Performance
(Text Pg 90 – 93)
It's important to note that certain muscles are more adapted to one energy production system than another.
There are Two Main Muscle Fibre Types
1. Slow Twitch Fibres
 Suited for repeated contractions during activities requiring a force output of < 20-25% of max force
output.
 Type I: Slow Oxidative
2. Fast Twitch Fibres (2 subdivision)
 Significantly greater force and speed generating capability than slow twitch fibres.
 They have the ability to tense and relax quickly, with low endurance levels
 Type IIA: Fast Oxidative Glycolytic
 Type IIB: Fast Glycolytic
The Fibre Type Continuum
Slow Oxidative
(Type I)
• Generates energy slowly
Fast Oxidative Glycolysis
(Type IIA)
• High speed of energy release
Fast Glycolysis
(Type IIB)
• Store lots of glycogen
• Stores lots of oxygen
(myoglobin)
• considered to be the
intermediate type muscle fibre
• High levels of enzyme activity
for quick contractions
• Dependant on aerobic
processes
• Can perform aerobically and
anaerobically (glycolysis)
• Do not require oxygen
• Very fatigue resistant (able to
maintain a lower level of tension
for a long duration).
• Moderate fatigue resistance
• Fatigue quickly
• With aerobic training, these
fibres can become Type IIA fibres
Sample Muscle Biopsy Slide