This is what it says in MIMS about l-carnitine:
"L-carnitine is an essential cofactor in muscle metabolism during exercise. L-carnitine forms the transport system which moves fatty acid molecules into the mitachondria for energy production.
L-carnitine also acts as a physiological buffer by inhibiting build-up of lactic acid in muscles. This helps delay the onset of fatigue in exercising muscles.
Carnitine deficiencies result in an inability to utilise fatty acids as an energy source, and causes skeletal muscle dysfunction and weakness, heart enlargement, heart failure and rhythm disturbances, and liver dysfunction. L-carnitine also assists in the oxidation of pyruvate and branched chain amino acids in the energy cycle, and prevents the build-up of fatty complexes within cells which can damage muscle cell membranes.
Supplementing with L-carnitine has a number of beneficial effects during exercise, including enhanced energy supply, increased utilisation of fatty acids, decreased buildup of lactic acid, and increased maximum work output. The level of L-carnitine in muscles plays a major role in determining the exercise capacity of muscles. L-carnitine is essential for normal heart function."