NUTRITION IN SPORT

(b) excitatory drive to lower motor neurones;
(c) motor neurone excitability.
In these three mechanisms there is a decrease in
neural drive. This can be detected by showing
that fatiguing muscle can still maintain power
output if the nerve is stimulated artificially or the
brain is stimulated.
2 Peripheral fatigue:
(a) neuromuscular transmission;
(b) sarcolemma excitability;
(c) excitation–contraction coupling.
Biochemical mechanisms for fatigue can also
be put forward:

• depletion of phosphocreatine in muscle;


• accumulation of protons in muscle;

154 nutrition and exercise

• accumulation of phosphate in muscle;


• depletion of glycogen in muscle;


• hypoglycaemia;


• changes in the concentrations of key amino
  acids in the blood leading to changes in the con-
  centrations of neurotransmitters in the brain.
  Some of these mechanisms have been dis-
  cussed elsewhere (Newsholme et al.1994). The
  first four are explanations for peripheral fatigue
  and the last two are explanations of central
  fatigue. The advantage of proposing a biochemi-
  cal mechanism is that, from this knowledge,
  ideas can be put forward for manipulations to
  delay fatigue. A hypothesis is suggested that
  links all of the latter three biochemical mecha-

Alanine

Glutamine and

asparagine

Cysteine

Proline

Proline

Glycine

Tryrosine

Tryrosine

Serine

Serine

Arginine

Aspartic acid

Glutamic acid

Valine

Valine

Tryptophan

Tryptophan

Histidine

Methionine

Phenylalanine

Threonine

Isoleucine

Leucine

Lysine

Arginine

Aspartate

Cysteine

Glutamate

Glutamine

Glycine

Hisitidine

Lysine

Methionine

Creatine

Nitric oxide

Pyrimidines

Taurine

γ-Aminobutyrate

Amino sugars

Purines

Pyrimidines

Creatine

Purines

Tetrapyrroles

Histamine

Carnitine

Creatine

Methyl derivatives

Choline

Ethanolamine

Sphingosine

Ornithine

Putrescine

Catecholamines

Melanin

Thyroxine

5-Hydroxytryptamine

Nicotinic acid

Pantothenic acid

+Peptides (e.g. carnosine, glutathione)

Non-essential

Essential

Fig. 11.1The amino acid composition of myosin, one of the two major proteins in muscle—and, therefore, in lean
meat—and the biosynthetic role of some of these amino acids. From Newsholme et al. (1994), with permission.