A subjective feeling of tiredness characterized by a lack of energy and motivation.

Pain in muscle.

A functional motor deficit where individuals whose responses to the challenges of exercise fail to achieve levels considered normal for their age and gender.

The occurrence of an unusually high amount of muscle pain following exercise.

A degree of kidney failure severe enough to require dialysis or kidney transplantation for survival characterized by a severe reduction in glomerular filtration rate (less than 15 ml/min/1.73 m2) and other manifestations including increased serum creatinine.

An increased concentration of lactic acid in the urine.

Hypertrophic cardiomyopathy (HCM) is defined by the presence of increased ventricular wall thickness or mass in the absence of loading conditions (hypertension, valve disease) sufficient to cause the observed abnormality.

Any structural anomaly of the heart.

A sensation that the heart is pounding or racing, which is a non-specific sign but may be a manifestation of arrhythmia.

Failure to thrive (FTT) refers to a child whose physical growth is substantially below the norm.

Forceful ejection of the contents of the stomach through the mouth by means of a series of involuntary spasmic contractions.

Increased susceptibility to fatigue.

Reduced strength of muscles.

A lack of strength of the proximal muscles.

An abnormal, increased fatiguability of the musculature.

Difficult or labored breathing. Dyspnea is a subjective feeling only the patient can rate, e.g., on a Borg scale.

An abnormal buildup of lactic acid in the body, leading to acidification of the blood and other bodily fluids.

An increased concentration of alanine in the blood.

Abnormally increased level of blood lactate (2-hydroxypropanoic acid). Lactate is produced from pyruvate by lactate dehydrogenase during normal metabolism. The terms lactate and lactic acid are often used interchangeably but lactate (the component measured in blood) is strictly a weak base whereas lactic acid is the corresponding acid. Lactic acidosis is often used clinically to describe elevated lactate but should be reserved for cases where there is a corresponding acidosis (pH below 7.35).

An abnormal increase in the molar ratio of lactate to pyruvate in the blood circulation.

The mitochondrial matrix refers to the substance occupying the space enclosed by the inner membrane of a mitochondrion, which contains enzymes, DNA, granules, and inclusions of protein crystals, glycogen, and lipid. Mitochondrial swelling refers to an increase in size of the mitochondrial matrix. This phenomenon is thought to be related to a permeabilized inner membrane that originates a large swelling in the mitochondrial matrix. Mitochondrial swelling may distend the outer membrane until it ruptures.

A reduction in the activity of the mitochondrial respiratory chain complex I, which is part of the electron transport chain in mitochondria.