Muscle weakness describes different conditions of reduced muscle strength, on the one hand true or primary muscle weakness or on the other hand muscle tiredness or muscle 'fatigability'. Muscle weakness is a symptom, not a disease!
Primary or true muscle weakness is a lack of physical muscle strength and the feeling that extra effort is required to move the limb, an inability to perform a motion or with simple words the muscle isn’t working properly. This can be caused on different levels of locomotor system.
A brain disease for instance a Stroke or a Multiple Sclerosis can detain the order to initiate a motion, although the following nerves and muscles are intact. At the level of the spine or spinal cord maybe a disc disease inhibits the order to reach the effector. Or at the level of peripheral nerve system an impinged nerve isn’t forwarding the electric stimulus to the muscle or at the end the target muscle is affected by a muscular dystrophy or myopathy. All these conditions result in weakened muscles which cannot move the usual load. It's a real change in muscle power.
Muscle tiredness or asthenia means an exhaustion or weariness when using the muscle leading to reduced speed. It takes more effort to do a motion, but the muscle isn't genuinely weaker. The muscle can perform. Conditions as chronic fatigue syndrome, fibromyalgia, depression, anaemia, diabetes, hypothyroidism, electrolyte disorders such as low levels on sodium or potassium, dehydration, rheumatism, soft tissue diseases, chronic pain syndrome, chronic heart, lung and kidney disease, ageing, medications as Statins (used to lower Cholesterol), antibiotics (Ciprofloxacin, Penicillin), long term use of oral steroids (Cortisone) and lack of exercises due to an inactive (sedentary) lifestyle are characterized by this type of weakness.
Muscle 'fatigability' defines diseases where the muscle starts off normally but tires very fast and takes longer to recover than normal. Typical diseases are myasthenia gravis and myotonic dystrophy.
Diagnosis starts by taking an extensive medical history addressing your symptoms, the trigger factors, the duration and the frequency, work environment, social habits, toxic exposure, risk of infectious diseases and family history of neurological disease. Check of vital signs such as blood pressure and pulse, followed by a proper physical examination focusing on gait and maintaining of balance, general and specific motor power testing, tabbing the muscle stretch reflexes should give first differential diagnosis. Additional tests such as laboratory tests, Electromyography (EMG), Nerve Conduction Studies (NCS) and imaging of brain are sometimes recommended for confirmation of diagnosis.