Weakness the next day after running may be due to physiological reasons such as excessive exercise intensity, excessive lactic acid buildup, insufficient rest or pathological reasons such as hypokalemia. 1. Physiological factors: (1) Exercise intensity is too large: due to the exercise volume and intensity is too large beyond the normal load range that the body can withstand, so that the body is in a kind of overloaded state, this state usually causes people to produce fatigue and weakness and other symptoms. (2) The body is not relaxed: because running will make the body sugar substances decompose to produce lactic acid, lactic acid produced by the accumulation of muscle tissue in the human body, not to relax after exercise so that too much lactic acid leads to weakness. (3) Insufficient rest: After running, you should take sufficient rest to restore your body to the normal level, otherwise you will easily feel weak. 2. Pathological causes: Hypokalemia can be the main cause. Due to sweating caused by running, a large amount of potassium ions are lost in the body, and the intake of water and potassium ions is too small, resulting in the potassium ions in the body being lower than the normal level, which leads to generalized weakness. If the symptoms such as generalized weakness do not improve significantly, it is recommended to consult a doctor in time to find out the cause of the disease and then actively treat it.