Hi. Strenous excercise sometimes causes dark coloured urine because of Rhabdomyolysis
which is a medical condition that results from direct muscle injury
or an altered metabolic relationship between energy production and energy consumption in muscle. In this case you should be also having muscle soreness or pains. You can go for a creatinine
kinase test. There are many other causes of dark urine. Please consult a doctor on the earliest for the required tests.