Muthuka won 100, 200 and 400 metres races at the 2005 Kenyan Athletics Championships and has won number of national championships since then. She competed at the 2006 African Championships in Athletics 400 metres race. She reached the semifinals at the 2007 All-Africa Games. At the 2008 African Championships she finished 7th in 400 metres final and helped the Kenyan 4*400 metres relay team to reach the final. However, she did not take part in the final, where Kenya finished second.