Standing in mud, using a wash out hose. EE went to move his feet - upper body moved, but right lower leg & foot did not, causing a ""popping"" in knee. EE did not report this until supervisor noticed him limping 4 days later. EE did not get med. attn. until 3 days following acc. EE has been working during physician's eval. - finally decided surgery required to replace meniscus.