Without seeing the MRI scan I cannot give you a definite answer
but from what you tell me, it sounds like the bulging disc may be comperssing some of the nerve fibers that come out of the spinal cord and travel down to the leg to supply the muscles in the leg. This is called a 'radiculopathy'
'S1' is the particular nerve root that is affected - this nerve root supplies the muscles involved in extending the foot at the ankle, bending at the knee and hip. It also carries sensation from the foot and back of the leg - that is why you get pain there
'psw' are positive sharp waves and imply that there is irritation in the muscle caused by damage to the nerve supplying that muscle.
If you have severe pain and an EMG showing some nerve damage you should consult with a spinal neurosurgeon to see if a surgical procedure could help you. Eventually the nerve could be damaged enough to cause leg weakness.
Good luck