Speed Harness (TM) Improve speed by training stride length and frequency in 2 athletes simultaneously. Stretch the Slastix tubing to the desired tension and have both athletes run in the same direction. The front athlete receives resistance training while the back athlete receives overspeed training. The fully sheathed Slastix resistance tubing provides a smooth, consistent pull for both athletes while offering maximum safety. Swivel waist belt allows athlete to change direction of pull easily without removing belt.