Reps as JD defines them(fast with long recovery emphasizing mechanics) works out the fast twitch muscles with a smaller contribution to force provided by the slow twitch muscles. What is the primary physiological benefit from these workouts? Is there a reduced fatigability of the FT muscles or a increase in peak force from the slow twitch or both. In addition JD has in his original book a maximum of 3 miles of reps per week. Could there be an additional benefit of doing more say 4-6 miles of reps even though they are "time consuming" as JD put it. When he wrote that was it an issue of practicality? Since economy is a major limiting factor in performance particularly in longer races would it be beneficial to spend a large amount of time on that. I remember shorter had a great economy something like a 65-70 vo2 max but could race with people that had much higher vo2 maxes. He would do 30 200s I believe. He had pretty form.