Chris had been experiencing some groin issues during the last week or so and during this run he had me look at his run stride and technique. I noticed that he was getting "lazy" with his leg lift or what I call knee drive. This was the same leg he was experiencing the groin pain, so I had him visualize or imagine he had to step over a log each time he took a running step. The faster he ran, the larger the log, the slower he ran, the smaller the log. We practiced this for the remainder of the run and his groin pain vanished - which was really a hip flexor issue from swinging his leg on each step rather than lifting it (over the log) properly.
More recently, I have been experiencing slight tightness in my upper hamstring attachment and hip flexor of my dominant right leg. I was writing this off to too much time spent sitting and catching up on my coaching computer work. But on today's trail run I decided to spend some time focusing on this with my own technique. Since this is my dominant leg, my good form awareness always shifts to my left leg and I noticed that my left leg was near perfect. This is pretty common, to have better form with our weaker or less dominant side or limb. I was a switch hitter in baseball and my left swing was always better than my right, but I had more power on the right side. So in discovering that I too had gotten lazy with my right leg knee drive, I focused on this during my 30 minute threshold trail run today and what do you know - my tightness released in both the hamstring and hip flexor.
So inspect my video at the 2:00 mark to understand how the knee drive helps promote stance leg stabilization, allowing the hip flexor to do it's job of lifting the leg and NOT act as a stabilizer - that is the job of the glute medius. If we do not lift that leg, the hip flexor will be recruited to do more work as a stabilizer, over working it, causing tightness.
And then focus on the 3:19 mark of the video to see how imagining stepping over a log might help with proper knee drive. Keep in mind that your knee drive height is relative to your speed. The faster you run the higher the knee and the slower you run the lower your knee will be. BUT the biomechanics and mindset if lifting the leg stay the same.