The quadriceps group (as the latin translation suggests) is made up of four muscle heads: vastus lateralis, vastus medialis, vastus intermedius and rectus femoris. These muscles primarily extend the knee while the rectus femoris also flexes the hip, which makes it a doubly important muscle for running.
If we remember that running speed and endurance is all about improving the three main components of locomotion: stance phase, stride frequency and stride length. The quadriceps will especially help with the stance phase. Rectus femoris (with the help of the other hip flexors and the lower abdominals) will also aid in improving stride frequency by bringing the leg into triple flexion more rapidly.
The reason an increase in strength in the quadriceps will help so much with the stance phase is that the quadriceps will now be able to exert more force on the ground with each foot contact. This means the runner will experience greater ground reaction forces and it will propel them forward at a greater pace. This will also indirectly increase stride length, as now you are moving further with each foot contact. Increased strength in the quadriceps will also enable a better ability to handle eccentric loading forces that again means that you will spend less amount of time of in the stance phase. This is very similar to the knee flexors role in the stance phase.
Probably the most important muscle of the quadriceps group that runners need to strengthen is the Vastus Medialis. There are two reasons: a) it is normally underdeveloped compared to the other quadriceps due to normal locomotion patterns and the predominant squat being performed in gym and strength programs being the half squat and b) because of it’s more distal insertion (e.g. it attaches further down the leg than the other quadriceps), it has greater responsibility for terminal range of motion (ROM) at the knee. This terminal ROM is the ROM that the knee goes through the most when running (e.g. your knees only bend a little bit when running right?) which makes it even more important to be strong in that particular ROM.
Exercises that you want to use to improve the strength of the quadriceps are almost all closed kinetic chain exercises and from anecdotal experience, you want to train them through either a full ROM or that terminal ROM I have just mentioned. This is especially true for the Vastus Medialis.
Here are two full ROM quadriceps exercises – the full squat and split squat
And if you are wondering about the safety of full squats, have a read of this. Ethically, I do not recommend anything that does not have a significantly greater reward to risk ratio for athletes or clients – meaning the potential rewards from performing a certain exercise has to drastically outweigh the risks associated with the exercise. In fact, if most exercise professionals knew that full squats improve 11 out of 13 knee stability markers (while there is no effect on the other 2) while half squats only improve 4 out of 13 (while 4 of the other markers actually get worse with the other 5 not having any effect on) there would be a lot more full squats being performed in gyms.
And here is a terminal ROM quadriceps exercise – the side step up (or single leg box squat)
Bringing up strength levels in the quadriceps with these exercises will also dramatically help reduce the amount of knee and hip related injuries runners will experience. This is especially true with Vastus Medialis’ involvement in preventing patellar tendonitis, and other patellar tracking problems in the knee. As I have already mentioned just above, other markers of knee stability will also increase which will aid in preventing knee injuries.