Many runners believe that by running more and/or running faster, their legs become stronger. To a limited extent this is true, but if you do not run at a faster speed all the time, or at least at the same speed, the strength that you gained will soon be lost.
Also important to keep in mind that every time you increase your speed of running, if you do not have the strength to withstand the forces that are involved in each touchdown, then you are opening yourself up for injury. Understand that you do not increase strength during the run. Increases in strength occur afterward during recovery but only if there was sufficient overload to bring about super compensation.
Doing more running or running faster is a very ineffective way of trying to develop greater leg strength. Undergoing a specialized strength training program to develop the muscles as they are used in the running stride, will not only give you stronger legs, but will enhance your run greatly. You will be able to run not only faster but also further and you will be able to do so with less effort.
To improve your strength, it is important that you select the correct exercises and know what each exercise will do for you. Learn to distinguish between general exercises which increase your general strength and level of fitness, and specialized strength exercises that duplicate the muscular action that occurs in each stride. For example, some coaches recommend that you only have to do the squat, lunge and calf raise to develop all of the major lower body muscles that are involved in running. But they have limited value.
The squat is an excellent exercise for strengthening the quadriceps and will help you from sinking too low during the support phase. The lunge is effective if you do a long lunge keeping the body erect in order to get a strong stretch of the hip flexors. This will give you a longer stride if you also have sufficient force to drive your thigh forward along with a strong push-off to get the necessary spread between the thighs.
The calf raise is excellent and is probably the best exercise to improve your push-off and if you have good running technique, to help withstand some of the forces experienced on touchdown. Some runners and coaches believe that the squat and lunge also help strengthen the hamstring and gluteus maximus muscles both of which are strongly involved in running. This is true but only if you go sufficiently deep in the squat and lunge.
Understand that for these muscles to become involved, you must maintain the normal curvature of the spine. If you round the spine, as typically occurs in the squat with most people, you give slack to the upper hamstrings and even though you get to the thigh level position, you are not strongly involving the glutes and hamstrings. But if you maintain proper spinal posture, you will find that it is extremely difficult to get to even the thigh level position at which point the glutes and hamstrings come into play. A similar situation exists with the lunge.
The most glaring omission when considering leg strength for runners is strengthening of the hip joint muscles, mainly the hip flexors which are important for driving the thigh forward and the hip extensors that are involved in driving the leg down and backward to make contact with the ground. Strengthening these muscles will do more to improving your running speed and distance than any other exercises.
Also important is strengthening the lower back with exercises such as the back raise done on the Glute-Ham/ Back Machine to get the strength and ability to hold the erect trunk position during the run. Stronger back muscles along with strengthening of the abdominal muscles –especially the obliques — will go a long way to enabling you to maintain effective technique during the run.
In addition, for most runners, it is also important to strengthen the muscles of the foot, especially of the foot arch, so that it can act not only in withstanding and absorbing some of the landing forces, but also to assist in each push-off. If you do not do strength exercises for the foot muscles and structures, then it is strongly recommended that you use the Barefoot Science Foot Strengthening insoles which strengthen the foot arch when you wear them when walking and running.
For more information on specific strengthening of the muscles involved in running and how you can run better with more effective technique, see Explosive Running.