The next accessory muscle group to be aware of in the lower extremity is tibialis anterior. Tibialis anterior is a dorsiflexor of the ankle meaning it pulls your toes towards your knees (along with the extensor group). Increased speed and force of dorsiflexion will shorten the lever arm of the recovering leg during sprinting. This means that the quicker the ankle can go into dorsiflexion, the quicker the leg can get through into the next stride. This will obviously increase stride frequency. This dorsiflexion ability is even more important if athletes have to run on uneven surfaces like sand or grass. Machine dorsiflexion exercises are the best way of training this function but you can also use a low cable.
From an injury point of view, if you have disfunction (whether it be strength or poor soft tissue) with tibialis anterior you are more at risk of chronic ankle sprains and shin splints which are all common conditions in runners. It can also lead to hyperpronation symptoms and when you have impaired dorsiflexion function, the lumbar spine has to go through more range of motion during each recovery phase of your stride as described here. (link to FMS)
The last lower extremity muscle that we really want to be aware of is the flexor hallucis longus (FHL). This muscle has a vital role to play in proprioception of the foot, propulsion off the ground and making sure too much pronation does not occur when the foot makes contact with the ground. If these properties are impaired during the stance phase, it will prolong the stance phase because excessive pronation will occur meaning it will take your foot longer to get off the ground and the foot will not be able to exert the same amount of force into the ground. I always remember a story from Mike Leahy who created ART and after treating a 100m sprinter’s FHL and he literally fell over next time he tried to run. But from memory two weeks later, this sprinter set the world record. So although small it is vitally important to running speed.
Like tibilias anterior, FHL is involved in the occurrence of shin splints and compartment syndrome. Again like tibilias anterior, it is also involved in hyperpronation. One quick and easy was of getting an indication of your FHL function is to raise both big toes off the floor as high as possible while the rest of the foot stays on the floor. If one big toe can be raised significantly higher than the other, this is termed functional hallux limitus and it may mean your FHL is fibrotic and/or weak. FHL will be trained somewhat with all calf raise work but you can increase the recruitment of FHL by shifting more weight over the big toe when performing these exercises. Band resisted and bodyweight flexion of the big toe also works to strengthen FHL. To do this effectively the action should be to “scrunch” your big toe into ball and then go into plantarflexion of the ankle.