How do you increase your speed if your fibers don't?
If you are a male and strength train for a significant amount of time progressively overloading as you work, the physiological cross-sectional area (PSCA) of the muscle will increase. The specific tension, that is the maximal force per unit PSCA will increase also.
What else occurs is that the fastest fiber Type IIX MyHC will shift toward a slower fast fiber MyHC Type IIA. MyHC is the abbreviation for myosin heavy chain. Myosins are the motor proteins involved in muscle contraction. Heavy chains are heavy, because they have double the chains of their slower light chain myosin counterpart. Heavy chains are stronger.
Taking a muscle biopsy following a comprehensive controlled training study and analyzing a single fiber you will find the max power and max velocity of the single fiber remains the same following training. You will also find that myofibrillar packing did not occur, even the specific tension of the single fiber remains the same.
The cross section of the muscle has increased and the specific tension of the entire muscle has as well... the athlete performs better....yet the muscle as a whole is actually slower.
If you have further interest read...'What causes in vivo muscle specific tension to increase following resistance training?'... Journal of Experimental Physiology
Scientist feel the explanation for all of this lies somewhere in the connective tissue not the fiber type. What they are still saying about training is that it is important to Get Strong.