Achieving a relaxed full range of motion with the arms in the top speed phase.
I have found that there is a problem with using the obvious muscles to move the arms fast. The obvious muscles used to move the arm forward are the deltoid and pectoralis. To move the arm backwards, the deltoid and latissimus are used. The problem seems to be in switching from a full contraction of one set of muscles immediately to a full contraction of the opposing muscles. Trying to do this leads to tightening of the muscles and shortening of the range of motion. This carries over into the legs being tight and having a shorter stride.
One way to enhance this system is to allow movement of the forearm to create force while the major muscles of the shoulder are switching from contraction to relaxation. This requires that the elbow can’t stay fixed at a 90 degree angle.
First let’s think about rotating the arm backwards. If the arm starts with the elbow bent at an angle less than 45 degrees and the hand near the near, the forearm can extend as the upper arm is brought downward. The elbow should remain loose and it should be allowed to open freely due to the rotation occurring at the shoulder. When learning this, don’t use the triceps, but try to create a loose whip out of the lower arm. The angle in the elbow can open to near 120 degrees and the forearm can accelerate faster than the upper arm. This should occur when the hand is near the waist. The force created by this acceleration can be suddenly countered by the biceps. This contraction bends the elbow and shortens the radius of the arm’s center of mass. The decreasing radius accelerates the arm back and upward to achieve a full stretch in the front deltoid without the need for the rear deltoid to be contracting. The biceps and lower arm movement create the necessary force and acceleration to complete the range of motion while the rear deltoid relaxes. This is a period of transition from contracting the rear deltoid to contracting the front deltoid. The front deltoid is already fully stretched and ready for its contraction to rotate the arm forward. Moving the arm forward isn't as complex. Maybe this is because the ideal forward position is not a full rotation of the shoulder’s range of motion. Just make sure that the hand is reaching the level of ear.