We (probably) all know that F = ma, meaning the net force exerted on an object equals the mass of the object times its acceleration.

Mass is a scalar quantity, but acceleration is a vector. It can be negative and positive. If the final velocity of the movement is less than the initial velocity, we have an acceleration in the negative direction (as opposed to the motion), so the net force exerted is subsequently negative.

The force exerted by a muscle, as reiterated by many coaches, if again ma, so we can apply the same force to a 500 lbs weight as a 200 lbs weight performed in different accelerations.

One thing I really wonder is this case: You perform a heavy bench press single. You lower the weight, push off the chest with relative ease, and your velocity greatly decreases near the lock out as your triceps shake, and in the end you complete the lift, but your final velocity was much less than your initial velocity.

Does this mean the muscle just exerted negative force to do work in the positive direction or is there something else happening at the neurophysiological level that prove the acceleration positive after all?