To be able to get the picture please read this post:
This is what guitarrista wrote in this topic:
For some reason this seems to happen unconsciously by ma but not by im where I constantly have to think about relaxing i otherwise the finger would curl or remain locked in the palm or form a claw when the higher tempi as guitarrista said demand the help of the extensor muscle.It is true that when you play at a faster speed, you are not going to see the first finger return before the next one strikes - however it does NOT follow that you are supposed to hold the first finger in at all times and all speeds. The reason the first finger SEEMS to stay in until the next one fires is speed - it is actually still relaxing immediately after its stroke and returning naturally (or with help from the extensor muscle at fast speeds but let's not worry about this now) to reset; however, when alternating at a faster speed, the time of the next finger firing has now intruded upon the time period when the first finger is relaxing and resetting.
How can this problem be fixed?