You can use IDENTITY (1,1) for the Log# so you do not to increment it every time you add a new Log.
Also you need to add GymMemberID and a Foreign Key constraint to Log Table.
This way when you insert as new Log you will need to have the GymMemberID and you will be able to retrieve the new Log#
ie:
declare @LogNum int
INSERT INTO Log
VALUES (@height,@Chest,@Forearms,@Arm,@Sholulders,@Thighs,@Calves,@date,@BodyMassIndex,@BeforePicturePath,@AfterPicturePath,@GymMemberID)
SET @LogNum = SCOPE_IDENTITY()