The Iso uses a power blocking scheme. An offensive lineman will have one of two jobs, if he has a defender lined up on top of him (this is called being covered), he will block that defender. If he does not have a defender on top of him (this is uncovered), he will take a moment to double team the closest defender before moving on to block a linebacker. This is what is called a combo block.
Combo blocks are in red.
This series of images shows a combo block in action:
The Iso can be run either between the center and guard (A gap), or between the guard and tackle(B gap). The only difference between the two is which linebacker will be isolated; A-Gap, the middle linebacker is isolated, and B-Gap, the outside linebacker is isolated. This is sometimes why you will see the O-Linemen talk to each other and point while getting lined up.
The QB is under center and begins the play by taking a 6 o’clock step and gaining as much depth as possible. Sometimes done with a reverse step by coach or player preference. One of the keys to this play is getting the ball to the HB as deep as possible, ideally 5 yards or more behind the line of scrimmage. After giving the handoff the QB runs a fake bootleg to either side.
The FB immediately runs toward the outside foot of the guard and will block the first linebacker he sees. The linebacker will attempt to stop the play in one of a few ways, working from the inside out, the outside in, or straight up. When the linebacker picks a direction, the FB will simply push him that direction and allow the HB to take the other direction. If the LB plays it straight up, the FB must get enough leverage to drive him back from the hole.
This gif shows a close up of the FB’s block on the isolated linebacker. Notice the large hole created by the linemen.
The HB begins the play with a slight delay, in the form of a back step or lead step, whatever the HB is comfortable with. This allows the QB extra time to get as deep as possible prior to the hand off. The HB then must make several reads very quickly. First, he looks at the defensive tackle closest to the play to tell if the DT is attacking the A gap or B gap,; if it is the A gap, the HB will run toward the B gap.
In this image, you can see the DT is attacking the A-gap, leaving the B-gap to be filled by the linebacker. The correct read for the HB is to run to the B-gap.
This gif shows the HB making the correct read and heading into the B-Gap
If the DT attacks the B gap, the HB must make another read. This time the HB will read the backside defensive tackle to see if he will attack the A gap, if so, the HB will cut back and run to the opposite side of the line. The FB always attacks the play side, through either the A or B gap, so cutting back to the other side is a last resort for the HB.
In these images you can see that the DTs have attacked the A and B gap, making the correct read a cutback across the line.
The gaps may look open, but the DTs are moving to the right to close them.
In the gif, you can see the FB still attacks the B gap, and the HB makes the cutback into the large hole on the other side of the line. Also note the combo block that stops #52 from making the tackle.
If the DT does not attack the A-gap, then that is where the HB will run. Once he commits to a hole, the HB will then read his FB’s butt. He always runs where the butt is pointing, because the defender is on the other side.
The goal of the defense on any running play is to plug all the gaps between offensive blockers, so that the HB cannot get through and gets tackled when he tries. If no defenders can shed their blocks, then the success of the defense depends on the isolated linebacker. If he can recognize the play, his goal is to move forward and meet the FB as close to the line as possible. If he can do this without being driven away by the FB then the HB loses his lead blocker and is much more vulnerable. The other option for the defense is to make the O-Line’s job as hard as possible. This is done by running slants and stunts, which is having the D-line and linebackers move in unexpected directions. This inhibits the O-Line’s ability to get the proper angle on their blocks and leads to confusion among the blockers. Teams that run the Iso often will see a lot of stunts and slants.
A very similar play to the Iso, the Power is another smash mouth running play sometimes called the hardest hitting play in football. The power can be run out of a variety of formations, from the I, to single back and shotgun sets, any formation with a running back.
The blocking scheme for the Power is incredibly varied and changes with every formation and defensive alignment. The basics are close to the Iso, with similar combo blocks. The piece that never changes, however, is having two lead blockers. These blockers are what makes it a Power play. One blocker is a pulling backside guard, where he steps back from the line and runs across the formation. The other lead blocker is often a FB, but is sometimes a TE or other player depending on the formation. The linemen typically leave the last defender unblocked and he is taken care of by one of the lead blockers.
In this play, #75 is the pulling guard and leads the HB through the hole. #49 is the FB and blocks the OLB on the far left.
Single back sets usually face a nickel defense, so have one less defender to block, and are run without a FB. The power is run generally run off tackle, but inside the edge of the defense. The reads for the HB can change with the formation, but the theory is the same as the Iso; he wants to avoid the defensive linemen and will attack a gap being covered by a LB.
This is a Power-O run by the Steelers out of a single back set.
#66 is the pulling guard; he will come across the line and block the edge defender. In this instance, the TE is the other lead blocker. Also notice the delay steps the HB takes so his blockers can get across the line.