Something happens to golfers when they see a forged muscle back iron, golf’s smallest, least forgiving type of iron that only a small percentage of golfers have the talent to use effectively.

The response is similar to how motorists feel when they see an accident on the freeway. They know that shouldn’t look, that it’s potentially hazardous for them to do so, but they just can’t help themselves.

Mizuno’s new MP-4 irons will take many golfers’ intrigue with muscle back irons a step farther. They have a classic shape and understated graphics that traditionalists will praise, and according to Chuck Couch, vice president of golf product for Mizuno, they feel as good if not better than any iron Mizuno has ever produced.

To say that a golf club has good feel sounds like a subjective statement, because good feel often means something different to different golfers. But Couch said that Mizuno has established a way to quantify feel and improve it scientifically, which is exactly what the company has done with the muscle back MP-4 irons, as well as with its new forged cavity back MP-54 irons.

Click here to see what members are saying about the MP-4 and MP-54 irons in the forums.

Mizuno MP-4 Irons

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The first thing most golfers will notice about the MP-4 irons is that they look small at address, and that’s no illusion.

In most iron sets, the blade lengths of the irons get longer as the clubs gets shorter. That’s because of the weight progression of the heads – short irons (which have shorter shaft lengths) have to be heavier than longer irons (which have longer shaft lengths) for the clubs to have a similar balance point, or swing weight.

For example, Mizuno’s previous muscle back iron, the MP-69, had a blade length that grew 1.5 millimeters from the 3 iron (74.5 mm) to pitching wedge (76 mm). That doesn’t sound like a big difference, but for the exacting golfers who tend to play muscleback irons, it’s noticeable.

Above: The “Pure Muscle” on a Mizuno MP-4 model 7 iron.

Instead of increasing blade length, engineers added weight to the MP-4 irons by increasing the size of the muscle pad behind the sweetspot of the irons, which is the reason for the irons’ pronounced bulge, or “Pure Muscle,” in the back. According to Couch, the added mass amplifies the MP-4’s “Harmonic Number,” the frequencies created at impact that golfers equate with feel.

By raising the harmonic number, and tweaking the shape of the muscle pad to ensure that the frequencies have even levels, Couch said engineers can create a more pleasing sound that translates into the “sticky, soft feel” at impact that golfers rave about.

Mizuno MP-54 Irons

If most golfers are honest with themselves, they’ll come to the conclusion that irons like the MP-4 aren’t for them. Couch speculated that not even Luke Donald, Mizuno’s highest-ranked professional golfer, will play the MP-4 irons. He’ll likely stick with the MP-64 irons, which are slightly larger and more forgiving (Click here to read our full review on the MP-64’s).

The MP-64 irons were one of our top-rated irons for 2013, but even for some of Mizuno’s staff players they’re still smaller and less forgiving than they’d like. That’s why Mizuno decided to create the MP-54 irons, which like the MP-4 and MP-64 are forged from 1025E “Pure Select” Carbon Steel to help create the soft, solid feel Mizuno irons are known for. But they’re slightly larger than the MP-64’s, which Couch said allowed engineers to take total advantage of modern iron technology.

The MP-54 irons have longer blade lengths, thicker top lines, more offset and a thicker sole than the MP-64 and MP-4 irons. Their most important characteristic, however, is the 16 grams of weight that Mizuno removes from the cavities of the 3, 4, 5, 6 and 7 irons (see photo 1). That discretionary weight was repositioned in areas that give the irons a higher peak trajectory and more forgiveness than Mizuno’s smaller-sized irons.

Photo 1

Photo 2

The MP-54’s 8 iron, 9 iron and pitching wedge do not have the weight removed from the cavity, however, which gives the clubs a flatter, more penetrating trajectory that better golfers prefer with their short irons (see photos 2).

Couch emphasized that the “Step Muscle” design – a.k.a. the hole that’s left in the cavity after milling out 16 grams of weight – does not compromise the feel of the irons.

“We left as much maximum thickness as we could behind the impact area, and use that to drive our feel,” Couch said. “And we used H.I.T. [Harmonic Impact Technology] to make the irons feel amazing.”

The MP-4 and MP-54 irons, which will hit shelves on Sept. 9, come stock with True Temper’s Dynamic Gold S300 shafts and will retail for $999.

Both sets have similar lofts – the 6 irons measure 30 degrees, the pitching wedges measure 46 degrees — to allow golfers to mix and match Mizuno iron models for a combo set.

Click here to see what members are saying about the MP-4 and MP-54 irons in the forums.

Click here to see what members are saying about the MP-4 and MP-54 irons in the forums.