"Musclebacks" are irons that have a full back of the clubhead, as opposed to a cavity back iron in which the the back of the clubhead has been hollowed out to some extent in order to help create perimeter weighting and forgiveness.
Musbleback irons are usually forged, although they can be cast. They provide greater feedback to a golfer, but require the golfer to contact the ball with the center of the clubface with great consistency.
Compared to cavityback irons, with muscleback irons there is a greater loss of distance and a worse feel on off-center strikes of the golf ball. Because the musclebacks have full backs on the clubhead, musclebacks tend to have a higher center of gravity location, which can produce a lower trajectory preferred by many highly skilled golfers. A muscleback iron will also have a lower moment of inertia (essentially, less forgiveness) than a cavityback, because of the cavityback's perimeter weighting.
As you can probably tell, musclebacks are more exacting irons, they require more precision to play well, compared to cavitybacks. Which is why muscleback irons are usually only played (or at least played well) by lower-handicappers.
For more about musclebacks, see the FAQs:
• How do cavity back and muscleback irons compare?
• How do cast and forged irons compare?
• Do musclebacks make it easier to "work" a ball than cavity backs?
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