Muscles are one of 8 benefits resulting from eating hazelnuts.
Almost all nuts have very high levels of omega 6 PUFA's in addition to other antinutrients. They can still be part of a healthy balanced diet but a general rule, nuts should be consumed in moderation.
The one exception is macadamia nuts. They are almost pure "good fat" and have very little in the way of antinutrients. They're almost pure fat though so as @Matt 11 points out, even with macadamias your diet should not be nut-centric.
Any nut is fine really, you should be eating so much of any type for it to be an issue. A nut-centric diet is not normal or natural.