Quantitative assessment of muscle fatigue is not commonly carried out when evaluating myopathic patients, perhaps because of the complex nature and difficulty of defining and measuring fatigue. Few studies have systematically examined the problem using objective measurement techniques (MRI with STIR sequences 
, exercise induced fatigue, vascular effect on post-exercise activity by nitric oxide production, blood flow pre- and post-exercise). For instance, insufficient relief of local vasoconstriction in active muscle can lead to muscle edema and Duchenne muscular dystrophy (DMD) and limb girdle muscular dystrophy (LGMD) patients show muscle edema on MRI. Furthermore, the variability of pathogenetic mechanisms underlying the various forms of muscular dystrophy is well known and it may play a different role in determining muscle changes. In myotonic dystrophy fatty degeneration and myxedema are localized to anterior thigh muscles and posterior leg 
The sarcolemma of striated muscle fibers is characterized by the presence of the dystrophin–glycoprotein complex (DGC) that is composed of cytoskeletal proteins (dystrophin, syntrophins), the dystroglycan complex and the sarcoglycan (SG) complex. The DGC provides a mechanical linkage between laminin in the extracellular matrix and the intracellular F-actin cytoskeleton. The structural and functional integrity of this connection is crucial to stabilize the sarcolemma during contractions. The SG complex is composed of four glycoproteins (namely α-, β-, γ-, and δ-SG), whose mutant genes cause a group of autosomal recessive Limb-girdle muscular dystrophies (LGMD) called “sarcoglycanopathies” (LGMD2D, 2E, 2C and 2F, respectively). When one gene is mutated all the other SG components are secondarily reduced, the assembly of the SG complex is compromised, and the sarcolemma integrity and stability are lost. A similar pathogenetic mechanism leads to DMD, which is caused by mutations in the dystrophin gene, while in Becker dystrophy there is a residual abnormal dystrophin.
The DGC has also signaling roles due to its interaction with other proteins including neuronal nitric oxide synthase (nNOS), which is anchored at the sarcolemma [4–11]
Nitric oxide (NO) is a messenger molecule that in muscle regulates development, contractility and blood flow. NO is formed by nNOS activity, which rapidly transduces signaling events in a calcium-dependent manner. In adult muscle, nNOS is usually localized at the sarcolemma, the neuromuscular and myotendineous junctions, while in the soluble fraction, significant amounts of nNOS are associated with the muscle phosphofructokinase isoform, although the nature of this interaction is unclear. Regulation of nNOS activity depends on the interaction with various proteins: it may be inhibited by caveolin-3, a component of the DGC that might displace the activator calmodulin.
One physiological role of nNOS in normal muscle is the production of NO that mediates the inhibition of sympathetic vasoconstriction. The secondary loss of nNOS has been suggested to contribute to fiber degeneration in muscular dystrophies, both in Duchenne and Becker dystrophies, because the reduced production of nNOS would reduce the normal protective action of NO against local ischemia during contraction (vascular hypothesis) and increase the cellular susceptibility to superoxides (oxidative stress hypothesis) ().
Cascade of events consequent to muscle exercise when normal or defective nNOS is present.
Muscle from DMD patients, dystrophin-deficient mdx mice and α1-syntrophin knock-out mice showed absent nNOS at the sarcolemma, which however remained in the cytosol. Absent sarcolemmal nNOS was observed also in Becker muscular dystrophy (BMD) with dystrophin gene deletions removing the central rod domain (exons between 10 and 53), a region which is crucial for the interaction between nNOS and α1-syntrophin 
. The possibility that the loss of sarcolemmal nNOS could be observed in disorders different from dystrophinopathies but having similar pathogenetic mechanism lead to an investigation in LGMD muscles. The sarcolemmal nNOS was found to be normal in an early study on α-sarcoglycanopathy, and reduced in a more extensive investigation on sarcoglycanopathies, suggesting that nNOS defect might contribute to muscle pathology. Among LGMD other than sarcoglycanopathies, only caveolinopathy (due to caveolin-3 gene mutations) has been investigated for nNOS expression in muscle and it was found to be either normal in patients with the rippling muscle disease phenotype, or severely reduced in patients with LGMD phenotype 
We analyzed the muscle biopsies from 32 patients affected with 7 different forms of molecularly defined LGMD and 5 patients with DMD, in order to investigate both the cytosolic nNOS expression and its sarcolemmal localization 
. By evaluation of the role of nNOS in muscle pathology and finally in disease progression, we found evidence supporting a role of nNOS in modulating the disease phenotype, this could be of great importance for future therapeutic interventions in LGMD.
One interesting and still controversial issue is whether the loss of sarcolemmal nNOS, compounded with the defect of another muscle protein (i.e. dystrophin or sarcoglycans) might modulate the course of the disease. Earlier studies demonstrated that normal nNOS activity could reduce dystrophic symptoms: forced expression of nNOS reduced muscle pathology in mdx mice; nNOS transgene in the myocardium of mdx mice prevented the ventricular fibrosis and greatly reduced myocarditis; sarcolemmal nNOS was restored by the use of minidystrophin vector in mdx mice. Moreover, the level of nNOS expression seemed to be inversely correlated with the severity of the disease in caveolinopathy and in BMD muscle (). However, in animal models some experimental results suggested that a reduction of NOS activity would not significantly contribute to the dystrophic pathology: mdx mice inbreeded with NOS-null mutant mice showed no difference in muscle pathology compared with mdx mice; nNOS-null mice do not show muscular dystrophy; α1-syntrophin knock-out mice with loss of sarcolemmal nNOS did not show muscle degeneration 
Loss of sarcolemma-localized nNOS and types of muscular dystrophies.