The pattern is weakness is limb girdle and truncal (including respiratory muscle). There also is camptocormia (bent spine syndrome) as the result of severe weakness of the paraspinal muscles. The best explanation is a myopathy.  Differential diagnosis of myopathies involving the proximal appendicular as well as truncal /respiratory muscles include inflammatory myopathies (inclusion body myopathy, polymyositis, dermatomyositis), muscular dystrophies (specially myotonic dystrophy type 2,  limb girdle dystrophy 1D [DNAJB6] and 2L [ANO5], myofibrillar myopathies, as well as calpainopathy and dystrophinopathy [the latter 2 conditions unlikely given the age of onset]), metabolic myopathies  (adult onset acid maltase deficiency, carnitine deficiency, branching enzyme deficiency, and mitochondrial myopathy), endocrine myopathies (i.e. Cushing syndrome, hyperparathyroidism), nemaline rod myopathy, central core and multiminicore myopathy,  and amyloidosis.

Differential diagnosis also includes dieases of neuromuscular junction (e.g. myasthenia gravis, Lambert Eaton myasthenic syndrome), and motor neuron diseases.

Click here for nerve conduction study and EMG, as well as other workup.