Biallelic variants in TTN, encoding titin, are known to cause rare forms of early-onset myopathy, manifesting with variable severity and progression of muscle weakness, contractures, respiratory insufficiency and cardiomyopathy in some. Muscle ultrasound is a highly sensitive, real-time imaging tool easily performed at bedside, used to visualize myopathic changes in the muscle. 30 patients with confirmed biallelic TTN – related myopathy were evaluated (16 Male, 14 Female; ages 6 months – 47 years old). Muscle ultrasound was performed at every visit. Two independent raters graded each of the three triceps brachii heads using a modified Heckmatt scale. Biceps ultrasound images were analyzed qualitatively.11/30 patients in the cohort demonstrated maximum involvement, as capturable on ultrasound, of all three of the triceps heads. Excluding these studies, 15/19 patients demonstrated a greater modified Heckmatt score in the long head compared to the lateral and medial heads of the triceps, indicating selective involvement, reminiscent of the known selective involvement of the semitendinosus muscle in the hamstring. Analysis of biceps ultrasound images also revealed a distinctive layering pattern, specifically greater increased echogenicity in the lower layer of the biceps, in 11/30 patients. Here we present an analysis of upper extremity muscle ultrasound findings in patients with biallelic TTN – related myopathy, revealing a consistent pattern of triceps involvement and characteristic appearance of the biceps brachii. These “TTN compatible imaging phenomena” will aid in variant interpretation and help substantiate a diagnosis of TTN – related myopathy.