This study aimed to determine whether amplitude modulation (A-mode) ultrasound (US) provides accurate and reliable measurements comparable to those obtained using brightness modulation (B-mode) US under diverse conditions. Thirty healthy participants (15 women and 15 men) underwent measurements of subcutaneous fat thickness (SFT), muscle thickness (MT), and muscle quality (MQ) in the trapezius and biceps brachii muscles using both US modes before and after exercises designed to stimulate the respective muscles. Among the three key indices, the results demonstrated the high validity of the A-mode, with minimal mean differences (MDs) between the two devices less than 0.91 mm and intra-class correlation coefficients (ICCs) exceeding 0.95 for all measures. In addition, the correlation coefficients between the error scores and average scores for the trapezius and biceps brachii suggested no evidence of systematic error. The trapezius MT and MQ significantly increased, and the biceps brachii MT significantly increased after the exercises (p < 0.05). Notably, both the A- and B-modes exhibited the same trend in these post-exercise changes in the muscle. This study suggests that low-cost and low-resolution A-mode US provides measurements of SFT, MT, and MQ similar to the more expensive, high-resolution B-mode imaging. A-mode US is an affordable and portable alternative for muscle assessment.