"Let's ミクササイズ！！" is a commissioned theme song written and produced by cosMo＠暴走P (cosMo@BousouP) for the Nintendo Switch title "Fit Boxing feat. 初音ミク -ミクといっしょにエクササイズ-," a fitness game starring Hatsune Miku as a workout coach. True to its exercise inspiration, the track rockets along at 239 BPM, a brisk tempo that mirrors the energetic, punchy aesthetic of the Fit Boxing series. cosMo＠暴走P, known for extreme-speed Vocaloid compositions such as "初音ミクの消失" (The Disappearance of Hatsune Miku) and "Grief," brings his signature rapid-fire melodic lines and driving synth work to this session, with Miku's voice pushed into an enthusiastic, almost breathless delivery that matches the workout theme. The song debuted in maimai でらっくす CiRCLE on December 5, 2025, categorized under niconico＆ボーカロイド. Its Master chart, designed by はっぴー, lands at level 13+ (13.8) and features a total note count of 1111, a playful numerical nod that stands out among the chart's dense Tap and Slide patterns. The Expert chart, crafted by サファ太, sits at level 12 (12.4) with 870 notes. Both charts are available across Japanese, international, and North American regions from release day. The in-game collection titles, "思い立った今日がmaimaiライフ一日目" (Today is the first day of my maimai life) and "筐体パンチはおやめください" (Please refrain from punching the cabinet), riff humorously on the exercise-and-gaming crossover theme.