The "Muscle-Muscle Fruit" (Muscles Muscles no Mi): A Paramecia-type Devil Fruit granting the user enhanced physical capabilities. At its core, the fruit allows for the targeted strengthening and augmentation of specific muscle groups. This isn't a blanket increase in strength across the board, but rather a precise control over muscular development.

Beginner Level (Comparable to a strong pre-timeskip Zoro): The user can initially focus on enhancing individual muscles, leading to improvements in speed, strength, and agility in specific areas. For example, they might temporarily enhance leg muscles for incredible sprinting speed, or arm muscles for powerful punches. This enhancement is temporary, lasting only for a short period (a few minutes at most) before requiring rest. The user can only focus on one muscle group at a time, and overexertion leads to significant muscle fatigue and soreness. The level of enhancement is limited; they wouldn't be able to lift a car, but could easily overpower several average humans. They might be able to jump higher or run faster than a normal human, but not to the extent of a superhuman.

Intermediate Level (Comparable to a post-timeskip Zoro, but still below Luffy): With increased mastery, the user can enhance multiple muscle groups simultaneously, albeit with reduced intensity in each. They could improve both speed and strength, but not to the same degree as focusing on a single area. The duration of the enhancement also increases slightly. They might develop a more refined control, allowing for subtle adjustments in muscle tension for improved precision and dexterity. They could potentially use this to enhance their defensive capabilities by hardening specific muscle groups.

Advanced Level (Approaching Luffy's power, but still weaker): At this stage, the user gains a deeper understanding of their body's biomechanics. They can achieve a more sustained enhancement, lasting longer and with greater intensity. They might develop techniques to channel the enhanced muscle power into specific attacks, creating powerful blows or rapid movements. However, even at this level, they remain significantly weaker than Luffy in terms of raw power and overall destructive capability. They might be able to match Luffy in a specific area of combat (like speed or precision), but not overall. The fruit's power is still fundamentally limited by the user's physical stamina and the need for rest between enhancements. Overuse can lead to serious injury.

Weaknesses: The fruit's power is tied to the user's physical condition. Fatigue, injury, or illness significantly weakens the effects. The user is still vulnerable to attacks that bypass their enhanced muscles, such as Haki-based attacks or powerful blunt force trauma. Like all Devil Fruits, the user is weak to seastone and water. The precise control over muscle enhancement requires training and practice; misuse can lead to injury or inefficient power application.