The island of Themascryia is a nation of amazons, an all-female race that spend their lives training for battle. They are currently under siege by foreign invaders. This army comes from a powerful empire who seek to take them as slaves. Technology evens the playing field between men and women in terms of war. In a world of guns, physical prowess matters much less on the battlefield. However, this is the ancient world. In an open field plain with sword and shield, natural strength and speed make the difference. Due to these factors, an amazonian army would not be able to stand up to an invading, experienced, and trained military force of males, pound for pound without other strategic or tactical advantages. However, these warriors have a secret.
The human body is capable of amazing physical feats, such as lifting up cars to save a child. The problem is that the brain normally suppresses these abilities. Our muscles can only be pushed to the max in flight or fight situations when we are in danger, or when the adrenaline is pumping. This is to prevent over straining our muscles and prevent them from tearing. These amazons have the ability to unlock this potential at will, giving them access to 100% percent of their muscles when they need it. The drawback with this in normal humans is that continuously using our bodies in this manner will eventually lead to tearing and degradation of those muscles.
How can I make the biology of these amazons to be capable of sustaining such constant usage to make it useful without the negative drawbacks?