One of the reasons you might struggle to lift a heavy object with your arm fully extended is because:.

the overlapping decreases between the thin and thick filaments.

Explanation:

When w extend our hand or arm to the full and try to lift any heavy object, we are unable to lift the object inspite of applying all our force. We struggle hard to lift the object with our fully extended arm because when we extend our arm fully it decreases the overlapping of our thin and the thick filaments of our muscles which makes it difficult to lift. In other words, the resting length of our arm is the optimal length to generate force.

i think that it is plants