Try negative reps for muscle confusion
What are negative reps? Think of your standard bicep curl. Well with negatives reps your focus is on slowing down the eccentric phase of the exercise (the lowering phase). Curl up at a faster pace (one second), when you go to the lowering phase you count slowly down, one one thousand, two one thousand, and end at five one thousand. Negatives will allow you to train using a slightly heavier weight, so take advantage of this! This style of training will fatigue your muscles beyond previous experience, while promoting growth and strength.