It's a common misconception that if you do endurance activities, you can't have muscle, be muscular, or build muscle. [Insert: "must use lighter weight if in an endurance phase"]. This narrative is taken too far, in my opinion. There is a point of meeting in the middle, but we'll get into that shortly.
What is an Endurance phase?
An endurance phase is typically done to increase muscular endurance and capacity. Most people, regardless of goals, visit this phase annually in one form or another. Endurance phases can be used as maintenance phases for those who typically focus on strength, as the bulk of the focus for those who are involved in endurance activities (hiking, running, skiing, etc.), or simply as part of their yearly program for overall wellness.
Endurance phases typically follow a format of 2-4 sets, 8-15 reps, and 30-90 sec rests per exercise. Although supersets can be found in any phase, it is common in the endurance phase.
Since the prescribed rep range is higher and rest times are lower, this is where the "must use lighter weight" is debunked. Yes, you're likely going to be using relatively lighter weight for a 12 rep exercise than you would for a 5 rep exercise. But, no, it's not because lighter weight automatically equals endurance. It's because a 5 rep exercise at an RPE 8 is going to look a lot different than a 12 reps exercise at an RPE 8. If you deadlift 5 reps at 135 lb for an RPE 8, deadlifting 12 reps at an RPE 8 is not going to be 135 lb. You would have to use relatively lighter weight with the same amount of effort to produce muscular endurance.
One of the first questions I often get when switching from a strength or power phase to an endurance phase is, "aren't I going to lose my muscle since I'm lifting less weight?" This leads us to our next order of business.
How do you maintain muscle in an endurance phase?
1. Lifting less weight doesn't necessarily mean your overall volume is down since you have increased the amount of reps. This is why following the RPE scale is so important. Working in the RPE 7-9 range most of the time, meaning leaving 1-3 reps left in the tank, can produce positive outcomes of increasing endurance without sacrificing muscle.
2. Nutrition is going to play a large part in maintaining muscle, as well. This is true throughout all phases of training. Keeping calories at maintenance while prioritizing protein can help maintain muscle. As a general recommendation, consider aiming for .8g/kg of bodyweight as a base. For more optimal recommendations, 1.4g-2.0g/kg of bodyweight would likely be more beneficial.
3. Include at least 2-3 full body lifts per week in addition to your cardio training. This will allow you to keep the frequency and volume of load at a manageable level throughout the endurance training.
Muscle will adapt to the environment you provide it. Continue to lift, fuel properly, and recover well to give yourself the best possible outcomes.