Lower body strength is the hardest to maintain with limited access to weight that you would normally use in the gym. This is because this is the strongest part of the body, and in order to build strength here, the weight lifted needs to, and will be much heavier than the weights used to increase upper body strength. Some of the best exercises to increase upper body strength, and muscle are pushup and pullup variations, movements that can basically be done anywhere. Reaching failure with these movements doesn't take very long. It’s also easy to increase the intensity when (and if) needed, and it doesn’t take very much to make those movements considerably more challenging.
However, for a generally fit/ strong person who may routinely squat, or deadlift/ hinge (in some form) they will likely NOT hit muscle failure, or close to it, without heavier weights simply because of the much larger and more fatiguing range of motion. General fatigue will likely happen before the lower body muscles achieve the desired stimulus that will improve muscle mass (hypertrophy) or at the very least, maintain strength.
With limited resources, or access to heavier weights, we still do have options though. For my April challenge I am only doing one set of front squats (AMQRP) per every day of training. Though one set doesn’t seem like much, this single set gets very close to failure. Failure in this case, is measured solely on my ability to maintain a controlled tempo, range of motion, and a proficient movement pattern. Since hypertrophy (increased muscle mass) is based on increasing volume over time, theoretically, doing this single set multiple times per week as long as the number of reps continues to progress, will create hypertrophy. It will not necessarily improve strength in terms of increasing your 1RM, but will considerably improve muscular endurance, as well as the potential to build back strength more quickly.
I wondered if what I explained above was supported by science. So, I reached out to Greg Nuckols, a well respected researcher in the “iron game” who is behind Stronger by Science. To my surprise, he responded! He confirmed what I explained above, and also suggested that when training does return to normal, the “comeback” with the heavier weights will be considerably faster. So, my plan is to give myself a cap; 40 reps. When I get there, I will add weight. This will likely drop my ability back to around the 20-rep range, but it will then be the same process...continue to progress that single set volume. More long term, I'll only do this for the length of the challenge (4-5 weeks) before having to change the movement so it doesn't become stale, and produce diminishing returns. So far, I’ve completed three days, day one being the worst! At 70kg, I went:
Day 1; 25 Reps
Day 2; 28 Reps
Day 3; 30 Reps
At the least here, depending on how many times a week I decide to do this, I may only get five total sets. At most, with other training included, I may get more like 8-10 sets in total for the weekly, knee dominant lower body training. This should at the very least produce *some hypertrophy, and improved muscular endurance.