Muscle strengthening activities are activities that make muscle do more work than they usually do. They can be part of unstructured play like tree climbing and playing tug-of-war or structured like push-ups and lifting weights. Muscle strengthening activities make muscles stronger and can provide additional benefits not found with aerobic activity. Musclular strength allows you to complete daily activities without excessive stress on joints and muscles. It is important to work all major muscle groups: legs, arms, abdomen, chest, back, and shoulders.

How much weight or force is used relative to how much a person is able to lift.

How often a person does muscle strengthening activity.

How many times a person lifts weight. This is similar to duration for aerobic activity.

United States Department of Health and Human Services
Physical Activity Guidelines for Americans - 2018
https://health.gov/sites/default/files/2019-09/Physical_Activity_Guidelines_2nd_edition.pdf