The squat and the leg extension test the strength of your legs and specific muscles in your leg. The deadlift remains the primary test of lower back strength, even though this lift also involves muscles from your legs and hips. Commonly used in a laboratory setting, both the squat and the deadlift remain part of the sport of powerlifting.
Squat
The barbell squat measures the strength of your hamstrings and quadriceps, the largest muscles in your legs, and generally requires you to squat until your knee reaches a 90-degree angle. Trainers often perform this test with a barbell, may they may also utilize a fixed device to control the path of the bar. Another type of testing, called isokinetic testing, involves not only controlling the path of the bar, but your acceleration as well. Testers primarily use isokinetic leg strength to measure muscle activation at a constant speed.
Knee Extension
The knee extension, or leg extension, test measures the strength of your quadriceps, the muscles on the front of your thigh. While versions exist that allow you to perform extensions with both legs, standard testing measures only the strength of one of your legs at a time. Testers commonly attach electrodes to read the conductivity and activation of your muscles. They refer to this process as electromyographic analysis, or EMG testing.
Deadlift
The deadlift tests the strength of your back by requiring you to squat down and pick a heavy barbell up off of the floor. Your back must remain flat or arched although sometimes, for testing purposes, athletes perform round-back lifting. When tested with a round back, you will always use a light weight due to the inherent risks to your spine. Unlike other methods of testing, trainers won't test the deadlift either isokinetically or in a fixed plane. When you perform a deadlift, you pull the bar in something of an arc, or even a slight S-shaped pattern. Attempting to force your body to do this in an unnatural movement pattern increases your risk of injury, particularly to your lower back.
Reasons for Testing
Testers may use the squat to measure leg strength, or limit strength -- the amount you can lift for a single maximum attempt. The squat may also test force generation, maximal power output or muscle activation and recruitment. The leg extension may also test recruitment, including recruitment of your hamstrings, to determine the effect they have when stabilizing your knee joint. The deadlift tests either the strength of your lower back, or the effect of shearing force generated by various styles of lifting. Never execute a maximum-weight lift without training, supervision and a spotter.
