You can easily improve your exercise performance and safety by maintaining an optimal quadriceps to hamstring strength ratio.
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Anatomy and Function
The upper part of the human leg is composed of two muscle-groups that work in opposition to each other. The posterior (rear) muscle group is called the hamstrings, and the anterior (front) is the quadriceps. As previously mentioned, they work in diametrically opposite fashion when it comes to moving the lower leg and hip.
The hamstrings consist of three independent muscles: the semitendinosus, biceps femoris and semimembranosus. The quadriceps includes four muscles: vastus lateralis, vastus medialis, vastus intermedius and rectus femoris.
Hamstrings muscles are responsible for hip extension and curling your lower leg towards your upper leg. The quadriceps literally do the exact opposite: pushing your lower leg away from your upper leg, and flexing your hip upwards, towards your body.
Muscle Imbalances
It is very important that the two muscle groups maintain a certain strength ratio. One side essentially relies on the other to maintain stability in the knee joint. Failure to do so can lead to poor posture and destabilization of the knee joint. Both of these effects can quickly lead to an injurious outcome for athletes, and really anyone who walks regularly.
Optimal Quadriceps to Hamstring Strength Ratio
Most people have hamstrings capable of producing 50-80% of the force that their quadriceps can, according to this study. This amount is not optimal, however.
The optimal relationship between your quadriceps and hamstring is a 1-to-1 ratio. That is, equal strength between the two muscle groups. The study linked above notes that stability in the knee joint continued to improve as the strength of the hamstrings rose to 100%.
It can be assumed that if hamstring strength surpasses quad strength, which is very uncommon, that the opposite instability would occur.
Determine and Achieve Balance
The easiest way to determine the strength balance between your hammies and quads is to use a leg extension machine, and a leg curl machine. Simply test your one-rep-maximum strength on each machine and compare.
If the machine lacks sufficient weight, do as many repetitions as possible with the maximum weight. Then enter the weight and repetitions into a one-rep-maximum calculator to generate an estimated maximum. Repeat for the quadriceps, and compare the numbers.
In order to achieve greater balance, many will have to strengthen their hamstrings through weight training.