Sometimes your legs just don't feel like working as hard as they normally do. Weakness in the quadriceps could be subjective due to fatigue or recovery from illness. Your whole body is recovering and you can't lift the weight you usually do. However, if weakness in the quadriceps is not accompanied by illness and is instead a measurable decrease in strength, something else may be going on.
Your quadriceps are actually a group of four muscles: rectus femoris, vastus lateralis, vastus intermedius and vastus medialis. Rectus femoris lies on top of the others and crosses both the hip and knee joint. The other three only cross the knee. All four come together into one common tendon -- the patellar tendon. The quads are responsible for flexion of the hip and extension of the knee joint and are a very powerful group of muscles.
Causes of Muscular Weakness
If you have had an injury, especially to the knee or hip, your muscle weakness could be due to disuse of your leg muscles. However, there are also other causes of quad weakness. It could be a metabolic disease, such as low sodium or Addison's disease. Measurable weakness can be due to neurological disease such as multiple sclerosis or Bell's palsy. Other causes of muscular weakness that may exhibit in the quads are toxins or muscular diseases. Only a doctor can diagnose one of the medical causes of quad weakness and, most likely, you will be exhibiting other symptoms as well.
Improving Quad Function
Often just adding daily activity can improve the strength of your quadriceps. Cardiovascular exercises such as cycling or walking use the quadriceps and will aid in improving muscular endurance. Add strength exercises such as squats, step ups, leg presses or leg extensions to improve the strength of your quads. Perform one set of eight to 12 repetitions with a weight that is challenging but allows you to complete your reps with proper form. Do the exercises two to three times per week on nonconsecutive days to see changes in muscular fitness, according to the American College of Sports Medicine.
If you have a medical condition or have had an injury, speak with your doctor and/or physical therapist about what type and how much exercise is appropriate for you to improve quad strength. If you are considered healthy, try increasing your physical activity in order to combat weakness in your quadriceps. If you do train a lot, it is possible that you are overtraining and not allowing your quads to heal properly, resulting in muscular weakness. Cut back on your workouts for improved quad function.