Final answer:
SMR and static stretching of the quadriceps are recommended for Susan's overactive quadriceps to help improve muscle imbalance. Muscles work in pairs, and balancing the quadriceps with its counterpart, the hamstrings, is crucial for leg function.
Step-by-step explanation:
If Susan's quadriceps have been identified as overactive, the type of flexibility training that should be used first to help improve this muscle imbalance is D) SMR and static stretching of the quadriceps. Muscles indeed come in pairs, and in the case of the legs, the hamstrings and quadriceps work together to facilitate movement. Overactive or tight quadriceps can lead to muscle imbalances that could affect overall leg function and movement efficiency.
The initial go-to method to address overactive quadriceps is often Self-Myofascial Release (SMR) followed by static stretching. SMR, usually done with a foam roller, helps to release muscle tightness, while static stretching involves holding a stretch for a period of time to elongate the muscle fibers. This combination can help in reducing muscle tension and correcting imbalances in muscle pairs that play roles in activities such as running, where the quadriceps and hamstrings move the leg back and forth.