Physical therapy can be an effective way to treat back muscle stiffness. Here are a few general ways that a physical therapist might approach this but please keep in mind that every treatment will be personalized specifically to your case:

Stretching exercises: Your physical therapist will teach you specific stretches that can help to reduce stiffness and increase flexibility in your back muscles. These stretches may include the hip flexor stretch, hamstring stretch, and cat-cow stretch.
Manual therapy: Manual therapy is a hands-on technique used by physical therapists to relieve muscle tension and improve joint mobility. This may include techniques such as massage, myofascial release, and joint mobilization.
Strengthening exercises: Weak muscles can contribute to back pain and stiffness. Your physical therapist will develop a customized exercise program to help you strengthen your core muscles, including your abdominals, glutes, and back muscles.
Posture education: Poor posture can lead to back pain and stiffness. Your physical therapist can educate you on proper posture techniques and help you make adjustments to your daily routine to avoid unnecessary strain on your back muscles.
Heat or cold therapy: Depending on your individual needs, your physical therapist may use heat or cold therapy to help reduce muscle stiffness and pain.
Patient education: Your physical therapist can provide you with information on how to maintain a healthy lifestyle, including tips on exercise, nutrition, and stress management. This can help you manage your back pain and prevent future episodes of stiffness.

It's important to note that the specific approach that your physical therapist takes will depend on the underlying cause of your back muscle stiffness. They will likely perform an evaluation and develop a treatment plan that is tailored to your individual needs.