When assessing the flexibility of a muscle, it is easy to just look at the muscle itself. But it is important to look at the response of joint(s) that the muscle acts on.
First we must understand that there are different types of tightness. There is a short muscle and there is a stiff muscle.
- Short muscle – a muscle that has a reduction of sarcomeres limiting its length
- Stiff muscle – a muscle that has appropriate length but maintains increased tension through that range
Now that this is addressed we can look further at assessing muscle flexibility. There can be different presentations of a short/stiff muscle. The 6 categories will be presented with assessment of the rectus femoris (RF) muscle. This muscle is most easily assessed in the prone position and by flexing the knee; normal motion will be between 115-125 degrees knee flexion.
- Normal – the knee is able to be flexed >115 degrees without compensatory pelvic motion
- Short RF- knee flexion is <115 degrees without compensatory pelvic motion. The anterior abdominal muscles have normal tension.
- Stiff RF – the knee is flexed >115 degrees with sensation of tension in the muscle and compensatory anterior pelvic tilt and when the pelvis is stabilized to prevent tilting the knee continues to its initial range.
- Stiff + Short RF – the knee is flexed >115 degrees with anterior pelvic tilt and when the pelvis is stabilized knee flexion <115 degrees. This shows that the abdominal muscles have less stiffness than RF but there is also a component of shortening of the RF.
- Stiff RF with automatic stabilization – passive knee flexion will cause compensatory anterior pelvic tilt. Active knee flexion does not have an associated anterior pelvic tilt. This is a sign that the body will maintain stability due to the stiffness of the RF.
- Deficient counter stabilization – the initial motion of knee flexion will cause a posterior pelvic tilt (the opposite anticipated reaction). Motor control issue.
Being able to differentiate between these 6 categories when assessing flexibility of a muscle will help appropriately guide treatment. I encourage you to use this when assess muscle flexibility for other areas of the body; such as the latissimus dorsi, hamstrings, pecs, gastrocs, etc.