In the trigger point & massage therapy communities, 5 muscles in particular are known to cause the majority of muscle induced pain. These 5 are the Quadratus Lumborum (QL), Psoas, Iliacus, Piriformis, and Gluteus Medius.
1. The Quadratus Lumborum (QL) muscle gets overworked in activity that requires spinal stability. Athletes and labor workers tend to respond well to QL release. Massage the QL if you are active in high impact sports or are physically active at your day job.
Quadratus Lumborum (QL) Placement
2. The Iliacus muscle tends to tighten after long periods of sitting. If you are a desk jockey or sit down for 8+ hours a day, release the Iliacus.
3. Psoas tightness goes hand in hand with Iliacus tightness. Massage the Psoas as well as the Iliacus if you sit for 8+ hours a day to loosen up those poor hip flexors.
4. The Piriformis muscle can lock up from high impact sports with lateral movement. When tight, it can cause duck feet (toes pointed outward by externally rotating the hip) as well as sciatica (by compressing the sciatic nerve).
5. Gluteus Medius massage goes hand in hand with QL release. Massage Glute Medius if you have pain from "overdoing it" (spending a day moving houses, doing yard work, playing hours of basketball, etc.).
Glute Medius Placement
Massage These Muscles - Buy Your QL Claw Today!
 Donnelly, Joseph M. Travell, Simons & Simons Myofascial Pain and Dysfunction: the Trigger Point Manual. 3rd ed., Wolters Kluwer Health, 2019.
 Davies, Clair, and Amber Davies. The Trigger Point Therapy Workbook: Your Self-Treatment Guide for Pain Relief. 3rd ed., New Harbinger Publications, Inc., 2013.