Hamstring Activation & Structural Constraints

Today’s guest is Zac Cupples, PT, DPT, OCS, CSCS, a physical therapist and strength coach known for bridging rehabilitation and performance. He’s the founder of ZacCupples.com and is respected for translating complex concepts around respiration and movement mechanics into practical tools coaches and clinicians can immediately apply to improve efficiency, reduce pain, and enhance performance. The bridge between sports performance rehab is an important one. In the midst of movement mechanics that drive good rehab, and high intensity lifting, lies the knowledge that can help athletes make continual gains while staying robust and healthy for their sport. On today’s show, Zac explores how an athlete’s structure influences movement, strength training, and even injury risk. He shares his track background and how it shaped his coaching, then unpacks concepts like narrow vs. wide “ISA” builds, why some athletes struggle to feel their hamstrings in traditional lifts, and how tools like front loading, box squats, machines, and sprinting can solve it. He also digs into long-duration isometrics, mobility vs. flexibility, and finishes with a fun lightning round. Today’s episode is brought to you by Hammer Strength. Use the code “LILAJUSTFLY10” for 10% off any Lila Exogen wearable resistance training, including the popular Exogen Calf Sleeves. For this offer, head to Lilateam.com Use code “justfly10” for 10% off the Vert Trainer View more podcast episodes at the podcast homepage. (https://www.just-fly-sports.com/podcast-home/) Timestamps 1:23 – Early Athletic Experiences 5:36 – Muscle Activation Challenges 11:22 – Structural Constraints and Movement 25:17 – Rethinking Traditional Strength Training 29:17 – The Role of Machines in Training 36:54 – Weight Shifts and Mechanics 40:45 – Long Hold Activities in Rehab 53:21 – Internal vs. External Rotation 59:27 – Flexibility vs. Mobility 1:07:06 – Lightning Round Questions 1:14:04 – Future Plans and Coaching Focus Zac Cupples Quotes "You got to preserve moving fast because that's how you catch yourself from falling." "It assumes everyone has the same body but no two people are going to perform both of those movements the same way, and it's not going to load the same way." "I start the majority of people with a box squat, because the way I think about a hinge is it's different from a squat because the hips are going to be moving more along that horizontal path." "It's way more useful to think, am I moving up and down? Am I moving side to side? And then just pick exercises within what a person has available." "If someone can't produce certain rotations, and I know that you need those rotations to do this movement, you probably got to find something else to train that pattern within their constraints." "You just have to find the hinge variation that they can execute. And if they don't have much to do that, you have to create constraints." About Zac Cupples Zac Cupples, PT, DPT, OCS, CSCS is a physical therapist, strength coach, and educator specializing in human movement, respiration, and performance optimization. He is the founder of ZacCupples.com and has become widely known for translating complex biomechanical and neurophysiological concepts into practical strategies that clinicians and coaches can immediately apply. Zac earned his Doctorate in Physical Therapy from Marquette University and is board certified as an Orthopedic Clinical Specialist. He has completed extensive post-graduate education through the Postural Restoration Institute (PRI) and integrates principles of respiration, pelvic mechanics, thoracic positioning, and neuromuscular control into both rehabilitation and performance training. Through his online courses, seminars, and educational content, Zac has influenced thousands of clinicians and coaches worldwide. His work bridges the gap between rehab and high performance, helping athletes move more efficiently, reduce pain, and unlock higher levels of strength and speed through better positional awareness and strategic breathing. Zac currently treats clients and consults internationally, while continuing to produce educational resources aimed at elevating the standard of movement practice in both clinical and performance settings.