Abstract
INTRODUCTION:
Hamstring strain injuries are common in sports that involve high-speed running. It remains uncertain whether the hamstrings are susceptible to injury during late swing phase, when the hamstrings are active and lengthening, or during stance, when contact loads are present. In this study, we used forward dynamic simulations to compare hamstring musculotendon stretch, loading, and work done during stance and swing phases of high-speed running.METHODS:
Whole-body kinematics, EMG activities, and ground reactions were collected as 12 subjects ran on an instrumented treadmill at speeds ranging from 80% to 100% of maximum (avg max speed = 7.8 m·s(-1)). Subject-specific simulations were then created using a whole-body musculoskeletal model that included 52 Hill-type musculotendon units acting about the hip and the knee. A computed muscle control algorithm was used to determine muscle excitation patterns that drove the limb to track measured hip and knee sagittal plane kinematics, with measured ground reactions applied to the limb.RESULTS:
The hamstrings lengthened under load from 50% to 90% of the gait cycle (swing) and then shortened under load from late swing through stance. Although peak hamstring stretch was invariant with speed, lateral hamstring (biceps femoris) loading increased significantly with speed and was greater during swing than stance at the fastest speed. The biarticular hamstrings performed negative work on the system only during swing phase, with the amount of negative work increased significantly with speed.CONCLUSION:
We concluded that the large inertial loads during high-speed running appear to make the hamstrings most susceptible to injury during swing phase. This information is relevant for scientifically establishing muscle injury prevention and rehabilitation programs.
My Comments:
This is a pretty interesting study. Most of the reports I have read of hamstring muscle injuries have been at the end of the swing phase, but there is some controversy with arguing that there is also risk at foot strike. This paper sought to settle that with regards to where forces through the hamstring muscle and tendon are greatest, but also found/reported other interesting findings which they and I agree, should have bearing on physical therapy exercise protocols for preventing and rehabilitating hamstring muscle and tendon injuries.
So what they did was have subjects sprint on a treadmill at speeds of 80%, 85%, 90%, 95% and 100% of their maximum. Hamstring forces at foot impact didn’t change significantly at higher speeds. Hamstring muscle length didn’t change either, because increased hip flexion at higher speeds was combined with increased knee flexion, keeping hamstring length at different speeds about the same. Hamstring forces however, particularly in the biceps femoris long head (the most frequently injured) did increase during the swing phase as sprint speed increased, particularly so up and above 90% with 95% and 100% being greater still. So this would lend credence to the idea that it’s at or near the terminal aspect of the swing phase, with the leg out in front that the injuries are most likely. Hamstrings were only eccentrically contracting at the very late swing phase, corresponding to where the forces are highest.
They referenced that hamstring moment arms are greater at the hip than at the knee, which I think is why Romanian Deadlifts (RDLs) feel like they work better than leg curls, and a newer exercise I have been working with, the Nordic Hamstring Curl (NHC). Based on the results of the study the authors suggested that resistance training be done with the knee fixed in extension and flexing the hip to end range (exactly what an RDL does). They also agreed that training the muscle eccentrically to be important. The RDL with a sufficiently loaded barbell trains the hamstrings (and glutes) hard both eccentric and concentrically, but does not quite overwhelm the hamstrings eccentrically like the NHC does. Unfortunately NHC acts more at the knee than the hip, and doesn’t stretch the hamstrings much at all like the RDL. So as of now I think the combination of the RDL and NHC are state of the art for the prevention and treatment of hamstring strains. If I had to pick one I’d go with the RDL, and I’m not sure that the NHC adds much to a program with the RDL already there. To play it safe I include the NHC since multiple large studies in professional soccer players have demonstrated it reduces the risk of hamstring injury and re-injury. The RDL, while I expect is better, hasn’t been specifically tested for prevention or rehabilitation of hamstring injuries. I think both are worth having in a program along with other good exercises like squats, lunges, and Olympic lifts/variants. Everything else, especially the various posterior bridging exercises, with or without a swiss ball, I think are trash.
Cheers.
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Chad Reilly is a Physical Therapist, obtaining his Master’s in Physical Therapy from Northern Arizona University. He graduated Summa Cum Laude with a B.S. Exercise Science also from NAU. He is a Certified Strength and Conditioning Specialist, and holds a USA Weightlifting Club Coach Certification as well as a NASM Personal Training Certificate. Chad completed his Yoga Teacher Training at Sampoorna Yoga in Goa, India.
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