It’s not news to draw attention to the Hamstrings in relation to ACL rehab, injury risk mitigation or especially in the context of a non-contact ACL injury. But, I do think it is interesting to draw attention to the magnitude of the forces expressed at the knee and reasoning behind the ACL/Hamstring relationship.
The ACL can potentially tear / break at forces at around 2000N, but outweighing this number is the fact that the quadriceps can generate upwards and towards 4000N of force. Critically the Hamstrings muscle group can generate force or torque on the tibia that essentially counters the quads force and the anterior tibial translation, a causative movement associated with ACL injury. Furthermore, the hamstrings have an influence on tibial or shank rotation as they can control or prevent some of the rotational moments.
In sports where ‘ACL Prevention’ is a commonly programmed injury risk mitigation goal, We’ve habitually gravitated in Physio / S&C towards assessing landing mechanics in search of the valgus/varus knee mechanics that are associated with the rotation component of ACL injuries. However, assessing or training the force capabilities and ratio between the quads and critically the hamstring force output/capacity should be given equal attention.
An emerging habit within strength and conditioning is to train the hamstrings with greater emphasis on training them as hip extensors and/or in the closed kinetic chain. My subjective assumption for this trend is with specificity in mind regarding the hamstrings role in running mechanics, jumping or co-contraction positions. Also these femur-on-hip exercises typically exhibit the ability to add more load and we often favour programming exercises that offer more ‘bang for your buck’ as seen in the heavier compound lifts. Whilst I agree hamstrings should be trained in this way, we need to also remember to incorporate their knee flexor or femur-on-tibia properties targeting their ability to counteract anterior tibial translation – With particular focus on the hamstrings motor control and force capabilities to eccentrically control the tibia anteriorly.
When assessing the hamstrings the insights on their peak force output, reactive-strength-index or rate-of-force-development are of course useful to understand the varying ‘strength’ characteristics occurring at the knee and hip. However, we should also incorporate testing their strength endurance or capacity. Especially given the correlation between fatigue and injury rate in sport, or the prevalence of injuries that occur during the later quarters/halves of team games. As such we must be thorough with how we assess, program and train the hamstring group to balance internal forces and minimise injury risk potential.