This week we discuss considerations when utilizing the limb symmetry index (LSI) as a metric for return to sport testing following ACL reconstruction. Limb symmetry index is a popular metric which compares the performance of the involved limb to the uninvolved limb during a test. Commonly, a 90% threshold is used to determine if there is acceptable levels of asymmetry. However, there are a few points to consider when utilizing this metric. First, after an ACL rupture the contralateral side also loses strength and functional capacity. Therefore, comparing to the post-surgical performance of the uninvolved side can give an artificially inflated LSI, so it is important to ground the test outcomes in normative data or benchmark data. Second, variability in movement is a "hallmark of normal function" and understand humnas, and sport, are not always symmetrical. Although, an argument can be made that the test is to see if you have the capacity to be symmetrical, which does not mean you have to be symmetrical all the time, especially at submaximal efforts. Is your maximal effort symmetric on each side? If not, then you may be more likely to be hurt. Third, biomechanical asymmetries can persist past 9 months post ACLR, so don't just look at the numbers: look at the movement strategy. Finally, assess the functional capacity across a load continuum via testing considering the end goal as the sport.

The abstract can be found here: https://journals.lww.com/nsca-scj/abstract/2024/08000/testing_limb_symmetry_and_asymmetry_after_anterior.3.aspx

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This week we discuss microscopic changes to muscle architecture following eccentric hamstring exercise. Twelve recreationally active participants in their mid-20s performed 9 weeks of eccentric hamstring training via Nordics: 3 session per week, progressively building from 4x6 to 5x8 at the end. After the 9 weeks of training, they had a 3 week detraining period. Eccentric hamstring strength during Nordics, passive fascicle length, and sarcomere length were measured at baseline, end of training, and end of detraining. Serial sarcomere number was calculated using fascicle length and sarcomere length. After the training block, there was significantly increased biceps femoris fascicle length, serial sarcomere number, volume, and knee flexion torque. There was a large effect for all of these metrics. There was a significant regional effect for fascicle length and serial sarcomere number: increased fascicle length at distal region compared to central, but increased serial sarcomeres centrally versus distally. During detraining, fascicle length decreased but remained logner than pre-training lengths. Muscle volume stayed the same and there was a decrease in strength but it was higher than pre-training. Takeaways from this article include evidence for a long-term adaptation of eccentric training being addition of sarcomeres in series which could be protective against hamstring strain injuries, and muscle architectural responses to detraining happen quickly! It would be interesting to investigate if addition of sarcomeres in series is an effect specific to eccentric training, or any specific hamstring muscle training would create this effect.

The abstract can be found here: https://pubmed.ncbi.nlm.nih.gov/39461588/

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This week we discuss tendon adaption. This review aims to define the mechanoresponses that lead to tendon adaptation, and whether or not these are positive or negative adaptations. All adaptations can be considered through a person- or tissue-level lens. Person-level adaptations involve a number of systems and are measured via athletic performance tasks, while tissue-level adaptations are isolated and unclear how they impact the whole system. Adaptation is relative to the amount of load to the tissue, and there is likely a moving, tissue-level threshold which determines if an applied load is adaptive or maladaptive. Some changes we can see in tendon are changes to cross-sectional area, stiffness, structural organization of collagen, water content, and vascularity. However, there is no clear link between changes to these characteristics and injury risk or athletic performance since these changes are seen post exercise in pathological and non-pathological tendons. Potentially, whether a tendon becomes pathological or not is more related to the systemic response to these transient, load-related changes?

The abstract can be found here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6737558/pdf/JMNI-19-300.pdf

As always, if you enjoy what you hear, be sure to follow us on your favorite podcast platform and on Instagram @readingrehabpod. If you have any article recommendations be sure to send them our way!