September 21, 2021 Newsletter

📝 Weekly paper summary

Lower extremity joint kinetic responses to external resistance variations (Flanagan and Salem, 2008)

Category

Within-subjects experiment

Context

Schmidt's Generalized Motor Program GMP) hypothesis suggests that the relative force and relative timing of actions are scaled proportionally to accommodate several different movement conditions. The main advantage of the GMP, relative to original motor program theories, is to alleviate previous theoretical concerns surrounding the "storage problem" (i.e., the brain does not have the capacity to store every possible motor program in memory) by asserting that certain features within different "classes" of movements are scaled for different contexts. For example, suppose an individual back squats 25% versus 75% of their one-repetition maximum (1RM). Rather than requiring a different motor program for each condition, the individual's recall schema would weigh the initial conditions with past outcomes from prior specific response specifications to adjust the relative force and timing of the ankle, knee, and hip extensors of the generalized motor program. Therefore, only one GMP is required for any squat because the relative forces and timing are scaled (i.e., remain consistent) to accommodate the varying demands of squatting different loads.

The purpose of this investigation was to assess the GMP hypothesis and quantify the relative contributions of the ankle, knee, and hip extensors during a barbell back squat when participants lifted 25%, 50%, 75%, and 100% of their 3RM. The authors hypothesized that the relative contributions should remain consistent regardless of the load lifted.

Correctness

There were a variety of strengths to this paper:

1. Their biomechanical model and analysis procedures were well-justified.
2. Their statistical approach to assessing this data appears acceptable (we could perhaps argue whether an individual's average from multiple trials is appropriate since the variability of joint loading is also important to consider). It would have been nice to have more information about the statistics used. Still, clearly, the authors were up against a word limit and (correctly, in my opinion) spent more time elucidating their biomechanical analyses.

There are a couple of delimitations to this study to consider when interpreting their results:

1. Participants were instructed to squat to a depth of 45% of their leg length (defined as the distance between the anterior-superior iliac spine to the medial malleolus) since the authors claimed this corresponds to about 90\(^{\circ}\) of knee flexion. More recent work has suggested that the knees are relatively loaded more when squat depth increases, but the general findings of this study are still supported (e.g., see Bryanton et al., 2012 and Goodman, 2020). It would be nice to see these studies expanded on with other nonlinear human movement analyses to expand upon the traditional biomechanical analyses used in these studies.
2. The 3RM loads that participants attained and the rest provided between their three sets of three repetitions with 25%, 50%, 75%, and 100% of their 3RM weren't reported. The only information about the participants reported was that they were young, "healthy," pain-free within the previous 6 weeks, and had experience performing a squat movement for at least 1 year. It's, therefore, harder to extrapolate these findings beyond this experiment.
3. The researchers only analyzed the lifting phase of the squat. Perhaps there is also important information to uncover in the descent of the squat that wasn't reported in this paper.
4. The paper did not provide much detail regarding the instructions and feedback provided to participants.

Contributions

• Except for the 25% 3RM condition, in every case, the contribution to the lower extremity work of the hips > knees > ankles (at the lightest condition, hips = knees > ankles).
• As load increases, the contribution of the work done by the hip extensors to the total lower extremity work increases while the contributions from the knee extensors decrease. The contribution of the work done by the ankle extensors to the total lower extremity work increased from 25% to 50% 3RM but remained consistent thereafter. This was likely due to an anterior displacement of the center of pressure found in each loading condition.
• There was a ~2% decrease in the maximum hip, knee, and ankle flexion angles when increasing %3RM load.
• The relative contributions of the hip, knee, and ankle extensors are not invariant, and thus the underpinnings of the GMP were not supported by this data. Therefore, the authors state that "modifications of the generalized motor program theory may be necessary to account for the control of resistance variations."
• At lighter loads, individuals displayed varying control strategies (i.e., different contributions of the hips, knees, and ankle extensors to the total lower extremity work). At heavier loads, they started converging on more similar control strategies. This is perhaps something to be conscious of during assessments and training/practice program design as limited motor solutions pools may impair the learning rate (e.g., Orth et al., 2018), the transfer of training (e.g., Seifert et al., 2019), or increase the risk of injury (e.g., the injury-variability overuse hypothesis, reviewed by Nordin and Dufek in 2019). (note: this isn't to suggest that you shouldn't lift heavy, but rather think about the additional constraints selected when lifting heavy to maximize transfer to sport/work/life).
• The fact that people "shift" the load to their hip extensors when squatting a larger mass may be another reason why this still transfers to improved vertical jump height (the literature suggests to jump higher you need to load your hip extensors more). Of course, there are other reasons at play as well (i.e., the intent to move fast even with heavier loads, increased motor unit recruitment with heavier loads, etc.), but the coordination piece that we can gather from this study is also particularly interesting.

🧠 Fun fact of the week

Water makes different pouring sounds depending on its temperature! The viscosity of water decreases with heat, and the way water splashes sound is affected by its viscosity. This is definitely something you were aware of subconsciously (try it out yourself and see!), but I'm guessing you didn't know you could consciously tell the difference. 😊  

🎙 Podcast(s) to check out

The CSB recently started creating podcasts. This interview with Dr. Jack Callaghan is a fantastic discussion about his research program at the University of Waterloo and lumbar spine mechanics in general. I highly recommend checking this one out!

🗣 Quote of the week

"First, practice not letting people know who you are- keep your philosohpy to yourself for a bit. In just the manner that fruit is produced- the seed buried for a season, hidden, growing gradually so it may come to full maturity. But if the grain sprouts before the stalk is fully developed, it will never ripen... That is the kind of plant you are, displaying fruit too soon, and the winter will kill you."

Epictetus, Discourses, 4.8.35b-37