Is Movement Variability An Individual Trait?

📝 Weekly paper summary

Exploring the role of task constraints on motor variability and assessing consistency in individual responses during repetitive lifting using linear variability of kinematics (Oomen et al., 2022)

Category

Cross-Sectional

Context

The degeneracy present in the human motor system yields variability between consecutive trials of the "same" movement. Nikolai Bernstein referred to this as "repetition without repetition." Previous research has linked limited movement variability to an increased risk of injury and earlier onset of fatigue. There is currently limited data examining individuals' movement variability during lifting-related tasks. Recent work suggests that variability is consistent across multiple days and task constraints, meaning it is primarily an individual trait for pipetting and assembly occupational tasks. It is also essential to test whether movement variability appears to be an individual trait during lifting-type tasks to understand workers' risk for sustaining injuries. This knowledge can help with workplace design and prepare workers to match the demands imposed by their job.

Correctness

• The authors had a relatively large sample of repetitions per individual (up to 105 per person, depending on their fatigue accrued during the collection, and 80 total cycles after data cleaning).
• The authors recruited 20 individuals for this study; this is quite commendable for a full-body motion capture study!
• Depending on the interpretation of movement variability one wishes to glean, we could debate the appropriateness of the methods used in this study. The researchers' goals were to reduce the dimensionality of the lower extremity, upper extremity, and lumbar spine movement variability across the entire lifting cycle into a single measure while still maintaining some interpretability from a practical perspective. Therefore, we can't say anything about how variability at a particular joint may be an individual trait, which could be relevant for assessing the risk of overuse injuries. For example, two people can have the same total lower extremity movement variability by accessing more or fewer movement solutions at different joints, which could be relevant for assessing specific injuries. This methodology certainly isn't a dealbreaker of the study but just something to consider when interpreting the findings.
• As the authors highlighted, other measures of coordination (e.g., relative phase) or variability (e.g., assessing the structure of movement variability [i.e., complexity] rather than just the magnitude) could change the interpretations of the data.

Contributions

• There were moderate to good ICCs between participants' lower extremity (summed standard deviations of the left and right ankle, knee, and hip joints across 101 time-normalized data points), lumbar spine, and upper extremity (summed standard deviations of left and right wrist, elbow, and shoulder joints across 101 time-normalized data points) movement variability across four constraints (two DOF constraints and two loads)
• In other words, movement variability appeared to be an individual trait even during a gross motor task, such as lifting, in which there are relatively more opportunities to exploit redundant degrees of freedom

🧠 Fun fact of the week

Penguins, like all other animals, must drink. When ashore, they'll typically drink fresh water from pools and streams. But what do they do at sea when surrounded by saltwater? They can drink it since they have special glands around their eye sockets (called supraorbital glands) to extract the excess salt from the blood before it travels through their bodies!

🎙 Podcast recommendation

I enjoy playing around with cold exposure (I find it makes me feel good), so I thought I'd share this podcast for this week.

🗣 Quote of the week

"Everything that can be counted does not necessarily count; everything that counts cannot necessarily be counted."

- Albert Einstein