How much can we infer about movement kinetics from kinematics?

📝 Weekly paper summary

Making Inferences about Muscle Forces From Clinical Observations (Herbert et al., 1993)

Category

Review/Commentary

Context

This paper is a classic for any strength coach, physiotherapist, chiropractor, athletic therapist, personal trainer, or anybody interested in observing human movement to guide their professional practice. As outlined by the authors in their introduction (pg. 195),

"While the observation and analysis of motor task performance provide a key role in assessment of people with movement disorders, there has been very little explicit discussion in physiotherapy literature about precisely what should be observed or how those observations should be analysed."

Although this paper is from 1993, I'd argue there's still little discussion about what exercise professionals should observe (visually or with technology) or how to interpret those observations from a physics-based perspective. Therefore, I highly recommend reading this paper for yourself!

Contributions

I'll leave key quotes for this paper below:

"When the accelerations of body segments are negligible, it may be possible to make reasonable inferences about muscle moments acting at a particular joint using models of analysis which assume the absence ofsegmental accelerations. However, when significant accelerations of body segments occur, more sophisticated models of analysis must be used if reasonable inferences about muscle moments are to be made."

• This point is essential to consider when conducting visual movement assessments. When people move fast, you'll probably need to do some calculations and not rely solely on your visual observations. As a mentor of mine has said, "it's why we do the physics!"

"When intersegmental dynamics dominate task kinetics it probably is not possible to make reasonable inferences about muscle moments from clinical observations alone. In these circumstances, physiotherapists must utilise findings from studies on the biomechanics of disordered in order to meaningful analyses."

• I like this quote because we're finally approaching a point now where, with emerging technologies, physiotherapists will be able to do more of these measurements themselves without relying only on studies with limited samples (both in terms of sample sizes and populations typically recruited in studies). Of course, this will require a new skillset from physiotherapists to become data literate.

"In the muscle co-contraction, it may be invalid to infer, on the basis of clinical observation alone, that a particular muscle group is not producing force."

• I like this quote because it illustrates two main points. First, people have been promoting this idea of a muscle group "not firing" or being "weak" based on visual observations for a very long time. Second, researchers have pointed out this faulty logic in clinical reasoning for longer than I've even been alive! We need better knowledge translation to bridge this gap moving forward :)

🧠 Fun fact of the week

Do you know which planet rotates the fastest in the solar system? You'd be forgiven if you didn't guess that it was Jupiter, our largest planet! Jupiter only takes about 10 hours to complete a full rotation, which is incredibly impressive given its massive size.

🎙 Podcast recommendation

I know I've given you a lot of Peter Attia lately, but this is another one worth your time. Ryan Hall was a former Olympic-level marathon runner, holds the current half-marathon record in the USA, and transformed himself into a respectable strength athlete upon retiring from professional running. Listening to him speak about his experiences in both sports has been fascinating, and I highly recommend giving this a listen!

🗣 Quote of the week

"I can accept failure, everyone fails at something. But I can’t accept not trying."

- Michael Jordan