What is a “Kinetic Chain”?

[Glenn Fleisig]

As biomechanists, we try to optimize the movement of athletes, patients, and others. We may be trying to maximize performance, minimize the risk of injury, or simply returning someone to “normal.” A term often used in biomechanics is “kinetic chain.” But what is a kinetic chain? Considering that this is a fundamental concept of biomechanics, there is a surprising lack of clarity on its definition. In his seminal textbook, Arthur Steindler defined a kinetic chain vaguely as “a combination of several successively arranged joints constituting a complex motor unit.”¹ More recently, the kinetic chain has been explained as “an interaction of body segments”², a “transfer of forces and motion”,³ and a “sequential transfer of energy.”⁴

So, here is my question for discussion. What is the most meaningful way to quantify the kinetic chain from our biomechanical data? That is, as biomechanists, do we ideally want to quantify the kinetic chain as the magnitudes and sequential timing of:

1. Joint angles?
2. Joint angular velocities?
3. Joint torques?
4. Segmental energy?
5. Muscle firing patterns?
6. Something else?

I welcome your expertise. Please vote and/or (more importantly) post your thoughts.

[Ben Hansen]

Re: What is a “Kinetic Chain”?

My vote: [4] Segmental Energy or [3] Joint Torques

Conceptually, I've always referred to the kinetic chain as a way to pass energy from proximal segments to distal segments in a summation, resulting in the most distal segments moving the fastest. Proper sequencing of that energy transfer yields the greatest velocity.

Even though I wrote extensively about [3] Joint Torques being a much better alternative than [2] Joint Angular Velocities for cross-subject kinetic chain optimization (https://motusglobal.com/tau-omega-ba...-biomechanics/), [4] Segmental Energies is probably the way to go. My only problem with segmental energies, is that is a very difficult concept to "feel" as a person. Velocities are hard to feel too. We're probably the best at feeling muscles activate, but an isometric contraction may not accelerate a segment or generate force (so that's out in my vote).

Often times in biomechanics, we have the hands down "right" way to measure something, but it may not always be the most "effective" way to communicate back to the person we analyze. For traditional research purposes, I'd probably use segmental energies. For coaching an athlete on moving properly, I'd probably use Joint Torques (perhaps we can feel a force/torque generation better than energy).

But maybe we should also practice making our clinical research more translatable to real-world training. This is a tough one. I'm excited to see other's responses on the matter.

-Ben Hansen

As biomechanists, we try to optimize the movement of athletes, patients, and others. We may be trying to maximize performance, minimize the risk of injury, or simply returning someone to “normal.” A term often used in biomechanics is “kinetic chain.” But what is a kinetic chain? Considering that this is a fundamental concept of biomechanics, there is a surprising lack of clarity on its definition. In his seminal textbook, Arthur Steindler defined a kinetic chain vaguely as “a combination of several successively arranged joints constituting a complex motor unit.”¹ More recently, the kinetic chain has been explained as “an interaction of body segments”², a “transfer of forces and motion”,³ and a “sequential transfer of energy.”⁴

So, here is my question for discussion. What is the most meaningful way to quantify the kinetic chain from our biomechanical data? That is, as biomechanists, do we ideally want to quantify the kinetic chain as the magnitudes and sequential timing of:

1. Joint angles?
2. Joint angular velocities?
3. Joint torques?
4. Segmental energy?
5. Muscle firing patterns?
6. Something else?

I welcome your expertise. Please vote and/or (more importantly) post your thoughts.

[Paul DeVita]

Re: What is a “Kinetic Chain”?

6. Something else

[Glenn Fleisig]

Re: What is a “Kinetic Chain”?

What "something else"?

[Mike Reinold]

Re: What is a “Kinetic Chain”?

Coming from the rehab and performance perspective, I can consider the kinetic chain simply the interlinking of joints. How the kinetic chain functions, vs the definition is what is impactful to me. We usually consider this in terms of force transfer, which is impacted by the magnitude and timing of the force. For example in a sports environment such as the baseball pitch, it's not just about how hard the legs drive the force, it's about the sequencing and timing of the transfer of the force. A poor kinematic sequencing of the kinetic chain could result in a poor overall transfer of force from one area of the body to another.

[Karl Zelik]

Re: What is a “Kinetic Chain”?

Force, power, work, acceleration, etc are all fundamental concepts derived from classical mechanics. And they all have identifiable and agreed upon units of measure. Is the term "kinetic chain" actually a fundamental concept as you suggest? Is this a term that appears in mechanics/dynamics textbooks? I'm not sure I've heard it used outside of biomechanics. It seems like your poll is asking what units should be used to quantify a term that has no agreed upon definition. How does one go about assigning meaningful units to a term with no known meaning? Why not start from a concept (force, power, etc.) that is well-defined?

[Karl Zelik]

Re: What is a “Kinetic Chain”?

To slightly expound/clarify, the term "kinetic chain" seems to be used to describe a mechanical system (or model of a system) that is comprised of segments connected by joints. Such systems can be described in terms of kinematics, energy, forces, torques, some combination of these, etc. Asking which is the most meaningful will depend on the question being asked.

[Glenn Fleisig]

Re: What is a “Kinetic Chain”?

Karl,
You are correct. This discussion is about the kinetic chain in biomechanics. Let me give you a little more tangible example - many biomechanics lab analyze athletes of a particular sport and wish to help them optimize their biomechanics. Should the biomechanists quantify the sequential timing of the peak angles, peak angular velocities, peak joint torques, peak segment energy, initiation or peak muscle activity, or something else?

[Dave Fortenbaugh]

Re: What is a “Kinetic Chain”?

The true answer may be that there is more than one acceptable definition of the kinetic chain. While biomechanists (and others analyzing the data) may understand the concept of proximal segments affecting distal segments, it may be possible that this concept can be expressed in different units. "Kinetic" makes me think of forces and torques whereas "kinematic" makes me think of displacements (linear or angular). That makes me want to say joint or segment force / torque is the answer. Since body segments move almost exclusively in an angular fashion, joint torques are more relevant than joint forces. Therefore, I can see joint/segment torques being appropriate.

However, for this application, it's quite possible that other units of measure commonly used to analyze systems, namely momentum and energy, may be perfectly acceptable alternatives.

Muscle firing patterns are a very interesting way to analyze human movement as well. They are also capturing how the sequence of motions progresses, particularly from the timing perspective. I would like to hear from someone who has significantly more experience than me in using EMG equipment, but I would likely dissuade others from picking muscle firing patterns as the primary way of collecting biomechanical data on the kinetic chain simply based on how cumbersome data collection can be and how difficult it is to express magnitudes of muscle contractions.

Again, there may be multiple solutions to this "problem," especially when you are talking about different biomechanical movements and different audiences to whom you are presenting the data.

To really throw a monkey wrench in the works, is it possible to analyze the data not only in terms of discrete time points (max X1 at time Y1 for segment 1, max X2 at time Y2 for segment 2, etc.) but to somehow consider the data more functionally?

[Duane Knudson]

Re: What is a “Kinetic Chain”?

I agree with Paul and Karl. Kinematic Chains are a well-established area of research in mechanical engineering, but Steindler (a physician) adapted the terms to the rather vague 'kinetic chain.' The vague use of the term kinetic chain is only common in sports medicine, kinesiology, and sport coaching literature. It is not a well-defined term or area of study. This has lead to considerable confusion in sports medicine referring to exercises as 'open kinetic chain' or 'closed kinetic chain' exercises depending on whether the distal body segment interacts with considerable resistance (whatever that means). Several scholars have recommended not using this vague terminology (Blackard et al 1999; Dillman et al 1994).

The reality is that a biomechanical systems that essentially move as a linked segment model of rigid bodies will transfer mechanical effects between the segments. The huge issue for biomechanics is uniquely defining these effects and how they happen. Given biomechanics' utility informing many applied problems, the majority of biomechanics faculty seek external funding that often tries to answer these applied problems rather than theoretical advancements in in the field.

One problem has been that our inverse dynamics models mix muscle/bone/ligament forces together to get net/resultant forces and moments that are difficult to interpret. A linked segment biomechanical system (kinematic or kinetic chain if you will) does transfer forces through muscles but also through joint forces. Betty Roberts and many of her students were convinced, and published data, that joint forces in high speed movements were transferred from the proximal to the distal segment (see Phillips et al. 1983 J Biomech or Chapman & Sanderson 1990 Multiple Muscle Systems) (some call this transfer passive dynamics). Unfortunately, Newton's Third Law makes life interesting and difficult in knowing what force is an action and what force in the linked segment system represents a reaction. About this time Carly Putman came out with another parsing of inverse dynamics (like Roberts) that concluded the transfer of force between segments at joints in high-speed movements originates from distal segments pushing on proximal ones (Putnam 1991 Med Sci Sports Exerc; 1993 J Biomech). These intersegmental reaction forces in linked segment models took another jump forward in complexity when Zajac and Gordon (1989 Exec Sports Sci Rev) showed that muscle forces have 'induced acceleration' effect at ALL segments of a system of linked rigid bodies. Since then, they and others have published induced acceleration analyses of walking, running, and high-speed movements like throwing.

I think we need to stay with the vector quantities (forces/impulse and moments/angular impulses) to try and understand multi-segment dynamics. Scalar quantities (work/energy/power) do not provide unique answers to these problems, nor do ill-defined qualitative quasi-theories of 'kinetic chains.'

[Glenn Fleisig]

Re: What is a “Kinetic Chain”?

Thank you, Duane. Great scholarly review of this question.

[Sherry Werner]

Re: What is a “Kinetic Chain”?

I would define it as a force/torque/energy proximal to distal sequencing.

[Arnel Aguinaldo]

Re: What is a “Kinetic Chain”?

Thanks for bringing this up, Glenn!

I’m going with the majority on this one and vote for #6 “something else,” which could conceivably also mean “all of the above.” Kinetic chain is a loosely defined term used mainly to describe the proximal-to-distal sequence of segmental motion during such tasks as a soccer kick and throwing. But outside this context the term is somewhat ambiguous, and Duane made a good point about how it is used by clinicians to describe “open kinetic chain” and “closed kinetic chain” movements in which the distal end is free or fixed, respectively. In the context of throwing, the kinetic chain is commonly used to describe the mechanism by which the kinematic and kinetic patterns of sequential motion of body segments occur in a “whip-like” fashion. This inherently makes all the parameters listed in this poll relevant, which could range from a simply analysis of timing in which relevant peak magnitudes occur (Putnam, 1993) to a partitioned segmental energy analysis to understand how energy flows through the kinetic chain (Aguinaldo & Escamilla, 2018).

From an injury risk perspective, joint torques would most likely be the most relevant and of most interest from coaches and players. However, we often discuss or hear about the efficiency of pitching and a discussion of efficiency is incomplete without talking about the energy transfer mechanisms associated with multi-segment motion during pitching. With regards to the energetics of sequential motion during pitching, I find that there are three “tiers” of analyses that attempt to explain these mechanisms:

1) Segmental/joint energy analysis where the contributions of net torques and forces to the energy of adjacent segments are calculated (Aguinaldo & Escamilla, 2018; Roach & Lieberman, 2014; Shimada, Ae, Fujii, Kawamura, & Takahashi, 2004). This is a simple analysis of how energy is generated, absorbed, or transferred between segments based on the work-energy relationship but does not take into account the contributions of joint torques distant to the segments whose energy/power is being calculated.
2) State-space power analysis where the contributions of net torques and interactive (velocity-dependent) torques to the work and power of a segment are decomposed (Naito, Takagi, & Maruyama, 2011), which is really an “energetic” extension of an induced acceleration analysis (Hirashima, Yamane, Nakamura, & Ohtsuki, 2008; Kepple, 2011).
3) Forward dynamic analysis where individual soft tissue and osseous contributions to net joint torques and energies are estimated (Buffi, Werner, Kepple, & Murray, 2015).

Rafael and I completed (and submitted for publication) a few studies examining these energy transfer mechanisms wrt elbow valgus loading utilizing methods 1 and 2, which is the more appropriate analysis when dealing with the dynamic coupling effects of pitching. I presented some of this data at World Congress, ASB, and ISBS over the summer but I’m hoping to share more of our findings sooner than later! We are currently attempting analysis 3 using Matlab and OpenSim.

References

Aguinaldo, A. L., & Escamilla, R. F. (2018). Relationship of segmental energy flow and elbow valgus load during baseball pitching. In Proceedings of the American Society of Biomechanics Meeting (pp. 190–191). Rochester, MN.
Buffi, J., Werner, K., Kepple, T., & Murray, W. (2015). Computing muscle, ligament, and osseous contributions to the elbow varus moment during baseball pitching. Annals of Biomedical Engineering, 43(2), 404–415.
Hirashima, M., Yamane, K., Nakamura, Y., & Ohtsuki, T. (2008). Kinetic chain of overarm throwing in terms of joint rotations revealed by induced acceleration analysis. Journal of Biomechanics, 41(13), 2874–2883.
Kepple, T. (2011). Application of induced acceleration analysis and computer simulation in sports. Portuguese Journal of Sports Sciences, 11(S3), 19–22.
Naito, K., Takagi, H., & Maruyama, T. (2011). Mechanical work, efficiency and energy redistribution mechanisms in baseball pitching. Sports Technology, 4(1–2), 37–41.
Putnam, C. A. (1993). Sequential motions of body segments in striking and throwing skills: Descriptions and explanations. Journal of Biomechanics, 26(S1), 125–135.
Roach, N. T., & Lieberman, D. E. (2014). Upper body contributions to power generation during rapid, overhand throwing in humans. Journal of Experimental Biology, 217(12).
Shimada, K., Ae, M., Fujii, N., Kawamura, T., & Takahashi, K. (2004). The mechanical energy flows between body segments during baseball pitching. Japanese Journal of Biomechanics in Sports & Exercise, 8(c), 12–26.

[Ross Miller]

Re: What is a “Kinetic Chain”?

Great question/topic/responses.

I find this topic is easiest to think about for movements that have a clear goal, e.g. throwing a ball for maximum velocity or jumping for maximum height (which is really just maximizing take-off velocity). There I'm using the linked nature of the chain to accelerate each successive segment to a faster velocity so that the "endpoint" velocity is faster than what I'd be able to achieve by an isolated joint motion. So I picked "Angular Velocity" in the poll. Where the extra speed comes from is more complicated and probably has lots of plausible answers depending on perspective and the level of inquiry.

I guess I think of "kinetic chains" as an extension of a "kinematic chain" design problem to a dynamics/control problem, i.e. how should I control the actuators of the chain to achieve a particular goal?

Ross

[Glenn Fleisig]

Re: What is a “Kinetic Chain”?

Thank you all who weighed in. In closing, I want to share that while watching the 2018 World Series a few days ago, commentator and Hall-of-Fame pitcher John Smoltz was talking about the effectiveness of the pitcher (Rich Hill) in varying his arm slot while still utilizing a "proper kinetic chain." They showed slow-motion, high-speed video of Hill while Smoltz [correctly] described the kinetic chain from the legs to trunk to arm. This exemplified what this discussion found, namely that the kinetic chain is a popular concept in sports of coordination through sequential body parts, but a consensus of specifics are lacking in biomechanics.