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Dr John Cronin: Velocity of Movement & Wearable Resistance: Mechanical Work & Kinetic Energy

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Lila Exogen Work Energy Relationship

Work-energy requirements for a 100 kg squat vs. 500 gm (0.5 kg) on each thigh. 


To really understand the effect of velocity of movement on WR, I am going to provide a rudimentary example of how squatting 100 kg requires similar additional muscular work at the hip as moving 500 gms on each leg during sprinting. 

Another first principles (established science and not assumptions) approach to discussing the overload provided by WR and the effects of velocity of movement is to look at the work-energy relationship. 

Most simply put, the amount of mechanical work performed by a muscle group is determined by the mechanical energy associated with the movement, or conversely, the energy determines the muscular work. 

In terms of the formula:

Mechanical Work = Kinetic Energy (KE) =1/2m.v2+ Potential Energy (PE) = m.g.h. As the net change in height for both squat and sprinting is zero, the PE need not be calculated.

Squat: So, let’s look at the squat. Let’s say this athlete’s 80% 1RM is 100 kg, the peak velocity associated with an 80% 1RM lift = 0.58 m/s (Zink et al., 2006). 

Note this is peak velocity and, theoretically, we should use an average velocity.

Squat KE: If you put the numbers into the equation (see Image above), you see we end up with around 17 kg.m.s of KE.

Sprinting:Now let’s do the math for 500 gms on each leg while sprinting. A well- trained sprinter’s hip extension angular velocity is ~1000 degrees per second (deg/s), whereas an untrained sprinter’s is ~400 deg/s. For this example, I took the middle ground and used a hip extension velocity of 700 deg/s, which I converted into a linear velocity = 6.1 m/s.

Sprinting KE: As you can see, the KE for moving the 1 kg load is slightly greater (18.6 kg.m.s) than the 100 kg load, so therefore the work performed by the hip musculature is slightly greater for the 1 kg loading.  

What is more influential in producing KE—and therefore muscular work—is velocity of movement and not mass.

This is because the effect of mass is halved, whereas velocity is squared. 

What are the implications and practical applications of this? Well, here are some key points to consider:

  1. Light loads (WR) moved fast result in substantial overload/muscular work.
  2. Such loading would seem ideal for sprinting, given the activity’s specific overload.
  3. Performing a movement with the same load at 50% vs. 90% of maximum velocity has very different KE and therefore muscular work requirements.
  4. Think about how you integrate WR into your sessions (e.g., you may well use WR in tempo runs that overload by % max velocity rather than changing mass, placement, and/or orientation).
  5. Scrutinize how you progressively overload before sprinting maximally with WR given what you know about KE now. 

However, remember this is only important depending on the mass you use, and the placement and orientation of the loading. 

If the load is light and placed close to the axis of rotation, then you can be less cautious. 

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