Blog By Dr John Cronin
More mechanical and muscular work at the hip moving 500 gm/17 oz on your thigh during sprinting compared to squatting 100 kg/220 lbs. You are having me on? No! Read on if you want to understand why wearable resistance (WR) is the real deal when talking resistance training.
To really understand the effect of velocity of movement on mechanical and muscular work you will see a rudimentary example of how squatting 100 kg/220 lbs requires similar additional muscular work at the hip as moving 500 gms/~17 oz on each leg during sprinting. Warning, if you are not into biomechanics, push fast forward and skip this next section and get to the take home messaging of the following section. However, if you’re wanting a deeper understanding of why WR works, then read on.
The work-energy relationship can help us understand and compare movements based on a first principles physics approach. This relationship, most simply put states the amount of mechanical and muscular 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 this relationship, let’s compare muscular work at the hip when squatting 100 kg/ 220 lbs to moving 500 gms/~17 oz on each leg during sprinting.
The workings are outlined below and the math you can see in Figure 1.
- 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 athletes 80% 1RM is 100 kg/220 lb, the peak velocity associated with a 80% 1RM lift = 0.58 m/s (Zink et al., 2006). Note this is a peak velocity and theoretically we should be using an average velocity.
- Squat KE: If you put the numbers into the equation you see in Figure 1 we end up with around 17.4 kg.m.s of KE.
- Sprinting: Now let’s do the math for 500 gms (17 oz) on each leg whilst sprinting. Well trained sprinter’s hip extension angular velocity is ~1000 degrees per second (deg/s) whereas an untrained is ~400 deg/s. For this example, I took the middle ground and used a hip extension velocity of 700 deg/s which converted to a linear velocity of ~ 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.
Figure 1: Kinetic energy calculations for the squat and sprint
How can this be so? Well let’s have a close look at the formula: KE = 1/2m.v2
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.
Note for the sake of simplicity I have given linear examples, however, to really understand this topic we should be looking at angular kinematics and kinetics.
Take Home Messages
So, what are some of your take homes? Well here are some key points to consider:
- Light loads (WR) moved fast, result in substantial overload/muscular work.
- Such loading would seem ideal for those interested in improving velocity of movement given the activity’s specific overload WR provides.
- Performing a movement with the same load at 50% vs 90% of maximum velocity has very different KE and therefore muscular work requirements.
- 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.
- Plan on how you progressively overload before sprinting maximally with WR, given what you now know about KE. However, remember this is only important depending on the masses you are using, 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. Refer to the previous article I wrote on rotational inertia.