Power to Weight Ratio… The Missing Link!

Power (force x distance/time) and physical therapy would seem to be a natural relationship, and indeed it has been studied and related to movement for some time in physical therapy. (walking speed, stair climbing, and sit to stand).  As physical therapists we are experts at helping patients with mobility problems regain their strength and function. Certainly as a part of the Power equation, strength is the variable that we as therapists can change to increase force output.  Why then is the Power to Weight Ratio the missing link in physical therapy? It is critical for the tests that we use to predict fall risk, ‘Timed Up and Go’, and ‘5 times Sit to Stand’.

 

Most athletes and coaches understand this principle yet it is still often overlooked.  To understand this principle, we must understand that an athletes’ performance in pure athletics which involve running and jumping is dictated by power to weight ratio. World class sprinters compared to elite sprinters have been shown to have a greater stride length and is determined by power output.

 

The equation below can pertain to an athlete or to your patient who is trying to climb steps or become more proficient at transitioning from sit to stand. If Power output is 440 watts and we increase our patient’s force output so that their power output moves from 440 watts to 480 watts the Power to Weight ratio (P/W) increases as well. Even if there is not an increase in force output but a reduction in body weight by 20 lbs. the P/W increases as shown below:  

 

P/W = 440 watts / 190 lbs. = 2.31        P/W= 480 watts / 190 lbs. = 2.52

P/W = 440 watts/ 170 lbs. = 2.58         P/W= 480 watts / 170 lbs. = 2.82

 

As a part of improving your patient’s function and ability to perform, if they are clearly overweight or obese, just a 5 – 10% reduction in their weight can improve their ability to negotiate steps, transition from sit to stand and increase their gait cadence.

 

Plan on providing your patient with this valuable information and discuss the many benefits of just reducing their body weight by 5-10%, which include reduced blood glucose and blood pressure, and a reduction in knee pain.

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