I hope that this may be of value to those who teach BM labs. I offered this demonstration and exercise to a group of lab students a few years ago. Using the wooden model (see attached) I introduced them to the Spring Mass model, the phases of the gait cycle and lead them through an activity to estimate their own leg-stiffness. Perhaps you may find it of value.

Following the demonstration with the model, students were instructed to run very, very slowly in the hall or outside while they counted and timed10 to 20 steps. . (Make certain that they “run” with a tiny aerial phase and do not “walk”.)

The slow run allowed them to find their Step Frequency and also introduced them to the units of Step Frequency; Hertz (Hz). (During the actual measurement they were reminded to count their steps by starting their stop watches and step count with the count of “zero” and not “one”.)

Most students found their Step Frequency to be between 2Hz and 3 Hz. From their Step Frequency (Hz) and their Body Mass (Kg) they were directed to a simplified version of the mass-bouncing-on-a-spring equation, so they could calculate their leg-stiffness.

f = (1/(2*PI))*SQRT(k/m) (1)

In biomechanical variables the equation becomes,

Step Frequency = (1/(6.28))*SQRT(Leg Stiffness/Body Mass)

Simplifying the equation for Leg Stiffness roughly yields,

Leg Stiffness = 40*Body Mass*Step Frequency^2

(You may need to help them with the squaring the Step Frequency (Hz) on their calculators.)

They should expect a Leg Stiffness roughly between 5,000 N/m and 18,000 N/m.

This lab exercise may help to introduce students to the ideas of modeling and hands-on measurements in the lab.

Note: If any students in your class are runners, you might suggest that they calculate their low speed cadence by multiplying the Step Frequency by 60. (It increases with speed.)

SMM_Model_JPG.jpg