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How the ERGYS Works:
An Automobile Analogy
| When discussing the technical details of the
ERGYS with riders and clinicians, our favorite analogy at Therapeutic Alliances is one of
a six-cylinder automobile with the cruise control set at 50 miles per hour. In the case of
the ERGYS, the analog of miles per hour is revolutions per minute (RPM) of pedaling, and
the 6 cylinders of the car can be thought of as the quadriceps, hamstrings, and gluteal
muscles of the rider’s legs. It follows in our analogy that the ERGYS "spark
plugs" are the electrodes placed over the motor nerves of the muscles. In an
automobile, the source of power is the combustion of a mixture of air and gasoline. The
ERGYS equivalent of combustion is the electrochemical reaction that takes place when the
muscle nerve is electrically stimulated via a surface electrode.
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The ERGYS sequences its electrical stimulus
much like an automobile fires its spark plugs. The automobile sequence is timed so that
the linear travel of the pistons in the cylinders is smoothly converted to the circular
motion of the drive train shaft, and, ultimately, to the rotation of the tires. Each
piston is restrained by a cylinder wall so that the movement of the piston is restricted
to a direction that will contribute to the turning of the crank shaft. With the ERGYS, the
Leg Restraints can be thought of as the cylinder walls. The Leg Restraints help to ensure
that the force generated by the muscle contractions is transferred to the pedal crank and
not lost through a "wobbling" of the legs at the hip and knee joints. As with
any piston-cylinder combination, the truer the motion, the more efficient the transfer of
power. |
Our bodies tend to naturally convert muscle contractions
into the circular motion necessary to accomplish tasks such as pedaling a bicycle. The
ERGYS computer sequences the contractions of the quadriceps, hamstrings, and gluteal
muscles in an order that allows for the hip, knee, and ankle joints to combine to create
the repetitive circular motion of pedaling.
Sensors in the ERGYS perform a variety of measurements that govern a session. One
sensor is used to track the angular position of the pedal crank, and hence the position of
the legs. This position is used by the ERGYS computer to select the muscle to be
stimulated. The intensity of the stimulus is dictated by the pedaling rate, which the
ERGYS computes by measuring the change in pedal position over a fixed unit of time. Rates
below 50 RPM will call for the ERGYS to increase the stimulus level. Conversely, rates
above 50 RPM will cause the ERGYS to lower the stimulus level. In this way, the
"closed-loop" control of the ERGYS is similar to the cruise control of a car.
The ERGYS will continuously raise and lower the stimulus intensity to keep the rider at 50
RPM, much like a cruise control will meter the flow of gasoline to maintain a preset
speed. |
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