Rate - Life Fitness CLUB Series Service Manual

Handlebars and Heart Rate
™
The Lifepulse contact heart rate system is designed to sense the electrical signals originating from the heart as it beats
and, using software, compute and display the corresponding heart rate value on the console readout. The two pairs of
electrodes on the Cross-Trainer Handlebars / Moving Handles (left and right) are the point of contact at which these
signals are picked up and transmitted from the body to the LifePulse circuitry. A hands-on detect circuit monitors for a
change in voltage corresponding to the input for each of the left and right sensor pairs to determine if either or both are
being held. Both sensors must be held in order for the electrical signal from the heart to be measured. This is similar to
measuring the voltage of a battery with a voltmeter. In order to measure a battery the voltmeter must be connected
across the battery terminals (one lead to the positive and one lead to the negative). Similarly, the heart can be thought
of as a voltage source or battery and in order to measure it's voltage you must measure across its terminals. Essentially,
when the left and right sensors are held they act like the leads of the voltmeter. Since an electrical path exists from the
electrodes held in one hand, up the arm, across the body (and the heart), down the other arm and into the other hand
holding the other pair of electrodes, a voltage can be measured. This is how the LifePulse signal is measured. And just
like measuring a battery, the polarity of the voltage is important so the left and right electrodes must correspond to the
left and right user hands.
To pick up the heart signal LifePulse must use a very sensitive high gain differential amplifier. This is primarily due to the
signal's initial low amplitude as it originates from the heart (typically less than 2 millivolts peak to peak) and the resultant
attenuation, or signal drop, as it travels through the body and into the electrodes. This type of amplifier is different from
a regular amplifier in that only the differences between the inputs are amplified. In this way common signals, typically
electrical noise, appearing on both inputs can be simultaneously ignored while the differences are amplified.
The output of this high gain differential amplifier, when neither or only one electrode pair is held, is meaningless because
the inputs to the amplifier basically act as an antenna picking up and amplifying stray electrical signals from the environment.
Ideally, once both electrode pairs are held, as detected by the hands-on circuitry, the heart signal can be isolated,
amplified and presented to the software for analysis. In practice however, additional unwanted signals exist. Some of
these signals come from other muscles which lay along the "voltage" path to the heart being measured (such as arm and
chest muscles). Similarly, hand to electrode contact problems which tend to weaken the signal or even introduce new
signals which hide the actual heart signal can occur. The LifePulse software attempts to isolate just the heart signal from
all other unwanted signals and noise using complex software techniques.
Basically, the LifePulse software samples the amplified signal picked up at the electrodes many times a second looking
for the heart pulses. Depending upon the amount of noise, size of signal and/or irregularity of the heart pulse, it may
take many seconds (from 4 to 20 or more) to confidently determine a value. And if a value cannot be confidently
determined, a heart rate will not be displayed. To increase the detection of a heart signal, the LifePulse software attempts
to expand the sampled signal to maximize the important features. A Gain Value which represents the relative amount
the signal was expanded is displayed in the LifePulse diagnostic screen and can range from 1 to 99. An assessment of
the strength of the signal can be directly related to this gain value.
Input signals already at maximum levels require a low gain because their features cannot be further expanded without
losing information. Very weak signals require more gain thus expanding them to full scale so their features can be easily
picked out. This scaling is done dynamically over consecutive blocks of samples with each gain computed relative to the
highest signal component within that block. The strength of the signal determines the effectiveness of the scaling. In
general, weak signals are less desirable than strong ones and very strong signals are less desirable than weaker ones.
Obviously, weak signals requiring high gain values means that the weak heart pulses will be competing with other low
level background noise when both are scaled up making it hard to determine the heart signal from the background
signals. On the other hand, strong signals, usually not from the heart pulse itself, will ultimately limit the amount of scaling
that can be applied. Typical gain values for low noise signals which produce good LifePulse heart rates are generally
between 10 and 30. Above 30 means the heart signal is weaker and below 10 means other noise signals are too strong
thus overpowering the heart pulses. Just as the gain value indicates the strength of the heart signal the Confidence
Number indicates the quality of the heart rate reading when one is displayed.
The LifePulse software uses many methods to analyze the heart signal and zero in on the heart rate reading. The Confidence
number which is also displayed in the LifePulse diagnostic display indicates the agreement in the computed heart rate
number among these different methods and therefore the confidence the heart rate displayed is correct. This is important
when Cardio workouts, which change the load based on the difference between the current and target heart rates, are
used. If a confident heart rate cannot be determined Cardio workout programs cannot automatically adjust the load to
reach the target heart rate. Confidence values can range from 0 to 9 with 0 being the least confident reading and 9 being
the most confident. Cardio program load changes occur when heart rate readings have confidence values of 5 or higher.
Factors that may affect the LifePulse software's ability to determine a heart rate include:
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