Life Fitness Weekly Service Manual page 19

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 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.
Some factors which affect Lifepulse's ability to determine a heart rate:
Hand slip and / or This can produce noise spikes which drives up gain values and lowers the ability to detect the
grip pressure features of the actual heart pulses.
changes on
electrodes:
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