Analysis of Accelerometer Signals and Weightlifting Performance

This report provides a comprehensive analysis of accelerometer signal processing, exploring its applications, particularly in weightlifting performance measurement. It begins with an overview of accelerometers, detailing their principles of operation, including different types such as piezoresistive and piezoelectric accelerometers, and their use in detecting vibrations and orientation. The report highlights the advantages of MEMS accelerometers, discussing their construction and applications in various fields, including mobile cameras and airbag sensors. The study emphasizes the correlation between accelerometer data and weightlifting techniques, citing previous research that validates the reliability of 3-axis accelerometers. Finally, it discusses the limitations of traditional methods for measuring weightlifting performance and highlights the advantages of using accelerometers for real-time training analysis and data collection, concluding that accelerometers offer reliable and accurate data in weightlifting analysis.

Running head: PROCESSING OF ACCELEROMETER SIGNALS
PROCESSING OF ACCELEROMETER SIGNALS
[Author Name(s), First M. Last, Omit Titles and Degrees]
[Institutional Affiliation(s)]

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Abstract
Weightlifting technique has remained a highly studied subject as far as standard biomedical
analysis is concerned including barbell velocity and barbell trajectory. There are numerous
techniques and/or devices that are used in measuring weightlifting performance in sports. All
these devices and/or techniques have each of the challenges or shortcomings that lead to
significant inefficiency in their use. Accelerometers sensors have been found to be an alternative
device for the measurement of the weightlifting performance in sports and can be used to
overcome the challenges recorded by the preceding devices or techniques. The purpose of this
study is to confirm the validity and reliability of the measures taken by commercial
accelerometer sensors by comparing how the accelerometer works to other measuring devices or
techniques. The hypothesis of the study is that there will be a high correlation between the data
taken by accelerometer and measurements of acceleration from other devices. The values of
accelerometer measurements and those of the contemporary technique were found to be highly
correlated and thus a conclusion can be arrived at on the basis of these results that accelerometer
sensors provide reliable and accurate data. The device can thus be used in estimating the
acceleration during real-time training sessions as opposed to only being used as a tool for data
collection in the laboratory setting.

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Accelerometer Sensors
An accelerometer is a device that is used in taking the measurements of acceleration.
Acceleration refers to the rate of change of velocity with time and is in most cases measured in
meters per squared seconds or in G-forces (Baron, 2012). Accelerometers are important in the
detection of vibrations that occur in systems. They are also useful in the orientation of
applications. Accelerometers are devices, which are electromagnetic in nature and work by
detecting either dynamic or static forces of acceleration. While dynamic forces include such
forces as movement and vibrations, static forces include gravity. Accelerometers can take
measurements of acceleration on one, two or three axes. The 3-axis units of measuring
acceleration are gaining popularity due to their cost-effective nature.
Sensors work on a principle of a mechanical sensing element, which is composed of a
proof mass that is linked to a mechanical suspension in relation to a reference frame. The proof
mass deflects due to gravity or acceleration resulting from the force of inertia. This is according
to Newton’s Second Law. The measurement of the acceleration is done electrically as per the
physical changes in the displacement of the proof mass in relation to the reference frame.
Piezoresistive and piezoelectric are the most commonly used types of accelerometers (Emilio,
2013).
In piezoelectric accelerometers, the applied acceleration causes the sensing element to
bend thereby resulting in a displacement in the proof mass and an output voltage proportional to
the acceleration applied is produced. These accelerometers do not respond to constant
acceleration components (Amadi-Echendu, 2010). Piezoelectric materials are able to change
part of the energy that comes with the internal mechanical strain into electrical energy that can be

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recovered, and the reverse is true. This property of photoelectric materials is said to be electro-
static coupling. Through photoelectric effect, photoelectric transducers change mechanical
energy into electric signals, which is normally proportional to the mechanical strain of the
piezoelectric material. The signals generated are proportional to the shock event or the system
vibration. The piezoceramic is the active element in the piezoelectric accelerometer and has one
of its side connected to the body of the accelerometer in a rigid manner while the other side has a
mass added. A force is created and acts upon the piezoelectric element when the accelerometer is
subjected to vibrations (Ceceri, 2012). A charge output is thus created that is of equivalence to
the force exerted by the vibration.
In piezoresistive accelerometers, the sensing element is composed of a proof mass and a
cantilever beam, which is formed through bulk micromachining. The piezoresistors in proof
mass and cantilever are used in the detection of the motion that occurs in the proof mass due to
acceleration. These resistors are responsive to DC and can be used in measuring constant
acceleration for example gravity (Ceceri, 2012). Lower output levels and temperature sensitive
drift in piezoresistors are the main drawbacks of this type of accelerometer sensors.
Piezoresistive accelerometers are designed for use in making measurements of high
shock and high frequency. Contrary to piezoelectric accelerometers, piezoresistive
accelerometers take advantage of the changes in the resistance of the piezoelectric material and
convert the resistance to a mechanical strain to a DC output. Most of the designs of
piezoresistive accelerometers are found to be either the type of a bonded strain gauge (fluid
damped) or MEMS type, which are gas damped. These accelerometers are most suitable and
accurate for taking measurements when the g level and the frequency ranges are relatively high.
Piezoresistive accelerometers find their applications mostly in the automobile safety testing that

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is composed of traction control system, anti-lock braking system as well as safety-air bags. Still,
the accelerometers find their application in weapon testing and seismic measurements
(Deodatis, 2014). Micromachined accelerometers are used in biomedical applications for
taking measurements of extremely small dimensions as in the case of submillimeter
piezoresistive accelerometers.
Accelerators have internal capacitive plates some of which are fixed in positions while
others are attached to the miniscule springs that experience motion internally when acceleration
forces act upon the sensors. The capacitance between the plates changes as they move in relation
to each other and the changes in the capacitance is used in the determination of the acceleration
of the moving plates (Baron, 2012). Other accelerometers are centered on piezoelectric
materials. Piezoelectric materials are tiny structures, which are able to output electric charge
should they be subjected to mechanical stress such as acceleration. Low power consumption,
swift response to the motion and low power consumption are among the advantages that come
with differential capacitive accelerometers. As a result of the low levels of noise of capacitive
detection, the better sensitivity of the accelerometers is also achieved.
MEMS Accelerometers
MEMS is an abbreviation for micro electro mechanical systems and is applicable to any
sensors that have been manufactured using the techniques of microelectronic fabrication. In
conjunction with microelectronic circuits, MEMS sensors are used in taking measurements of
physical parameters including acceleration. The capacitive type is one of the most commonly
used types of MEMS accelerometer due to its unique properties including high accuracy at high
temperatures and high sensitivity (Dorf, 2010). These device does not alter values based on the

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nature of the base materials used but instead only depends on the capacitive value which exists
due to the change in the distance between the plates.
MEMS Accelerometer
Shown in the diagram above is a typical MEMS accelerometer which can also be referred
to as a simple one-axis accelerometer. 2 or 3-axis accelerometer can be made by having more
capacitor sets kept at 90 degrees to each other. A MEMS transducer is made up of a
microstructure that is movable or a proof mass that is linked to a mechanical suspension system
and hence on to a reference frame (Bentsman, 2016). The mobile plates and the fixed outer
plates serve as the capacitor plates such that should acceleration be applied, the proof mass
would move accordingly thereby generating a capacitance between the fixed outer plates and the
movable plates.
A displacement is experienced as X1 and X2 between the two plates upon the application of
acceleration which turns out to be a function of the generated capacitance. The circuit used in the

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calculation of the acceleration is as shown below. The acceleration is a derived by finding the
change of the distance between the capacitor plates.
Applications of MEMS Accelerometers
 Used as a tit sensor in mobile cameras
 Provision of stability of images in camcorders
 Used in airbag sensors in a car crash
 Military monitoring, projectiles, missile launching and other real-time applications
(Frangopol, 2013)
 Protection of hard disk drives in laptops
Accelerometers are sensors used in taking the measurements of the accelerations of moving
objects along a reference axis. The measurement of physical activity is using accelerometer
signals is preferred since acceleration is directly proportional to the external force producing it
and hence can be used in the reflection of the frequency and intensity of human movement. From
the accelerometer data gathered, information on the displacement and velocity can be derived
through integrating the data with respect to time (Gamble, 2011). Some accelerometers can
respond to gravity and thereby provide tilt sensing in relation to the reference planes during the
rotation of the accelerometers with objects. The resultant inclination data from the tilt can be

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used in the classification of the orientations of the body. As such, accelerometry and
accelerometer signals can be used in the generation of enough information that can be used in the
measurement of physical activity as well as a range of human activities (Jones, 2014).
Measurements based on sensors have been used to measure human activities and have
provided for the assessment of physical activity. The use of accelerometry techniques allows
continuous, automatic and long-term measurement of activities of subjects that interact with the
free-living environment. Accelerometer signals allow for basic steps in counting and intensity,
which are usable in the estimation of the energy spent during physical activity. This method is
widely used in the management of weight and diet (Hoffman, 2014).
The relationship between 3-axis accelerometer and weightlifting techniques
One of the previous works that have a significant contribution to the success of this
project was a study that was done to establish the validity and reliability of 3-axis accelerometer
for measuring weightlifting movements. In this study, the comparison was made between 3-axis
accelerometer and kinematic data derived from 3D videography (Knudson, 2013). The data
used was obtained from 11 track and field throwers who did three trials with each of the trials
having different loads in the power clean, jerk and power snatch. The study conducted the
validity and reliability tests separately in which for the case of validity test, ANOVA analysis,
the coefficient of variation of the method error Pearson product-moment correlation and method
error were used in each of the phases of the study.
All the methods demonstrated a good correlation between the criterion measures and
accelerometers since the analysis of the variance revealed insignificant difference (p>0.05). The
findings from the study established a strong correlation between the 3-axis accelerometer

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measures and those derived from 3D videography data. Using these findings, it could be deduced
that 3-axis accelerometers are very reliable and valid for the measurement on the z-axis on
weightlifting movements. 3-axis accelerometer is thus a useful hand tool that is easy to use in the
measurement of the acceleration for weightlifting performance and training sessions (Lee,
2011).
Ideas on intelligent weight training emerged as early as 1984 when Aerial presented ideas
on various weight training machines. These machines operated on principles that mainly
revolved around force, displacement as well as the time of movement for the cases of a
framework that is controlled by a feedback. These ideas and technologies presented a variety
from which the most suitable method of exercise was selected. There are very many instruments
that are used in taking measurements of weightlifting performance (Larence, 2011). Among
these instruments include 2D/3D motion capture, video analysis, potentiometer/ encodes, the V-
scope TM among other instruments. Despite the achievements that these devices have managed
to offer in the field of measuring weightlifting performances, most of these devices if not all
come with shortcomings that make their efficiency as instruments for measuring weightlifting
performance greatly reduced.
Cost, extensive engineering expertise, extensive scientific expertise as well as a
considerable delay in the time between collection and return of data are among the shortcomings
of most of these devices. This makes it important for further study and research to be done so as
to establish devices that can overcome these among other challenges. By overcoming these
challenges, the efficiency of the instruments for measuring weightlifting performance is
increased (Lu, 2011). This literature review is thus important as does an in-depth analysis of the
various many instruments that are used in taking measurements of weightlifting performance,

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their advantages, and shortcomings. The literature is then used as a basis for establishing a
research gap on what can be done in order to improve the efficiency of weightlifting
performance.
Following the findings of the study, it will be established to whether advancements can
be done on the existing instruments or otherwise a total overhaul would be deemed fit. By total
overhaul, it means coming up with completely new devices which do not bear the shortcomings
of the existing devices. The existing devices are extensively discussed below.
Potentiometer
A potentiometer is a resistor with three terminals having a sliding or rotating contact that
forms a voltage divider that can be adjusted. The potentiometer acts as a rheostat or a variable
resistor in case only two of the three terminals are put in use. In essence, a potentiometer is a
voltage divider that is used in taking a measurement of electric potential i.e. voltages and are
commonly used in control devices among them those that control volumes on audio equipment.
The output voltage of the potentiometer is determined by the position of the wiper (Margolis,
2011).
The potentiometer can be treated as two distinct resistors that have been connected in a
series in which the position of the wiper is the determinant of the ratio of the resistance of the
first resistor to the second resistor. The rotary potentiometer is the most commonly used form of
the potentiometer. Thus potentiometer is in most cases applied in the control of audio volume
besides numerous other applications (Miller, 2013). Various materials are used in the
construction of potentiometers among them cermet, metal film, conductive plastic, wire wound
and carbon compositions.

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Measurement of the unknown voltage in the potentiometer is done by determining a
position in the sliding contact at which the reading of the galvanometer is zero. By indicating a
zero reading on the galvanometer it means no current flows through the path hence there is a
potential drip in the sliding contact. The voltage drop is determined using the expression
E1=working current*resistance of the segment of the wire
Determining this requires the determination of the operating current of the potentiometer
as well as the resistance of the segment of the wire used when the reading of the galvanometer is
zero. It is assumed that the resistance of the wire is uniform since the wire has a uniform cross-
sectional area (McComb, 2011). Assuming that the resistance of the whole length of the wire is
R, then the resistance of the chosen segment will be determined by R*length of segment/total
length of the wire
From this calculation, it is possible to adjust the working current using the variable
rheostat that is chosen by the process of standardization. During the standardization process, a
cell of a standard voltage of 1.0186 V is picked and the witch connected to calibrate. The sliding
contact is positioned at 101.86 cm from the starting end of the wire. Adjustments are made to the
rheostat Rh so as to achieve a zero galvanization at the point G. At S, the voltage across the
101.86 cm of wire remains 1.0186 V (Baron, 2012). Assuming that the total length of the wire
is 200 cm and that the resistance of the wire is 200 ohm, then the resistance of the 101.86 cm
length of the ire segment will be 101.86 ohms. The working current is thus determined from the
equation I=V/R=1.0186/101.86=0.01A as the working current.

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Potentiometers are classified using various categories and properties. One of such ways
of classifying potentiometers is based on whether they are adjustable or preset. Preset
potentiometers are mostly used when there is need to set a value within a circuit especially
within the production set-up and test stage when it is being manufactured. Some of the presents
are made up of a single turn adjustment hence can be coursed in cases where the accurate setting
is needed (Murthy, 2011). Adjustable potentiometers, on the other hand, have a spindle and can
be used with the help of a knob. This type of potentiometer is in most cases used for applications
such as control of tones and volumes on radios. They can also be used in cases where there is
need to set a value by the user.
The relationship between the potentiometer and weightlifting techniques
A potentiometer was used in assessing weightlifting performance in a study that involved
selected characteristics of kinematics during a bench press in which disabled powerlifting
athletes were involved. In the study, the potentiometer was used in taking records of the time of