\section{Methodology}
\label{sec:methodology}
\subsection{Research Approach}
Because of the controversial findings about the impact of key actuation forces
on speed \cite{akagi_keyswitch, loricchio_force_speed} and the fact, that
keyboard related work can increase the risk for \gls{WRUED} \cite{ccfohas_wrued,
  pascarelli_wrued}, we decided to further investigate possible effects of
different actuation forces and even a keyboard equipped with non-uniform
actuation forces on speed, error rate and satisfaction. To our best knowledge,
to this date, there is no published work about the influence of a keyboard with
non-uniform actuation forces on these metrics. Therefore, we first asked
seventeen people about their preferences, experiences and habits related to
keyboards to get a better understanding on what people might prefer as a
baseline for the design of the adjusted keyboard (keyboard with non-uniform
actuation forces) and to complement the findings obtained through our literature
review. Further, we collected information about available mechanical keyswitches
on the market. Additionally, we conducted a small preliminary experiment with 6
subjects, where we measured the peak forces each individual finger of the right
hand was able to apply to distinct keys in different locations. We then created
the design for the adjusted keyboard based on those measurements. Lastly, an
experiment with twenty-four participants was conducted, where we compared the
performance and user satisfaction while using four different keyboards,
including our adjusted keyboard, to values obtained with the participant's own
keyboards.

\subsection{Preliminary Telephone Interview}
\label{sec:telephone_interview}
Some of the studies we found that researched implications of actuation force on
speed, preference or other metrics were published between 1984 and 2010. That is
why we wanted to ascertain if and how, with the advance of technology in recent
years and especially the capabilities modern smartphones offer, keyboard usage
has changed. Further, we wanted to gather information about the preference of
key resistance, keyswitch type and experiences with \gls{WRUED}. Therefore, we
conducted a structured interview with seventeen volunteers (59\% females) via
telephone. The age of the subjects ranged between 22 and 52 with a mean age of
29 years. The professions of subjects were distributed among medical workers,
students, office employees, computer engineers and community workers. The first
question we asked was \textit{``Which keyboard in terms of actuation force would
  be the most satisfying for you to use in the long run?''}. Thirteen (76\%) out
of the seventeen subjects mentioned, that they would prefer a keyboard with
light actuation force over a keyboard with higher resistance. The next question
\textit{``Have you ever had pain when using a keyboard and if so, where did you
  have pain?''} yielded, that 41\% of those polled experienced pain at least
once while using a keyboard. The areas affected described by the seven who
already experienced pain were the wrist \underline{and} forearm (3 out of 7),
wrist only (2 out of 7), fingers (1 out of 7) and forearm only (1 out of 7). The
results for the third question \textit{``Which keyboard are you currently using
  and for how many hours a day on average?''} were in line with the statements
we found during our literature review \cite{ergopedia_keyswitch,
  peery_3d_keyswitch}. Nine answered that they use a notebook (scissor-switches,
membrane), six stated that they use an external keyboard with rubber dome
switches and only two responded that they use a keyboard featuring mechanical
keyswitches. The average, self-reported, usage ranged between half an hour and
10 hours with a mean of 4.71 hours. It is important to note, that a study by
Mikkelsen et al. found, that self-reported durations related to computer work
can be inaccurate \cite{mikkelsen_duration}. The last question \textit{``Which
  tasks do you still prefer to perform with a keyboard rather than your mobile
  phone?''} revealed, that all of the subjects preferred to use a keyboard when
entering greater amounts of data (emails, applications, presentations,
calculations, research), but also surprisingly 41\% preferred to use a keyboard
to write instant messages (chatting via Whatsapp
Web\footnote{\url{https://web.whatsapp.com/}}, Signal
Desktop\footnote{\url{https://signal.org/download/}}, Telegram
Desktop\footnote{\url{https://desktop.telegram.org/}}).

\subsection{Market Analysis of Available Mechanical Keyswitches}
\label{sec:market_forces}
To gather information about available actuation forces, we collected the product
lines of keyswitches for all well known manufacturers, namely
Cherry\footnote{\url{https://www.cherrymx.de/en/mx-original/mx-red.html}},
Kailh\footnote{\url{https://www.kailhswitch.com/mechanical-keyboard-switches/}},
Gateron\footnote{\url{http://www.gateron.com/col/58459?lang=en}},
Matias\footnote{\url{http://matias.ca/switches/}},
Razer\footnote{\url{https://www.razer.com/razer-mechanical-switches}} and
Logitech\footnote{\url{https://www.logitechg.com/en-us/innovation/mechanical-switches.html}}. Since
some of the key actuation forces listed on the manufacturers or resellers
websites were given in cN and most of them in g or gf, the values were adjusted
to gram to reflect a trend that is within a margin of ± 2 g of accuracy. The
results shown in Figure \ref{fig:keyswitches_brands} are used to determine the
minimum, maximum and most common actuation force for broadly available
keyswitches. According to our findings, the lowest commercially available
actuation force is 35 g ($\approx$ 0.34 \gls{N}) the most common one is 50 g
($\approx$ 0.49 \gls{N}) and the highest resistance available is 80 g ($\approx$
0.78 \gls{N}).

\begin{figure}[H]
  \centering
  \includegraphics[width=0.9\textwidth]{images/keyswitches_brands}
  \caption{Available actuation forces for keyswitches of major keyswitch manufacturers}
  \label{fig:keyswitches_brands}
\end{figure}

\subsection{Preliminary Study of Finger Strength}
To evaluate the impact of an adjusted keyboard (keyboard with non-uniform
actuation forces) on performance and satisfaction we first needed to get an
understanding on how to distribute keyswitches with different actuation forces
across a keyboard. Our first idea was to use a similar approach to the keyboard
we described in Section \ref{sec:lr_sum}, were the force required to activate
the keys decreased towards the left and right ends of the keyboard. This rather
simple approach only accounts for the differences in finger strength when all
fingers are in the same position, but omits possible differences in applicable
force depending on the position a finger has to enter to press a certain key.
To detect possible differences in peak force depending on the position of the
fingers, we conducted an experiment with six volunteers (50\%
females). Subject's ages ranged from 20 to 26 with a mean age of 24 years. The
subjects were all personal contacts. Subjects professions were distributed as
follows: computer science students (3/6), physiotherapist (1/6), user experience
consultant (1/6) and retail (1/6). All Participants were given instructions to
exert maximum force for approximately one second onto the key mounted to the
measuring device described in Section \ref{sec:force_meas_dev}. We also used a
timer to announced when to press and when to stop. We provided a keyboard to
every participant, which was used as a reference for the finger position before
every measurement. To reduce order effects, we used a balanced latin square to
specify the sequence of rows (top, home, bottom) in which the participants had
to press the keys \cite{bradley_latin_square}. Additionally, because there were
only six people available, we alternated the direction from which participants
had to start in such a way, that every second subject started with the little
finger instead of the index finger. An example of four different positions of
the finger while performing the measurements for the keys \textit{Shift, L, I}
and \textit{Z} can be observed in Figure \ref{fig:FM_example}.

\begin{figure}[H]
  \centering
  \includegraphics[width=1.0\textwidth]{images/FM_example}
  \caption{Prototype of the force measuring device used to gather data about the
    maximum applicable force to a key with different finger positions. The
    positions for certain keys are simulated by aligning the wrist pad (left
    picture) to the scale of the device. The four different positions for the
    keys \textit{Shift, L, I, Z} (right pictures) are color coded according to
    the keys on the scale}
  \label{fig:FM_example}
\end{figure}

The results of the measurements are given in Table \ref{tbl:finger_force}. The
median of the means (15.47 N) of all measurements was used to calculate the
actuation forces in gram for the keyswitches later incorporated in the layout
for the adjusted keyboard. We used Eq. (\ref{eq:N_to_g}) and
Eq. (\ref{eq:actuation_forces}) to calculate the theoretical gram values for
each measured keyswitch.

\begin{equation}
  \label{eq:N_to_g}
  GFR = \frac{50 g}{M_{maf}} = \frac{50 g}{14.47 N} = 3.23 \frac{g}{N}
\end{equation}

\begin{equation}
  \label{eq:actuation_forces}
  AF_{key} = GFR * MAF_{key}
\end{equation}

With $M_{maf}$ the median of the means of applicable forces, $50 g$ the most
commonly found actuation force on the market (Section \ref{sec:market_forces}),
$GFR_{key}$ the gram to force ratio, $MAF_{key}$ the median of applicable force
for a specific key and $AF_{key}$ the actuation force for that specific key in
grams.

An example where we calculated the theoretical actuation force for the \textit{P}
key can be seen in Eq. (\ref{eq:force_example}).

\begin{equation}
  \label{eq:force_example}
  AF_{P} = GFR * MAF_{P} = 3.23 \frac{g}{N} * 10.45 N \approx 33.75 g
\end{equation}

We then assigned the each theoretical actuation force to a group that resembles
a spring resistance which is available on the market or can be adjusted to that
value. We matched the results from Table \ref{tbl:finger_force} to the groups
representing the best fit shown in Table \ref{tbl:force_groups}.


% Custom spring stiffness
% https://www.engineersedge.com/spring_comp_calc_k.htm
% https://www.eng-tips.com/viewthread.cfm?qid=198360


\begin{table}[H]
  \centering
  \ra{1.3}
  \begin{tabular}{?l^l^l^l^l^l^l^l^l^l^l}
    \toprule
    \textbf{Bottom Row} & \multicolumn{2}{c}{\emph{F5}} & \phantom{.} & \multicolumn{1}{c}{\emph{F4}} & \phantom{.} & \multicolumn{1}{c}{\emph{F3}} &  \phantom{.} &\multicolumn{3}{c}{\emph{F2}}\\
    \cmidrule{2-3}\cmidrule{5-5}\cmidrule{7-7}\cmidrule{9-11}
    \rowstyle{\itshape}
    Key                        & ↑     & -     && :     && ;     && M     & N     & B    \\
    \midrule
    \emph{Mean Force (N)}      & 11.23 & 10.84 && 14.22 && 15.34 && 16.38 & 15.60 & 14.36\\
    \emph{Actuation Force (g)} & 36.27 & 35.01 && 45.93 && 49.55 && 52.91 & 50.39 & 46.38\\
  \end{tabular}
  \begin{tabular}{?l^l^l^l^l^l^l^l^l^l^l}
    \\
    \textbf{Home Row} & \multicolumn{2}{c}{\emph{F5}} & \phantom{.} & \multicolumn{1}{c}{\emph{F4}} & \phantom{.} & \multicolumn{1}{c}{\emph{F3}} &  \phantom{.} &\multicolumn{2}{c}{\emph{F2}}\\
    \cmidrule{2-3}\cmidrule{5-5}\cmidrule{7-7}\cmidrule{9-10}
    \rowstyle{\itshape}
    Key                        & Ä     & Ö     && L     && K     && J     & H     &\\
    \midrule
    \emph{Mean Force (N)}      & 11.88 & 12.27 && 15.84 && 18.56 && 17.78 & 18.43 & \phantom{69.69}\\
    \emph{Actuation Force (g)} & 38.37 & 39.63 && 51.16 && 59.95 && 57.43 & 59.53 &\\
  \end{tabular}
  \begin{tabular}{?l^l^l^l^l^l^l^l^l^l^l}
    \\
    \textbf{Top Row} & \multicolumn{3}{c}{\emph{F5}} & \phantom{.} & \multicolumn{1}{c}{\emph{F4}} & \phantom{.} & \multicolumn{1}{c}{\emph{F3}} &  \phantom{.} &\multicolumn{2}{c}{\emph{F2}}\\
    \cmidrule{2-4}\cmidrule{6-6}\cmidrule{8-8}\cmidrule{10-11}
    \rowstyle{\itshape}
    Key                        & +     & Ü     & P     && O     && I     && U     & Z    \\
    \midrule
    \emph{Mean Force (N)}      & 10.80 & 10.70 & 10.45 && 14.34 && 17.95 && 17.00 & 16.80 \\
    \emph{Actuation Force (g)} & 34.88 & 34.56 & 33.75 && 46.32 && 57.98 && 54.91 & 54.26\\
    \bottomrule
  \end{tabular}
  \caption{Maximum force measurements for all digits of the right hand in
    different positions. The mean force of six participants is shown in the
    first row of each table and the resulting actuation force for the
    corresponding keyswitch in the following row. The columns indicate the label
    of the scale on the measuring device which can be seen in Figure
    \ref{fig:FM_example}. \textit{↑} stands for the shift key. \textit{F5} :=
    little finger, ..., \textit{F2} := index finger}
  \label{tbl:finger_force}
\end{table}

\begin{table}[H]
  \centering
  \ra{1.3}
  \begin{tabular}{?l^c^c^c^c^c^c^c}
    \toprule
    \rowstyle{\itshape}
    \textbf{Spring Stiffness:} &              35 g & 40 g  & 45 g  & 50 g  & 55 g  & 60 g \\
    \midrule
    \emph{\textbf{F5:} Key (g)} & \centered{P&(33.75)\\Ü&(34.56)\\+&(34.56)\\-&(35.01)\\↑&(36.27)}& \centered{Ä&(38.37)\\Ö&(39.63)}&&&&&\\
    \midrule
    \emph{\textbf{F4:} Key (g)} &&& \centered{:&(45.93)\\O&(46.32)} &\centered{L&(51.16)}&&\\
    \midrule
    \emph{\textbf{F3:} Key (g)} &&&&\centered{;&(49.55)}&&\centered{I&(57.98)\\K&(59.95)}\\
    \midrule
    \emph{\textbf{F2:} Key (g)} &&&\centered{B&(46.38)}&\centered{N&(50.39)\\M&(52.91)}&\centered{Z&(54.26)\\U&(54.91)\\J&(57.43)}&\centered{H&(59.53)}\\
    \bottomrule
  \end{tabular}
  \caption{Categorization of theoretical actuation forces acquired with
    Eq. (\ref{eq:actuation_forces}), into groups of more commonly available
    stiffnesses of springs. The rows indicate which finger is used to press the
    key. \textit{F5} := little finger, ..., \textit{F2} := index finger}
  \label{tbl:force_groups}
\end{table}

We simply mirrored the results of the right hand, for keys operated by the left
hand and copied the values to keys which are out of reach without lifting the
hand. Finally, we created the adjusted keyboard layout that can be examined in
Figure \ref{fig:adjusted_layout}. This layout was used in our main experiment
where we compared it to four different keyboards with uniform actuation forces
which is discussed in more detail in the following section.

\begin{figure}[H]
  \centering
  \includegraphics[width=1.0\textwidth]{images/adjusted_layout}
  \caption{Adjusted keyboard layout based on the measurements conducted in this section}
  \label{fig:adjusted_layout}
\end{figure}

\subsection{Main User Study}
\label{sec:main_study_meth}
\subsubsection{Hypotheses}
\label{sec:main_hypotheses}
Based on the literature review and preliminary telephone interviews, we derived
the following hypotheses concerning the impact of actuation force on different
metrics related to performance and user experience to ultimately answer our
research question―\textit{``Does an adjusted actuation force per key have a positive
impact on efficiency and overall satisfaction while using a mechanical
keyboard?.''}

\begin{longtable}{p{0.3cm} p{0.5cm} p{13cm} p{0.5cm}}
  & \textbf{H1}	& Actuation force has an impact on typing speed (efficiency - speed). & \\
  \\
  & \textbf{H2}	& Higher key actuation force decreases typing errors compared to lower key actuation force (efficiency - error rate). & \\
  \\
  & \textbf{H3}	& Keys with lower actuation force are perceived as more satisfactory to type with than keys with higher actuation force. & \\
  \\
  & \textbf{H4}	& An adjusted keyboard (non-uniform actuation forces) improves typing speed compared to standard keyboards (uniform actuation forces) (efficiency - speed).& \\
  \\
  & \textbf{H5}	& An adjusted keyboard decreases typing errors compared to standard keyboards (efficiency - error rate).& \\
  \\
  & \textbf{H6}	& An adjusted keyboard is perceived as more satisfactory to type with compared to standard keyboards. & \\
  \\
  & \textbf{H7}	& Differences in actuation force influence muscle activity while typing. & \\
\end{longtable}

\subsubsection{Method}
\label{sec:main_method}
In our laboratory study, twenty-four participants were required to perform two
typing test with each of the four keyboards provided by us and two extra typing
test with their own keyboards as a reference. The four keyboards differed only
in actuation force and were the independent variable. The dependent variable
were, typing speed (\gls{WPM} and \gls{KSPS}), error rate (\gls{CER}, \gls{TER})
and satisfaction (preference, usability, comfort, forearm muscle activity
measured via \gls{EMG}, post experiment semi structured interview and ux-curves)

\subsubsection{Participants}
\label{sec:main_participants}
There were no specific eligibility criteria for participants (n=24) of this
study beside the ability to type on a keyboard for longer durations and with all
ten fingers. The style used to type was explicitly not restricted to schoolbook
touch typing to also evaluate possible effects of the adjusted keyboard on
untrained typists. All participants recruited were personal contacts. 54\% of
subjects were females. Participant's ages ranged from 20 to 58 years with a mean
age of 29. Sixteen out of the twenty-four subjects (67\%) reported that they
were touch typists. Subjects reported the following keyboard types as their
daily driver, notebook keyboard (12, 50\%), external keyboard (11, 46\%) and
split keyboard (1, 4\%). The keyswitch types of those keyboards were distributed
as follows: scissor-switch (13, 54\%), rubber dome (8, 33\%) and mechanical
keyswitches (3, 13\%). We measured the actuation force of each participants own
keyboard and the resulting distribution of actuation forces can be observed in
Figure \ref{fig:main_actuation_force}. The self-reported average daily usage of
a keyboard ranged from 1 hour to 13 hours, with a mean of 6.69 hours. As already
mentioned in Section \ref{sec:telephone_interview} it is important to note, that
a study by Mikkelsen et al. found, that self-reported durations related to
computer work can be inaccurate \cite{mikkelsen_duration}. All participants used
the \gls{QWERTZ} layout and therefore were already used to the layout used
throughout the experiment.

\begin{figure}[H]
  \centering
  \includegraphics[width=0.79\textwidth]{images/main_actuation_force}
  \caption{Distribution of actuation forces from participant's own
    keyboards. The colors represent the type of keyboard. \textit{EXT:} external
    keyboard, \textit{NOTE:} notebook, \textit{SPLIT}, split keyboard}
  \label{fig:main_actuation_force}
\end{figure}

\subsubsection{Experimental Environment}
\label{sec:main_environment}
The whole experiments took place in a room normally used as an office. Chair,
and table were both height adjustable. The armrests of the chair were also
adjustable in height and horizontal position. The computer used for all
measurements featured an Intel i7-5820K (12) @ 3.600GHz processor, 16 GB RAM and
a NVIDIA GeForce GTX 980 Ti graphics card. The operating system on test machine
was running \textit{Arch Linux}\footnote{\url{https://archlinux.org/}}
(GNU/Linux, Linux kernel version: 5.11.16). The setup utilized two 1080p (Full
HD, Resolution: 1920x1080, Refresh-rate: 144Hz) monitors were participants were
allowed to adjust the angle, height and brightness prior to the start of the
experiment. The only two applications that were used during the experiment were
the typing test application described in Section \ref{sec:gott} inside of the
\textit{Chromium}\footnote{\url{http://www.chromium.org/Home}} browser (Version:
v90.0.4430.93-r857950) and \textit{FlexVolt
  Viewer}\footnote{\url{https://www.flexvoltbiosensor.com/software/}} (Version:
0.2.15, Chrome App). The FlexVolt Viewer app was used to collect \gls{EMG} data
via a bluetooth dongle (\textit{Plugable USB 2.0 Bluetooth®
  Adapter}\footnote{\url{https://plugable.com/products/usb-bt4le/}}) from the
\textit{FlexVolt 8-Channel Bluetooth Sensor}. Because of the ongoing COVID-19
pandemic\footnote{\url{https://www.who.int/emergencies/diseases/novel-coronavirus-2019}},
we ensured proper ventilation of the room and all participants including the
researchers were tested with antigen tests prior to every appointment.

\subsubsection{Independent Variable: Keyboards}
\label{sec:main_keyboards}
Additionally to the reference tests conducted with the participant's own
keyboards, we provided four keyboards which only differed in terms of actuation
force (Appendix \ref{app:equipment}). We decided to assign pseudonyms in the
form of Greek goddesses to the keyboards to make fast differentiation during the
sessions easier and reduce ambiguity. The pseudonyms for each keyboard and the
corresponding actuation force can be found in Table \ref{tbl:kb_pseudo}.

\begin{table}[H]
  \centering
  \ra{1.3}
  \begin{tabular}{?l^l^l^l}
    \toprule
    \rowstyle{\itshape}
    Pseudonym & Actuation Force && Description\\
    \midrule
    \textbf{Own}       & 35 g - 65 g & $\approx$ 0.34 N - 0.64 N & Participant's own keyboard (Figure \ref{fig:main_actuation_force})\\
    \textbf{Nyx}       & 35 g & $\approx$ 0.34 N & Uniform\\
    \textbf{Aphrodite} & 50 g & $\approx$ 0.49 N & Uniform\\
    \textbf{Athena}    & 80 g & $\approx$ 0.78 N & Uniform\\
    \textbf{Hera}      & 35 g - 60 g & $\approx$ 0.34 N - 0.59 N & Non-uniform / Adjusted (Figure \ref{fig:adjusted_layout})\\
    \bottomrule
  \end{tabular}
  \caption{Pseudonyms used for the keyboards throughout the experiment.}
  \label{tbl:kb_pseudo}
\end{table}

All keyboards used the standard ISO/IEC 9995 \cite{iso9995-2} physical layout
and provided keycaps representing the German \gls{QWERTZ} layout, which all
participants were already familiar with. All four keyboards used in the
experiment were
\textit{\gls{GMMK}}\footnote{\url{https://www.pcgamingrace.com/products/gmmk-full-brown-switch}}
equipped with \textit{Gateron} mechanical
keyswitches\footnote{\url{http://www.gateron.com/col/58459?lang=en}}. The order
in which participants would use the four keyboards during the experiment was
defined by a balanced latin square to reduce order effects. Additionally, the
mentioned reference tests with \textit{Own} were conducted at the start and end
of each session to detect possible differences in performance due to
exhaustion. The resulting groups used during the whole experiment were as
follows:

\begin{itemize}
  \item \textbf{Group 1:} \textit{Own $\rightarrow$ Hera $\rightarrow$ Athena $\rightarrow$ Nyx
    $\rightarrow$ Aphrodite $\rightarrow$ Own}
  \item \textbf{Group 2:} \textit{Own $\rightarrow$ Athena $\rightarrow$ Aphrodite $\rightarrow$ Hera
    $\rightarrow$ Nyx $\rightarrow$ Own}
  \item \textbf{Group 3:} \textit{Own $\rightarrow$ Aphrodite $\rightarrow$ Nyx $\rightarrow$ Athena
    $\rightarrow$ Hera $\rightarrow$ Own}
  \item \textbf{Group 4:} \textit{Own $\rightarrow$ Nyx $\rightarrow$ Hera $\rightarrow$ Aphrodite
    $\rightarrow$ Athena $\rightarrow$ Own}
\end{itemize}


\subsubsection{Experimental Design}
\label{sec:main_design}
\textbf{Preparation and Demographics}

The whole laboratory experiment was conducted over a total time span of 3
weeks. Participants were tested one at a time in sessions that in total took
$\approx$ 120 minutes. Prior to the evaluation of the different keyboards, the
participant was instructed to read the terms of participation which included
information about privacy, the \gls{EMG} measurements and questionnaires used
during the experiment. Next, participants filled out a pre-experiment
questionnaire to gather demographic and other relevant information e.g., touch
typist, average \gls{KB} usage per day, predominantly used keyboard type,
previous medical conditions affecting the result of the study e.g.,
\glsfirst{RSI}, \glsfirst{CTS}, etc. The full questionnaire can be observed in
Appendix \ref{app:gott}. Further, participants could adjust the chair, table and
monitor to a comfortable position.

\textbf{\gls{EMG} Measurements}

Since we measured muscle activity during all typing tests, electrodes were
placed on the \glsfirst{FDS}/\glsfirst{FDP} and \glsfirst{ED} of both
forearms. As already discussed in Section \ref{sec:meas_emg}, the main function
of the \gls{FDS} and \gls{FDP} is the flexion of the medial four digits, while
the \gls{ED} mainly extends the medial four digits. Therefore, these muscles are
primarily involved in the finger movements required for typing on a keyboard
\cite[650-653]{netter_anatomy}. We used ECG-Electrodes (Ag/AgCI/Solid Adhesive,
Pregelled, Size: 43mm) from TIGA-MED Deutschland
GmbH\footnote{\url{https://www.tiga-med.de/Diagnostik-Geraete/EKG-Elektroden-Zubehoer/EKG-Klebeelektrode-Festgel-50-Stueck-Pack}}.
To identify the correct locations for the electrodes, participants were
instructed to wiggle their fingers till contractions of the \gls{FDS}, \gls{FDP}
or \gls{ED} could be felt \cite{kim_typingforces}. A reference electrode was
placed next to the pisiform bone onto the dorsal side of the wrist. The
locations were then shaved and subsequently cleaned with alcohol before applying
the electrode. The distance between electrodes was 20mm. The correct placement
was then confirmed, by observing the data received by the \textit{FlexVolt
  8-Channel Bluetooth Sensor} in the \textit{FlexVolt Viewer} application while
the participant performed flexion and extension of the wrist. The
\textit{FlexVolt 8-Channel Bluetooth Sensor} used following hardware settings to
record the data: 8-Bit sensor resolution, 32ms \gls{RMS} window size and
Hardware smoothing filter turned off. To gather reference values (100\%\gls{MVC}
and 0\%\gls{MVC}), which are used later to calculate the percentage of muscle
activity for each test, we performed three measurements. First, participants
were instructed to fully relax the \gls{FDS}, \gls{FDP} and \gls{ED} by
completely resting their forearms on the table. Second, participants exerted
maximum possible force with their fingers (volar) against the top of the table
(\gls{MVC} - flexion) and lastly, participants applied maximum possible force
with their fingers (dorsal) to the bottom of the table while resting their
forearms on their thighs (\gls{MVC} - extension). We decided to also measure
0\%\gls{MVC} before and after each typing test and used these values to
normalize the final data instead of the 0\%\gls{MVC} we retrieved from the
initial \gls{MVC} measurements.


\textbf{Familiarization with \glsfirst{GoTT} and the Keyboards}

Participants could familiarize themselves with the typing test application
(\gls{GoTT}) for up to five minutes with a keyboard that was not used during the
experiment. Further, representative of the other keyboard models used in the
experiment (\gls{GMMK}), participants could familiarize themselves with
Aphrodite (50 g). Additionally, because of a possible height difference between
\gls{GMMK} compared to notebook or other keyboards, participants were given the
choice to use wrist rests of adequate height in combination with all four
keyboards during the experiment. If during this process participants reported
that an electrode is uncomfortable and that it would influence the following
typing test, this electrode was relocated and the procedure in the last section
was repeated (Happened one time during the whole experiment).

\textbf{Texts Used for Typing Tests}

As described in Section \ref{sec:gott}, we acquired ten, non-overlapping, texts
so that every keyboard could be tested twice. The texts were labeled T0\_1,
T0\_2, T1\_1, ..., T4\_1, T4\_2 and could be selected before each typing
test. The order of the texts did not change during the experiment. All texts had
almost identical \gls{FRE} scores (mean = 80.10, SD = 0.48).

\textbf{Questionnaires}

To receive feedback about several aspects that define a satisfactory user
experience while using a keyboard, we decided to incorporate two questionnaires
into our experiment. The first questionnaire was the \glsfirst{KCQ} provided by
\cite[56]{iso9241-411} and was filled out after each individual typing test
(\glsfirst{PTTQ}). The second survey, that was filled out every time the keyboard
was changed, was the \glsfirst{UEQ-S} \cite{schrepp_ueq_handbook} with an
additional question―``How satisfied have you been with this keyboard?''―that
could be answered with the help of an \gls{VAS} ranging from 0 to 100
(\glsfirst{PKQ})\cite{lewis_vas}. The short version of the \gls{UEQ} was selected, because of
the limited time participants had to fill out the questionnaires in between
typing tests (2 - 3 minutes) and also because participants had to rate multiple
keyboards in one session \cite{schrepp_ueq_handbook}.

\textbf{Post Experiment Interview \& UX-Curves}

To give participants the chance to recapitulate their experience during the
whole experiment, we conducted a semi-structured interview, after all typing
tests were completed. We recorded audio and video for the whole duration of the
interviews and afterwards categorized common statements about each
keyboard.

Further, we prepared two different graphs were participants had to draw
UX-Curves related to subjectively perceived typing speed and subjectively
perceived fatigue for every keyboard and corresponding typing test. The graphs
always reflected the order of keyboards for the group the current participant
was part of. Furthermore, before the interview started, participants were given
a brief introduction on how to draw UX-Curves and that it is desirable to
explain the thought process while drawing each curve \cite{kujala_ux_curve}. An
example of the empty graph for perceived fatigue (group 1) can be seen in Figure
\ref{fig:empty_ux_g1}.

\begin{figure}[H]
  \centering
  \includegraphics[width=1.0\textwidth]{images/empty_ux_g1}
  \caption{Empty graph for participants of group 1 to draw an UX-curve related
    to perceived fatigue during the experiment}
  \label{fig:empty_ux_g1}
\end{figure}

\textbf{Main Part of the Experiment}

Each subject had to take two, 5 minute, typing tests per keyboard, with a total
of 5 keyboards, namely \textit{Own (participant's own keyboard)}, \textit{Nyx
  (35 g, uniform), Aphrodite (50 g, uniform), Athena (80 g uniform)} and
\textit{Hera (35 g - 60 g, adjusted)} (Table \ref{tbl:kb_pseudo}). As described
in Section \ref{sec:main_keyboards}, the order of the keyboards \textit{Nyx,
  Aphrodite, Athena} and \textit{Hera} was counterbalanced with the help of a
balanced latin square to reduce order effects. The keyboard \textit{Own} was
used to gather reference values for all measured metrics. Thus, typing tests
with \textit{Own} were conducted before (one test) and after (one test) all
other keyboards, to also capture possible variations in performance due to
fatigue. Participants were allowed, but not forced to, correct mistakes during
the typing tests. The typing test application allowed no shortcuts to delete or
insert multiple characters and correction was only possible by hitting the
\textit{Backspace} key on the keyboard. The \textit{Capslock} key was disable
during all typing tests, because there was only visual feedback in form of
coloring of correct and incorrect input and no direct representation of entered
characters (Figure \ref{fig:gott_colorblind}), which could lead to confusion
when the \textit{Capslock} key is activated on accident.

\subsection{Summary}
\label{sec:meth_summary}
The preliminary telephone interview and the market analysis of available
mechanical keyswitches allowed us to gather profound information concerning
user's preferences and availability of hardware components. Additionally, the
preliminary study, where we measured the maximum applicable force onto a
keyswitch for each finger of the right hand in different positions, yielded
necessary data for the design of the adjusted keyboard layout. Throughout the
main user study, where we compared five different keyboards, we were able to
obtain various qualitative and quantitative data regarding performance and
satisfaction. The statistical evaluation of this data will be presented in the
next Section.
