# Influence of floor inclination on handle push and pull forces production of the upper limb

Matthieu Abautret, Benjamin Goislard De Monsabert, Chantal Maïs, Guillaume Rao

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## To cite this version:

Matthieu Abautret, Benjamin Goislard De Monsabert, Chantal Maïs, Guillaume Rao. Influence of floor inclination on handle push and pull forces production of the upper limb. Applied Ergonomics, 2024, 119, 10.1016/j.apergo.2024.104322. hal-04942813

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# Influence of floor inclination on handle push and pull forces production of the upper limb

Matthieu Abautret<sup>a, b, *</sup>, Benjamin Goislard De Monsabert<sup>a</sup>, Chantal Maïs<sup>b</sup>, Guillaume Rao<sup>a</sup>

<figure><img src="image_3.png" /></figure>

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## ARTICLE INFO

**Keywords:**

Muscle strength

Isometric contraction

Postural balance

Ground reaction force

Working conditions

Man-machine systems

## ABSTRACT

Floor inclination can alter hand force production, and lower limb kinetics, affecting control operations, and threatening operator safety in various domains, such as aviation, naval, construction industry, or agriculture. This study investigates the effects of different floor inclinations, on handle push or pull force production. Participants performed maximal isometric contraction tasks requiring to exert a maximal voluntary force either by pulling or pushing a handle, at different floor inclinations from -30° to +30° about the transverse and longitudinal axes. Maximal hand force and Ground Reaction Forces about both feet were recorded. The results revealed non-equivalent variations in hand and feet responses as a function of inclination angle. Specifically, there was a significant reduction in handle push-pull force production, up to 70% (p < 0.001) for extreme inclinations, around both axes. This study provides critical data for design engineers, highlighting the challenge of production forces at steep angles.

## 1. Introduction

Working conditions can be drastically affected by ever-changing and dynamic environmental situations, such as in aviation and naval sectors, to the point where both passengers and crews operating systems are exposed to significant safety risks (Peters et al., 2019). Beyond modifications of the environment, the unpredictability of these conditions can also be triggered by navigational orders such as submerging or surfacing for submarines. In such cases, the inclination of the floor can vary relatively rapidly and reach amplitudes of ±30° for both the Forward-Backward angle, i.e., the inclination around the ship's transverse axis and the Left-Right angle, i.e. the inclination around the ship's longitudinal axis (Bureau Veritas, 2016).

Such large floor inclinations can induce significant challenges for crews in terms of operating their systems safely and efficiently (Ford et al., 2015). Military vessels, including surface ships and conventional and nuclear submarines, are complex integrations of systems thus requiring constant need for preventive and corrective maintenance of the vast array of equipment's on board (Murthy and Asgharizadeh, 1999). Similar to the industry, 50% of these maintenance activities are manual handling tasks requiring push and/or pull forces (Baril-Gingras and Lortie, 1995). More importantly, operational tasks on board naval

vessels are directly related to the safety of the vessel and its crews and/or to nuclear security. These manual operational tasks include actuation of valves, steering wheels, handles, and levers. For instance on submarines, an operator may have to exert significant muscular effort within a limited time frame such as blow ballast tanks to bring the ship to the surface urgently. Despite those important safety aspects, the influence of floor inclination on the operators' balance and on their ability to quickly and effectively produce forces remain poorly known.

Various studies have investigated the diverse biomechanical effects of floor inclination along both the lateral and anteroposterior axis. Many authors studied adaptations during lifting tasks, especially standing balance control and associated kinematic and kinetic strategies (Hu et al., 2013, 2015; Jiang et al., 2005; Shin and Mirka, 2004; Simeonov et al., 2003). These studies showed that floor inclination increases postural instability, thus requiring adaptation of postural strategies to maintain the task performance. Other authors investigated the many factors influencing hand force production. Those studies investigating the capacity to produce pushing and pulling forces have documented the effect of variables such as body weight, force application height, posture, and handle position, on the maximum force production capacity of the hand (Chaffin et al., 1983; De Looze et al., 2000; Granata and Bennett, 2005; Grieve and Pheasant, 1981; Kumar, 1991, 1995; Lin

<footnote><sup>a</sup> Aix-Marseille Univ, CNRS, ISM, 163 Luminy Avenue, 13288 Marseille, Cedex 09, France</footnote>

<footnote><sup>b</sup> Naval Group, 199 Pierre-Gilles De Gennes Avenue, 83190 Ollioules, France</footnote>

<footnote>* Corresponding author. Aix-Marseille Univ, CNRS, ISM, 163 Luminy Avenue, 13288 Marseille cedex 09, France.</footnote>

<footnote>E-mail address: matthieu.abautret@univ-amu.fr (M. Abautret).</footnote>
et al., 2013). For instance, Kumar (1995) found that push-pull force production is maximal for a medium-height handle and decreases with higher or lower force application point. More interestingly, Pheasant et al. (1982) showed that, for a given handle height, force in the sagittal plane was strongly related to foot position. On a similar topic, Hoffman et al. (2008) observed that participants tend to increase the extent of their base of support and to move away its position from the handle as the magnitude of the force applied by the hand increased. This strategy allows to generate a greater moment of body weight to increase hand force generation. By changing the base of support, the mass of the participants' lower limbs is indeed strategically used to counterbalance against the force generated by their hands. While this principle is well established for a horizontal support surface, the influence of floor inclination on this strategy is not known.

Beyond the interplay of hand force and foot position, several researchers (Grieve and Pheasant, 1981; Kerk et al., 1994; Okunribido and Haslegrave, 2008) have found that during horizontal pushing and pulling tasks, subjects exerted force vectors that deviated significantly from the optimal force orientation. De Looze et al. (2000) showed that the deviation of the direction of the hand force relative to the horizontal axis increases with force intensity during pushing actions. Similarly, Granata and Bennett (2005) observed that the upward direction deviation during pushing force increased with intensities and handle heights. While established studies have explained force vector deviations in push-pull force production, the potential modulation of force direction by floor inclination remains an understudied biomechanical aspect. In particular, the existing literature has not directly studied the effect of floor inclination on both handle push-pull forces and ground reaction forces. Understanding this relationship is essential to refine the sizing and positioning of controls during the design phases of aerial and naval vehicles, and is relevant to various industries that frequently deal with inclined surfaces, such as construction, and agriculture.

The aim of this study is thus to investigate the effects of floor inclination, both along the Forward-Backward and Left-Right axes, on the maximum hand force production and ground reaction forces under each foot during isometric pushing and pulling tasks. We hypothesize that floor inclinations lead to a reduction in maximum handle push-pull forces production. This decrease is potentially due to deviations in hand force orientation from the initially required direction, and to adaptive changes in the lower limb contributions to task performance. More specifically, we anticipate changes in the use of body weight and variations in the distribution and intensity of ground reaction forces exerted under each foot.

# 2. Material and methods

## 2.1. Participants

Fifteen healthy, right-hand dominant male participants were recruited from the university community (height: 1.78 ± 0.07 m, mass: 73.2 ± 8.5 kg, age: 22 ± 2 yrs). This demographic was specifically selected for its relevance to the predominant male population within the military naval context (Centre d'Etudes Stratégiques de la Marine, 2018). All participants were uniformly outfitted with running shoes for the duration of the experiment. All participants were free from any recent upper-extremity and torso injuries or disorders. All participants were asked to read and sign a written consent form before participating in this study. This study was reviewed and approved by the CERSTAPS ethics committee (IRB00012476-2023-23-03-240).

## 2.2. Instrumentation and data acquisition

Hand Forces (HF) were measured using a six-axis load cell (Delta SI-660-60, ATI Industrial Automation, USA). Participants grasped a cylindrical handle with a 3.2 cm diameter that was mounted on the load cell. This assembly was mounted on a horizontal length of slotted rail (80/80,

Elcom, France) that was part of the test apparatus (Fig. 1). The force sensor was manually adjusted in the 3 dimensions, to each participant to ensure the forearm longitudinal axis was aligned with the antero-posterior, xH axis, of the sensor (Fig. 1) for the horizontal floor inclination, later referred as 0°. The force sensor orientation and position remained the same throughout the test, despite changes in floor inclination.

Ground Reaction Forces (GRF) were measured under the left and right feet using two force platforms (Kistler, Switzerland). These platforms were mounted on an aluminum profile frame (40/40, Elcom, France). This assembly allowed the force platforms to be inclined and a digital level was used to check the inclination. R13-anti-slip (AFNOR, DIN 51130:2014) strips were strategically placed on the platforms to ensure maximum traction for participants.

Hand Force and Ground Reaction Forces were recorded synchronously at 1000 Hz on a PC computer with two 16-bit analog to digital (A/D) conversion cards (Qualisys, Sweden and Kistler, Switzerland respectively). All collected instrument data were processed using Qualisys-Track-Manager software (Qualysis, Sweden).

## 2.3. Experimental procedures and protocol

First, anthropometric measures were taken, including height, mass, olecranon-ground height, C7-right acromion distance, Epicondyle-3rd metacarpal head distance (Table 1). These anthropometric measurements were essential in customizing the handle position for each subject in the experimental setup. This approach ensured that the handle position was tailored to each individual's physical dimensions, thereby eliminating any potential influence of the handle position on task performance. While the handle position was adjusted by the investigator, participants performed a targeted warm-up routine. This included exercises such as dynamic abdominal planking, dynamic core rotations, and dynamic back rotations, such as "Superman" exercises, shoulder, and back warm-ups (hip sways). This routine was designed to prepare their joints to maximal force contraction and minimize the risk of injury by performing active movement, exploring their range of motion, and activating their muscles progressively. The participants completed their warm-up with a familiarization phase involving the experimental task described below. This phase involved gradually increasing force levels.

As part of the main experiment, participants were required to perform an isometric task consisting of grasping a handle and exerting a maximal voluntary force along the xH axis (Fig. 1) by either pushing or pulling the handle. The task involved standing in a free-standing position, which has been shown to allow for the greatest hand forces (Daams, 1993). Maximal trials were 6 s in duration (Hoffman et al., 2008) and were preceded by a practice trial. During the practice trials, subjects were asked to explore different postures and to identify their ideal one. Consistent verbal encouragement was provided to maintain consistency between trials. Two repetitions (Maleki-Ghahfarokhi et al., 2022) were conducted for each factorial condition to reduce the total number of trials conducted. To minimize the risk of fatigue, a minimum 3-min rest period was allowed between each trial.

Floor inclination changed either by rotation along either the Left-Right axis, or the Forward-Backward axis (Fig. 1), and for each of these axes, inclination angle could take 5 values, which are respectively designated as 30 Left (L)°, 15L°, 0°, 15 Right (R)°, 30R° and as 30 Forward (F)°, 15F°, 0°, 15 Backward (B)°, 30B°.

Participants were asked to exert force in two separate directions (anterior: push, and posterior: pull). Because of the number of inclinations (10), the number of force exertion direction (2), this experimental design results in twenty conditions, each repeated twice. To mitigate the risk of physical fatigue and weariness, we aimed to reduce the total number of trials per participant. Consequently, we employed the Balanced Incomplete Block Design (BIBD) method for our experimental design (Okunribido and Haslegrave, 2008). According to the BIBD table in Cochran and Cox's (1992), 15 participants performing 12
<figure><img src="image_4.png" /><figcaption>Fig. 1. (a): Laboratory configuration with six-axis load cell, force platforms for measuring forces at the hand and feet respectively; (b): Typical force output waveform at the hand level and time window definition.</figcaption></figure>

<table><caption>**Table 1**
Participant's anthropometric data, used in part for handle positioning.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Age<br>(yrs.)</th><th rowspan="2">Height<br>(m)</th><th rowspan="2">Mass<br>(kg)</th><th colspan="3">Anthropometric Data for Handle Positioning</th></tr><tr><th>Olecranon-<br>ground<br>Height (m)</th><th>C7-right<br>acroinion<br>Distance<br>(m)</th><th>Epicondyle-<br>3rd<br>Metacarpal<br>Head<br>Distance (m)</th></tr></thead><tbody><tr><th>Mean</th><td>22</td><td>1.78</td><td>73.2</td><td>1.09</td><td>0.19</td><td>0.36</td></tr><tr><th>SD</th><td>2</td><td>0.07</td><td>8.5</td><td>0.05</td><td>0.01</td><td>0.02</td></tr></tbody></table>

out of the 20 conditions originally planned was required to obtain the full program.

To assess fatigue, the isometric push and pull maximum exertions at 0° inclination were taken again after the test conditions.

## 2.4. Data analysis

Post-processing was carried out using Python (version 3.9). All data were filtered using a fourth order zero-lag lowpass Butterworth filter with a cut-off frequency of 3 Hz.

### 2.4.1. Hand force

The total force exerted on the handle during the experimental task was determined from the three force components recorded by the sensor. Hand Force (HF) intensity was calculated as the norm of the three force components at each time point. We then calculated the maximum value for each of these components, as well as for HF, by averaging the data over a specified time window. This time window was determined by identifying the time points where HF first exceeded 1.5 times the overall trial average and when it became less than 1.5 times the overall trial average (Fig. 1). This processing window was consistently used for all subsequent analyses.

For further analysis, we selected the trial with the highest average Maximal HF value out of the two trials conducted per condition. We then normalized Fx, Fy, Fz, and Maximal HF values by dividing them by the Maximal HF value observed in the 0° condition and expressing them as percentages.

The magnitude of the HF vector in the sagittal plane was calculated

similarly to the above method, but excluded the y (lateral) component. The sagittal orientation of the vector ($HF_{\theta}$) was calculated in degrees from the horizontal axis using the following formula:

$$HF_{\theta} = atan2(Fz/Fx) \times 180^{\circ}/\pi \quad (1)$$

Where Fx and Fz represent the components of the force applied to the handle along the antero-posterior and vertical axes, respectively.

### 2.4.2. Ground reaction force

Both Left-foot GRF and Right-foot GRF intensities (Fig. 2) were calculated using Euclidean norm of components.

To study the global lower limb actions, the Resultant GRF vector was determined as the sum of the components from each force platform at each time point, as follows:

$$\overline{GRF} = (Fx_{Right}(t) + Fx_{Left}(t)) \cdot \vec{i} + (Fy_{Right}(t) + Fy_{Left}(t)) \cdot \vec{j} + (Fz_{Right}(t) + Fz_{Left}(t)) \cdot \vec{k} \quad (2)$$

Where Fx Right, Fy Right, Fz Right represent the components of the force applied to the right platform over time, with Fx Left, Fy Left, Fz Left represent the components of the force applied to the left platform over time, and with $\vec{i}$, $\vec{j}$, $\vec{k}$ unit vectors, respectively in $x_L$, $y_L$, and $z_L$ directions (Fig. 2). The magnitude of the Resultant GRF vector in the sagittal plane was calculated (eq. 2) excluding consideration of the y component of the force, and its orientation was calculated in degrees from the horizontal.

To study the kinetic strategies used at the foot level, a Total GRF intensity was defined as the sum of Left-foot GRF and Right-foot GRF intensities. Then, to determine the respective contributions of each foot, the Left-foot GRF and Right-foot GRF were normalized by the Total GRF and expressed as a percentage, as follows for the Left-foot GRF contribution:

$$LGRF_c = \frac{\text{Left foot GRF}}{\text{Total GRF}} \times 100 \quad (3)$$

With Left-foot GRF, the magnitude of the Left-foot GRF vector and with Total GRF representing the sum of the magnitudes of the Left-foot GRF
<figure><img src="image_5.png" /><figcaption>Fig. 2. Representation of dependent variables from sensors.</figcaption></figure>

vector and the Right-foot GRF vector.

Additionally, Total GRF intensity was normalized by the 0° condition and was also expressed as a percentage for further analysis.

## 2.5. Statistical analysis

The effect of floor inclination on the different hand and feet force variables was tested using a specifically BIBD-adapted repeated measures ANalysis Of VAriance (ANOVA). To compare the experimental conditions, we then performed Tukey post hoc tests. All significance values were set at $p < 0.05$.

Statistical post-processing was performed using R (version 4.2.2) and the *Agricolae* package (De Mendiburu and Simon, 2015), https://cran.r-project.org/web/packages/agricolae/index.html.

# 3. Results

The comparison of the Maximal HF for 0° inclination at the beginning and end of the experimental protocol did not reveal any significant difference for both pushing ($p = 0.426$) and pulling ($p = 0.052$) tasks. The values were on average around 340 N during pulling and around 290 N during pushing. Furthermore, the participants reported no feelings of fatigue at any time during the test sessions.

## 3.1. Intensities and orientations of HF and GRF vectors

The intensity and orientation of Maximal HF and Resultant GRF vectors varied with floor inclination, with different trends along Forward-Backward and Left-Right axes. Furthermore, those variations also differ between push and pull tasks.

### 3.1.1. Forward-Backward inclination

The results regarding the influence of Forward-Backward inclination revealed that both HF and GRF vectors exhibited significant reductions in force intensity and alterations in orientation for specific inclinations compared to 0° inclination (Fig. 3). During pushing tasks, both HF and GRF vectors demonstrated a substantial decrease in intensity ($p < 0.01$) and a change in orientation ($p < 0.001$) for the extreme Backward inclination (30B°). A comparable reduction in force intensity and alteration in orientation was observed during pulling tasks for the extreme Forward inclination (30F°; $p < 0.001$) (see Table s1 of Supplementary Material).

### 3.1.2. Left-Right inclination

The influence of Left-Right inclination on the intensity and

orientation of Maximal HF and Resultant GRF vectors was found to have comparable effects for both pushing and pulling exertions (Fig. 3). Furthermore, lateral inclinations demonstrated parallel impacts on hand and foot kinetics across these two types of tasks. The results revealed that extreme Left and Right inclinations significantly impacted the intensity of both HF and GRF vectors compared to a neutral 0° inclination (Table s1). In pushing tasks, there was a marked decrease in HF vector intensity for the extreme Left inclination (30L°; $p < 0.001$), though the orientation remained relatively stable. A similar pattern was observed in the GRF vector, which demonstrated a reduction in intensity ($p < 0.01$) and a shift in orientation ($p < 0.01$) for the extreme Left inclination (30L°). Pulling tasks exhibited comparable trends, with the most pronounced differences observed for the extreme Right inclination (30R°), where significant differences in both intensity and orientation were observed.

## 3.2. Hand force description

### 3.2.1. Forward-Backward inclination

In both tasks, the intensity of Maximal HF exhibited a progressive decrease as the inclination deviated from the neutral 0° position (Fig. 4). The most pronounced losses were observed at extreme inclinations, with over 50% for the extreme Backward inclination (30B°) in pushing tasks and over 70% for the extreme Forward inclination (30F°) in pulling tasks (Table s2). In pushing tasks, the antero-posterior component (Fx) exhibited a similar decline with increasing inclination, whereas the lateral (Fy) and vertical (Fz) components remained largely stable. In pulling tasks, in addition to the decrease in Maximal HF and antero-posterior component, there was also a slight reduction in the vertical component. The lateral component demonstrated consistency across inclinations.

### 3.2.2. Left-Right inclination

The results demonstrated that Maximal HF intensity exhibited a progressive decline with all Left-Right inclinations in comparison to a neutral 0° inclination in both pushing and pulling tasks (Fig. 4), with statistical significance ($p < 0.001$). The greatest reductions were observed at extreme inclinations, with the highest loss occurring at extreme Right inclination (30R°) in the pulling condition, where the decrease exceeded 40% (Table s2). Similarly, the antero-posterior component (Fx) demonstrated reductions in line with increases in Left-Right inclinations for both tasks. Notably, the lateral component (Fy) remained stable in pushing tasks but exhibited a progressive decrease in pulling tasks, from extreme Left (30L°) to extreme Right (30R°) inclinations. The vertical component (Fz) demonstrated
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Maximal Hand Force (HF) and Resultant Ground Reaction Force (RGRF) vectors (intensities and orientations) in the sagittal plane for the Forward-Backward inclination axis (upper panels) and for the Left-Right inclination axis (lower panels) during both for the push task (right panels) and for the pull task (left panels). Corresponding statistical significance tests are presented in Table s1 of Supplementary Material.</figcaption></figure>

consistency across all inclinations in both conditions.

### 3.3. Left and right foot contributions

#### 3.3.1. Forward-Backward inclination

In the push condition, there was a progressive decrease in Total GRF intensity across all inclinations when compared to the neutral 0° inclination (Fig. 5). The most significant reduction was observed for the extreme Backward inclination (30B°; p < 0.05). The distribution of GRF between the feet remained largely consistent, though a significant variation was noted between the extreme Backward (30B°) and 0° inclinations (Table s3). Conversely, in the pulling task, the Total GRF intensity did not demonstrate significant changes, maintaining a stable level across different inclinations. However, a trend was observed (p =

0.08), suggesting a slightly greater contribution from the right foot in the neutral position.

#### 3.3.2. Left-Right inclination

During the push condition, Total GRF intensity exhibited a progressive decline with all inclinations compared to the neutral 0° (Fig. 5). This decline was most notable for the extreme Left inclination where the highest loss was observed (30L°; p < 0.01). For the inclinations between 30° Left (30L°) and 15° Right (15R°) indicated, a disproportionately larger contribution was noted for the right foot, approximately 2.5 times than that of the left. At extreme Right inclination (30R°), the force distribution approached symmetry. In contrast, during the pulling task, Total GRF intensity remained stable across inclinations. While no significant differences were noted, a clear trend showed a decrease in the
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Maximal Hand force (HF) and components (Fx, Fy, Fz) as a function of floor inclinations for, Forward-Backward inclination (upper panels) and Left-Right inclination (lower panels) during push task (right panels) and, pull task (left panels). Corresponding statistical significance tests are presented in Table s2 of Supplementary Material.</figcaption></figure>

intensity for the right foot and an increase for the left foot as the angle shifted from extreme Left (30L°) to extreme Right (30R°) inclinations, culminating in nearly equal contributions from both feet at the latter inclination (Table s3).

## 4. Discussion

This study investigated the effects of floor inclination on the capacity to produce force when pushing or pulling handles, as well as the adaptive changes in lower limb contributions to task performance along the Forward-Backward and Left-Right axis. This previously unexplored topic represents an important question for industrialists in various sectors, including aerial and naval transportation, where the environment or navigation order can significantly affect floor inclination relative to a gravitational reference frame. Primary results indicated that the force produced at the handle decreased by up to 70% at extreme Forward-Backward inclinations (Figs. 3 and 4). During Forward-Backward inclination, the force required to pull the handle decreased sharply at extreme forward inclination (–72% at 30F° relative to 0° condition), while the force required to push the handle was lowest at extreme backward inclination (–58% at 30B° relative to 0° condition). During Left-Right inclination, handle pushing forces decreased for both left (–40.5% at 30L° relative to 0° condition) and right (–23.6% at 30R° relative to 0° condition), while handle pulling forces only decreased for right inclination (–42.5% at 30R° relative to 0° condition). The results suggest that individuals may face difficulties in generating significant pulling or pushing forces when confronted extreme floor inclination. This could potentially restrict their capacity to perform certain tasks that necessitate mechanical control actuation.

The force values obtained for no floor inclination, i.e., 0° condition, are consistent with those reported in the literature for both the maximum HF intensity (Kerk et al., 1994; Boocock et al., 2006) and the useful push-pull force (Okunribido and Haslegrave, 2008), deduced from the Fx component contribution to Maximal HF in Fig. 4. Additionally, it was observed that the handle push-pull force (Maximal HF) decreases sharply compared to the 0° condition for extreme inclinations. Floor inclination also influences the distribution of the contributions of the individual components Fx, Fy, and Fz. In particular, the useful push-pull force (Fx) decreases more than the other components with floor inclination. This leads to a decrease in intensity as well as a change in the orientation of the Maximal HF vector in the sagittal plane (Fig. 3). These observations can be interpreted as adjustments in the direction of force at the onset of a push or a pull and may serve as a protective mechanism to assess the frictional properties of the floor before initiating a maximal effort (Grieve, 1983). In our context, participants may have instinctively maintained non-required hand force components for the task to counterbalance the destabilization of their whole-body posture induced by an inclined floor.

The analysis showed a relationship between the handle push-pull force production and the orientation of the Resultant GRF (which can be linked to the moment of force from the use of body weight). This relationship demonstrates the combined influence of floor inclination on the ability to produce force and the ability to maintain standing balance. Upon examination of the pulling force produced in the Forward-Backward direction, it was observed that a smaller angle between the Resultant GRF vector (sagittal plane) and the horizontal occurred at the 0° inclination. This resulted in an increase in the force produced, specifically, Maximal HF at hand level (Fig. 3). As described by several
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Total Ground Reaction Force (GRF) and contribution of both feet as a function of floor inclination for Forward-Backward inclination (left panels) and Left-Right inclination (right panels) during both push task (upper panels) and pull task (lower panels). Corresponding statistical significance tests are presented in Table s3 of Supplementary Material.</figcaption></figure>

authors (Hoffman et al., 2008; Hoozemans et al., 1998), utilizing body weight while performing pushing or pulling tasks can result in whole body postures that cause an antero-posterior shift in the Center of Mass (CoM) (anterior for pushing and posterior for pulling). This shift leads to a greater horizontal distance between the CoM and Center of Pressure (CoP) under the feet, thereby creating a larger moment around the CoP. The examination of dynamic strategies at the feet based on GRF vectors can achieve an analysis of handle push-pull force production with floor inclination from the perspective of balance control. Our observations in Forward-Backward configurations revealed a variation in the orientations of the resultant GRF vector in the sagittal plane, with orientations closer to the Earth's gravitational axis for extreme inclinations. These findings are consistent with previous reports in the literature. In a study on the effect of dynamic perturbations of the platform inclination during balance posture, Gurfinkel et al. (1981) found that human subjects always maintain their trunk aligned with the vertical axis. Similarly, studies on cats (Lacquaniti et al., 1990) have observed that these quadrupeds maintain the orientation of their trunk parallel to the floor, even when the floor is inclined. Trunk stabilization plays a critical role in maintaining postural orientation and balance (Leroux et al., 2002). This approach is essential in preventing significant deviations of the CoM from the base of support, thereby ensuring balance. Given that the trunk is the heaviest segment of the body, its stabilization is of utmost importance.

The results of our analyses also showed a relationship between handle push-pull force production and GRF which can be interpreted in terms of either the ability to enhance force production or the ability to maintain standing balance. In most configurations, participants produced a greater Total GRF production when the inclinations were closer to the 0° level floor. This greater Total GRF would allow for more force transfer through the body and enhance force production capability at the handle. This Total GRF is determined by participant's global

muscular capacity, the friction between the feet and the floor, and the constraints on our standing balance. By strategically distributing the Total GRF, it is possible to optimize the use of muscular capacity and increase the force that can be applied to the handle. The present study's findings suggest that asymmetrical distribution of force under each foot may be associated with maximum hand force production, which is consistent with the observations made by Hoffman et al. (2008). When performing push-pull tasks, the applied force creates a moment of force around the body's longitudinal axis. This moment of force causes the body to rotate in the opposite direction to the applied force, requiring a body response to maintain stability. Applying additional pressure to the right leg can generate an extra moment of force around the body's longitudinal axis, in the opposite direction to the force exerted by the hand. This mechanism compensates for the initial momentum and helps stabilize the body. At the same time, this asymmetrical distribution of force under each foot might induce asymmetrical muscular effort between the two sides of the body. The increased muscular effort on the right side might generates more force, that can be transferred through the kinetic chain to the handle, thus increasing the pulling force. Additionally, we observed in most configurations, the distribution of forces between the two feet tended to be more symmetrical as the floor became more inclined. This finding highlights the importance of reaching an equal contribution of the feet to maintain postural balance in extreme inclinations. According to a 2012 study (Wang and Newell, 2012), authors indeed observed that a balanced body weight distribution, with 50% of the weight on each foot, appears to be a configuration that promotes stable balance when standing. These findings are supported by the work of Simeonov et al. (2003), which studied roofing tasks and demonstrated that an inclined floor effectively reduces the support surface beneath the feet. This reduction, in turn, limits the range of motion of the body's CoM, thereby increasing postural instability. Our results thus confirm that when the environment does endanger our
standing balance, e.g. for extreme inclination of the floor, both lower limbs are used equally to maintain that balance, to the detriment of the force production task at the handle. On the contrary, when our standing balance is not threatened, e.g for inclination at level floor, both lower limbs are available to enhance force production capacity at the handle.

The findings suggest that when attempting to maintain balance on an inclined floor while producing a push-pull force, the Central Nervous System (CNS) experiences a conflict. On the one hand, the use of body weight coupled with asymmetrical contribution of each foot to ground reaction forces could enhance handle force production during a pushing or pulling task. On the other hand, postural balance requires to keep the body weight close to the absolute vertical and to equally distribute ground reaction forces under each foot. From a motor control perspective, the CNS must prioritize tasks to optimize performance and safety (Lee et al., 2019). According to Stoffregen et al. (1999), the primary (if not sole) objective of the postural control system is to maintain balance by one or more means, such as controlling the center of mass position (Collins and De Luca, 1993) or organizing strategies across body segments (Nashner and McCollum, 1985). In situations where the balancing task becomes challenging, all available resources are directed towards maintaining balance, leaving little room for facilitating other tasks that require the same resources. In fact, in the event of imminent balancing failure, the task may even be interrupted. Our results align with these conflictual situations, which can explain the tradeoff between generating maximum strength and maintaining balance. When operating mechanical controls in submarines that require force generation by pulling or pushing, a significant floor inclination may hinder the operator's ability to maneuver the control due to the need to maintain balance. The present study indicates that this loss of force production capacity can be as much as 70% compared to a non-inclined floor. Our study provides preliminary guidelines on the force generation capacity related to floor inclination. This information is important for designers seeking to optimize control dimensions, positions, and orientations to reduce biomechanically disadvantageous configurations. By doing so, operational efficiency and safety can be ensured for both operators and vehicles. Similarly, in aviation, this knowledge can be used to position controls and equipment efficiently, making it easier for pilots and crew to operate in a variety of flight conditions. It could also help in the design of aircraft carts, optimizing them for easier handling by flight attendants. In the construction sector, particularly for roof work, our data could be helpful for recommendations associated to tasks requiring manual handling on slopes. In agriculture, with its varied terrain, our results could help optimize the design of manually handled tools and machines operated on inclined terrains. Overall, our work seems to promise advances in safety, efficiency, and effectiveness of operators from diverse working sectors.

The study's limitations are mainly attributed to the test apparatus design. Specifically, the support surface was restricted to keep the feet on the force platforms. This may have constrained the postural strategies of the subjects, for instance preventing an optimal feet positioning on the antero-posterior axis. Additionally, the rotation axis used to create floor inclination was below the support surface while the hand position was fixed. This design choice may have introduced two complications: an imperfect simulation of a typical submarine environment where the entire system (floor and handle) is inclined, and a variable distance between the center of the support surface on the floor and the gripping handle. Despite these limitations, our apparatus represents a relevant simulation of floor inclination situation and allowed to provide first data to understand its influence on force production. It should also be noted that the postural adjustments, i.e. change in joint angles, in response to floor inclinations were not recorded during this study. Hence, it is not possible to conclude on the possible role of the force-length relationships of the muscles of the whole body regarding the observed decreases in hand force production capacity. Further studies focusing on floor inclination should thus include motion capture recordings to accurately characterize postural adjustments associated with hand and foot kinetic

variations.

## 5. Conclusions

The aim of this study was to investigate the effects of floor inclination on the ability to produce maximal handle push-pull force, as well as the adaptive changes in lower limb kinetics. The main findings suggest a significant reduction in hand pulling force during forward inclination (72% decrease) and a significant decrease in pushing force for backward inclination (58% decrease) when compared to 0° condition. During Left-Right inclination, pushing forces decreased for left (40.5% reduction) and right (23.6% reduction) inclination, with a significant decrease in pulling force (42.5% reduction) for right inclination, compared to 0° condition. In addition, the orientation and distribution of Ground Reaction Forces (GRF) showed significant variations with inclination. This modulation of GRF directly affects the ability to generate force at hand level, specifically through the way the body distributes force between both feet to maintain balance. These results corroborate the idea that the CNS prioritizes maintaining balance when it is challenged over other task aspects. In the context of maritime ships, such data thus suggest that individuals may struggle to generate significant pushing or pulling forces under extreme floor inclination, potentially limiting their performance in tasks requiring mechanical control actuation.

## Funding

This work was financially supported by the ANR as part of the France Relance plan of the French government (ANR-21-PRRD-0024-01), in collaboration with Naval Group (France).

## CRediT authorship contribution statement

Matthieu Abautret: Writing – review & editing, Writing – original draft. Benjamin Goislard De Monsabert: Supervision. Chantal Maïs: Project administration. Guillaume Rao: Supervision.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

This work was financially supported by the ANR as part of the France Relance plan of the French government, in collaboration with Naval Group (France). However, the authors do not derive any commercial benefit from this work.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apergo.2024.104322.

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