Falls from ladders are a leading cause of workplace injury. In 2020, over 22,000 injuries and 160 deaths at work cited ladders as the main cause for injury (U.S. Bureau of Labor Statistics, 2023). Falls with a ladder can occur from the ladder slipping or tipping (Chang et al., 2005; Deschler et al., 2023; DiDomenico et al., 2013; Milligan et al., 2020; Simeonov et al., 2017; Williams et al., 2022). Falls from a ladder, where the climber decouples from the ladder, is an emerging area of research. One important source of falls from a ladder is slipping, which accounts for 14 % of all falls involving ladders (Cohen and Lin, 1991). Descending a ladder has been associated with a higher number of falls than ascending (Cabilan et al., 2018; Lombardi et al., 2011; Moore et al., 2009), yet slip risk during ladder descent is not well understood.

One metric to quantify slip risk is the required coefficient of friction (RCOF). It is a person- and task-specific metric that quantifies the amount of friction needed for a person to not slip while completing a task (Chang et al., 2011). RCOF has been quantified extensively for walking (Anderson et al., 2014; Beschorner et al., 2016; Chang et al., 2011; Hanson et al., 1999; Kim et al., 2005) and in a variety of tasks like turning while walking, stepping in and out of bathtubs, or stair climbing (Christina and Cavanagh, 2002; Sekiguchi et al., 2017; Siegmund et al., 2010; Yamaguchi et al., 2018; Yamaguchi et al., 2013). In walking, RCOF is positively correlated with slip events when encountering a slippery surface (Beschorner et al., 2016). A higher RCOF has been associated with increased heel contact velocity, higher walking speed, and the center of mass (COM) farther from the foot (Anderson et al., 2014; Lockhart et al., 2003; Yamaguchi and Masani, 2016; Yamaguchi et al., 2018). Understanding the relationship between biomechanical factors and RCOF can allow for training of safer techniques and identification of people at an increased slip risk.

The use of RCOF in ladder climbing is emerging and has only been described for some climbing tasks. Notably, RCOF during ladder descent has not yet been characterized. Descending has been investigated in roof-to-ladder transition and in the transition between two sections of an extension ladder (Griffin et al., 2023; Williams et al., 2023). Two biomechanical metrics have been identified as contributing to RCOF: foot angle and body angle (positioning of the trunk relative to the foot). In both steady-state ascent and descending transitions, the RCOF is significantly associated with the foot orientation, where a horizontal foot orientation is associated with a lower RCOF. The RCOF is related to the body angle during ascent (Martin et al., 2020), where the RCOF is lowest when the trunk is aligned above the foot. In the roof-to-ladder transition the medial–lateral body angle was associated with medially directed friction (Griffin et al., 2023).

Other biomechanical factors may also be relevant to RCOF during ladder climbing since they influence slip risk during climbing or RCOF during walking. Heel contact velocity and walking speed are correlated with RCOF during level walking (Anderson et al., 2014; Lockhart et al., 2003). Thus, the foot contact velocity and climbing speed for ladder climbing may also be associated with RCOF. Additionally, foot placement is significantly related to the risk of a slip and to RCOF during ascent, where the rung contacting closer to the toe is associated with a higher slip occurrence (Martin et al., 2020; Pliner et al., 2014). Therefore, foot placement is a potential predictor of the RCOF during descent.

One key difference of ladder climbing versus overground walking is that the slipping plane is defined by the foot orientation instead of the ground. Therefore, the foot orientation defines the direction of shear and normal force vectors (Griffin et al., 2023; Martin et al., 2020; Williams et al., 2023). The reaction force vector, in contrast, is influenced by the location of someone’s trunk COM relative to their contact foot (Griffin et al., 2023; Yamaguchi et al., 2013). In an ideal scenario for minimizing slip risk, the applied force would be normal to the slipping plane, resulting in no required friction. Therefore, the synchronization of foot orientation and body positioning is a potential predictor of the RCOF. In this study we propose a novel metric called the foot-body coupling angle which quantifies the relationship between the body and foot orientation by measuring the angle between the body vector (i.e. the vector from the rung contact point to the trunk COM) and the vector normal to the foot plane (Fig. 1).

The purpose of this study was to determine the relationship between the RCOF and foot and body orientation during descent of a ladder. There were two main hypotheses: 1) RCOF is negatively associated to the foot angle (i.e. a foot oriented toe-down is associated with a lower RCOF) and positively associated with the body angle (i.e. positioning the body further from the foot is associated with a higher RCOF), and 2) RCOF is positively associated with the foot-body coupling angle. In addition to these hypotheses, the impacts of foot contact velocity, climbing velocity, and foot placement on RCOF were also quantified.

The average RCOF was 0.25 with a standard deviation of 0.085. The direction of the anterior/posterior frictional forces, on average, suggests the risk of a forward slip (Fig. 3). No slips occurred during the baseline trials that are reported in this study. Central tendency and spread for the considered biomechanical variables are presented in Table 1.

The RCOF had a strong, positive association with the foot-body coupling angle (t₃₂₃ = 21.8, p < 0.001, R² = 0.606). Also, the foot angle had a strong negative association with the RCOF (t₃₆₂ = −19.8, p < 0.001, R² = 0.534). The body angle had a weak, positive association with the RCOF (t₃₂₃ = −3.4, p = 0.001, R² = 0.061). Finally, weak, negative associations were found between the RCOF and the foot contact velocity (t₃₅₉ = 5.2, p < 0.001, R² = 0.097), the RCOF and trunk COM velocity (t₃₂₁ = −2.4, p = 0.015, R² = 0.047), and the RCOF and foot placement location (t₃₄₇ = 2.2, p = 0.030, R² = 0.043) (Fig. 4). Ladder angle was significant in all models. The 90° angle was generally associated with a higher RCOF than the 75° angle. The size of this effect (the difference between the least squares mean RCOF for 90° and 75°) was 0.153 for foot angle, 0.105 for body angle, 0.017 for foot-body coupling angle, 0.054 for foot contact velocity, 0.024 for trunk COM velocity, and 0.034 for foot placement.

The multivariate model (F_3,322 = 168.0, p < 0.001, R² = 0.610, Fig. 5) and all of the factors (foot angle (t₃₂₂ = −21.31, p < 0.001), body angle (t₃₂₂ = 8.17, p < 0.001), and ladder angle (t₃₂₂ = −2.87p = 0.004)) were significant predictors of RCOF. The effect of ladder angle in this model was small. The significance of both the foot angle and body angle indicates that they provide unique information and explains the high correlation observed for the foot-body coupling angle (Fig. 4C). In this model, foot orientation was negatively correlated to RCOF where the lowest RCOF was found when the foot was in dorsiflexion (toe-up). A 10° increase in the foot angle was associated with a decrease of 0.135 in RCOF. The body angle was positively associated with RCOF where a 10° increase in the body angle was associated with an increase of 0.113 in the RCOF. Thus, each angle had a similar contribution to RCOF further supporting combining them to a single metric.

As predicted, all metrics (foot angle, body angle, foot-body coupling angle, foot contact velocity, trunk COM velocity, and foot placement) were significantly associated with RCOF. The effects for the models with body angle, trunk COM velocity, foot placement, and foot contact velocity were small. However, the effect of foot angle and foot-body coupling angle was large. The foot-body coupling angle had an advantage over the foot angle since it better generalized across the two ladder angles. In the multivariate model, both foot and body angle were significantly associated with RCOF indicating that both variables (and thus the foot-body coupling angle) provide unique contributions to predict the RCOF.

The results indicate that the foot-body coupling angle is a promising predictor for RCOF (during ladder climbing). When used as the sole regressor for RCOF (i.e. a model that excludes the ladder angle), the model had an R² value equal to 0.596, indicating that the foot-body coupling angle can explain a majority of the variance in RCOF. Theoretically, it follows that an ideal climbing position (where the foot-body coupling angle is 0°) occurs when the body vector is perpendicular to the foot vector. Another promising aspect of the coupling angle is that the ladder angle has a small effect on the coupling angle’s relationship with the RCOF. This small effect in both the controlled bivariate analysis and the multivariate test indicates that the foot-body coupling angle is robust to changes in conditions. This work indicates that while the individual metrics do provide meaningful information, the foot-body coupling angle is the best individual predictor of the RCOF.

In level walking, the RCOF is highly correlated (R = 0.94) to the tangent of the angle formed by a vector traveling from the center of pressure (COP) to the whole-body COM (Yamaguchi and Masani, 2016). This can be explained mathematically by a two-legged inverted pendulum model. In ladder climbing, the foot-body coupling is more appropriate than the COP-COM angle because it also considers the shear contact plane orientation. In a post-hoc analysis, we determined that the tangent of the foot-body coupling angle was correlated with the RCOF (R = 0.77), but this correlation was not as strong as that reported in walking (R = 0.94) (Yamaguchi and Masani, 2016). Typically, the strong correlation between the COM angle and force angle is because conservation of angular momentum requires the ground reaction force to be oriented toward the COM to minimize angular acceleration, especially during single stance. However, the arms act as a support during ladder climbing, which may enable the ground reaction forces to deviate more from the body’s COM without inducing angular acceleration. Future work can develop a mechanical model which considers the upper limbs in ladder climbing.

Foot angle was negatively associated with RCOF in both its bivariate correlation model and the multivariate model, meaning that orienting the foot toe-up in space (dorsiflexing) decreases RCOF. This is consistent to what was observed in a roof-to-ladder transition and while descending an extension ladder (Griffin et al., 2023; Williams et al., 2023). However, the opposite relationship was present in ladder ascent, where the lowest RCOF was captured when someone had their foot in plantarflexion (Martin et al., 2020; Pliner et al., 2014). The difference may be due to the fundamental differences in the movements associated with climbing up and down a ladder.

There are some limitations within this study that should be noted. Due to the height restrictions of the laboratory, the participants could only descend from the fifth rung of the ladder, meaning that the maximum height of their feet was around 1.53 m high, which is lower than what is typically seen on a worksite. The perceived risk of climbing to a lower height may have altered the participants’ behavior (Simeonov et al., 2003). Further, the safety equipment (i.e. the shin guards, elbow pads, and knee pads) may have altered the way that the participants were able to move, or they may have altered their behavior due to being observed by the researchers. Finally, more perturbation studies are needed to confirm the relationship between RCOF and slipping risk for ladder climbing.

The significance of the foot-body coupling angle could provide an opportunity for biomechanical training to reduce slip risk. Future work can identify a meaningful way to cue workers to lower their foot-body coupling angle, potentially through biofeedback or training on proper technique. Additionally, this metric could be used to monitor the safety of ladder climbing. With the expansion of safety monitoring systems including camera- and wearable-based technology, foot-body coupling angle could be targeted to identify individuals and work tasks associated with an elevated risk of slip. Finally, minimizing the foot-body coupling angle offers unique benefits for reducing slip risk. Individuals of different sizes or strengths may not be able to climb with a specific body angle. However, many different combinations of foot and body positionings can minimize the slip risk and a person can customize their strategy to their climbing style (climbers who prefer to climb with a large body angle can adapt their foot angle and vice versa). Thus, this is a potentially more flexible intervention than training climbing technique based on foot or trunk angle alone.

This study found that the foot-body coupling angle is a strong predictor of RCOF while descending a ladder. A person who synchronizes their trunk COM location and their foot orientation to minimize their foot-body coupling angle while climbing down a ladder will reduce their slipping risk. These results indicate that there may be an opportunity to train safer ladder climbing techniques to reduce the frequency of ladder slip and fall events.