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. Author manuscript; available in PMC: 2025 Aug 25.
Published in final edited form as: Knee. 2024 Jul 22;49:217–225. doi: 10.1016/j.knee.2024.07.006

Differences in knee joint moments between individuals who are living with obesity and those of a healthy weight when negotiating stairs

Todd J Leutzinger a,*, David C Kingston a, Danae M Dinkel b, Elizabeth Wellsandt c, Brian A Knarr a
PMCID: PMC12376230  NIHMSID: NIHMS2104341  PMID: 39043017

Abstract

Background:

Individuals who are living with obesity often adopt alternative lower limb walking mechanics compared to persons with a healthy weight. Stair negotiation is a common activity of daily living that, when used consistently with diet and other physical activity, can help promote the reversal of health-related risk factors associated with people who are obese. The purpose of this study was to determine how stair negotiation affects normalized and non-normalized peak knee extension and abduction moments in young adults who live with obesity (BMI between 30 and 40 kg/m2) compared to adults with a healthy weight (BMI between 18.5 and 25 kg/m2).

Methods:

Fifteen young adults living with obesity and fifteen with a healthy weight performed stair ascent and descent walking trials on a 3-step instrumented staircase at a self-selected walking speed. A one-way ANCOVA (covariate: gait speed) was used to compare knee moment variables between groups.

Results:

No significant differences were found between groups in peak knee joint moments normalized to body mass. The individuals living with obesity demonstrated significantly larger non-normalized peak knee extension moments during stair ascent and descent but no differences in the non-normalized peak knee abduction moments for stair ascent or descent.

Conclusion:

Results of this study indicate differences in non-normalized peak knee extension moments between BMI groups. The young age of the obese group may have contributed to minimal differences overall. Future research should determine how these findings differ in an older obese population and how using a handrail would affect these results.

Keywords: Stairs, Knee joint moment, Obesity, Kinetics

1. Introduction

Obesity is an epidemic in the United States—more than 42% of adults report a body mass index (BMI) greater than 30 kg/m2—with projections estimating nearly half (48.9%) of US adults will become obese (BMI > 30 kg/m2) by the year 2030 [1,2]. Obesity is a concern amongst many health professionals because it is a risk factor for various comorbidities such as diabetes, osteoarthritis (OA), heart disease, and certain cancers [3]. Obesity is one of the leading causes of knee OA development [4] and may result from increased loading and inflammation [5]. Individuals who are living with obesity often employ alternative walking strategies compared to individuals with a healthy weight, and it is believed these altered mechanics are an attempt to reduce knee joint loading. For example, individuals who are living with obesity walk at slower gait speeds, with shorter stride lengths [6], increased work at the ankle joint, and decreased knee extensor moments compared to persons with a healthy weight [7]. Additionally, there appears to be a relationship between gait biomechanics and knee OA development [8]. Individuals who walk with higher knee adduction moments are more likely to develop knee OA, signifying a potential link between obesity, walking mechanics, and knee OA progression [4,8].

Stair negotiation is a common activity of daily living, but the lower limb joint mechanics of how individuals who are living with obesity traverse stairs are largely unknown. Stair negotiation, when performed regularly, helps promote quadriceps strength, cardiovascular health, and weight loss, which are important factors in the maintenance of a healthy knee joint [9,10]. During stair negotiation, Egret et al. (2019) found that individuals who are living with obesity produce significantly lower normalized peak hip adduction and knee abduction moments during stair descent [11]. Conversely, Egret et al., (2019) found individuals who are living with obesity present a higher peak hip adduction moment and peak knee anterior shear force during stair ascent compared to individuals of a healthy weight when these measures are normalized to body mass [11]. However, Egret et al. (2019) studied a group of individuals who are living with obesity that were nearly 15 years older than the group of persons with a healthy weight, limiting the conclusions that can be made from this study. Older individuals have been shown to experience lower leg strength and altered walking mechanics compared to younger individuals [12]. Additionally, step data were analyzed on the transition step between the staircase and the floor at the bottom of the staircase, which requires higher neuromuscular control and may have augmented differences between groups. Therefore, it is important to build upon this previous work and investigate knee joint mechanics in the obese population during consistent stair walking compared to individuals with a healthy weight who are of a similar age.

External knee extension and abduction moments are biomechanical variables that are adequate surrogate measures for predicting knee joint loading during walking [13-15]. Manal and colleagues (2015) demonstrated that peak knee adduction moment alone accounted for 63% of the variance in predicting peak medial loading at the knee joint [14]. Peak knee flexion moment was also a significant predictor, accounting for an additional 22% of the variance in peak medial knee joint loading. Kutzner et al. (2013) also demonstrated a moderate correlation between knee adduction moment and medial joint contact force throughout all of stance phase (R2 = 0.56) and during late stance phase (R2 = 0.51) of gait, indicating its efficacy as a surrogate measure for knee joint loading [16]. Therefore, the purpose of this study was to investigate how stair negotiation affects knee joint moments in individuals who are living with obesity and individuals with a healthy weight. We assessed normalized peak knee extension and knee abduction moments during stair negotiation in young obese adults (BMI between 30 and 40 kg/m2) and persons with a healthy weight (BMI between 18.5 and 25 kg/m2). We hypothesized that individuals who are living with obesity would produce significantly greater normalized and non-normalized peak knee extension and abduction moments compared to persons with a healthy weight for stair ascent and stair descent.

2. Methods

2.1. Study participants

This study was approved by the Institutional Review Board at the University of Nebraska Medical Center (IRB# # 777-19-EP), and research participants provided written informed consent to participate. Fifteen individuals who are living with obesity and fifteen persons with a healthy weight were recruited for this study [11,17](Table 1). Inclusion criteria for the group of individuals who are living with obesity included 1) age 19–60 years old, 2) BMI greater than or equal to 30 kg/m2 but less than 40 kg/m2, 3) the ability to go up and down stairs without the use of a handrail in a step-over-step fashion. Inclusion criteria for the healthy weight control group were: 1) age 19–60 years old, 2) BMI greater than or equal to 18.5 kg/m2, but less than 25 kg/m2, 3) the ability to go up and down stairs without the use of a handrail in a step-over-step fashion. Exclusion criteria for both groups included: 1) a history of major surgery to a lower extremity joint such as but not limited to a joint replacement or reconstruction surgery 2) systemic inflammatory arthritis 3) the presence of neurological disease 4) lower extremity injury or pain that would inhibit or alter walking 5) the inability to provide consent for study participation.

Table 1.

Study participant anthropometric data.

Healthy Weight Obese Weight
Mean ± Stdev Max Min Mean ± Stdev Max Min
Height (m) 1.76 ± 0.11 1.98 1.6 1.80 ± 0.08 1.97 1.62
Body Mass (kg) * 68.68 ± 10.88 87.54 52.16 108.35 ± 12.49 131.54 84.82
BMI (kg/m2) * 22.01 ± 1.48 3.98 19.48 33.32 ± 1.61 35.54 31.22
Age (years) 27 ± 8 58 20 28 ± 7 49 21
Sex (M/F) 8 Males, 7 Females 13 Males, 2 Females
Waist Circumference (cm) * 79.17 ± 7.78 92 68 106.25 ± 7.62 119 88
Right Knee Extensor Strength (Nm/kg) 3.08 ± 0.71 3.99 1.97 2.89 ± 0.62 4.6 2.03
Left Knee Extensor Strength (Nm/kg) 3.04 ± 0.69 4.05 2.02 2.75 ± 0.73 4.58 1.87
Physical Activity (MET-minutes) 5344 ± 8528 30,840 360 2502 ± 2505 8040 0
Stair Descent Speed (m/s) * 0.71 ± 0.11 0.99 0.51 0.63 ± 0.07 0.81 0.54
Stair Ascent Speed (m/s) 0.59 ± 0.06 0.7 0.48 0.57 ± 0.08 0.75 0.47
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*

indicates significant differences between groups.

2.2. Study procedures

All study participants completed five stair ascent and five stair descent walking trials at their self-selected pace. Study participants began stair ascent trials and completed stair descent trials approximately 3–4 m away from the stairs. Study participants were asked to negotiate the stairs facing completely forward (perpendicular to the staircase) in a step-over-step fashion at a comfortable walking speed. For stair ascent and stair descent, the stance phase of the step occurring on the second stair was analyzed, as this is considered consistent stair walking according to previous literature [18,19]. Heel strike was classified as the first spike in force plate data when the participant’s foot contacted the second stair. Toe off was considered to be the last frame in which the participant was in contact with the second stair and force data were present. Participants were instructed to step on the second stair using their right leg. A picture of the staircase setup can be seen in Figure 1. The stairs measured 0.165 m high, 0.91 m wide, and 0.3 m deep. Both obese and healthy weight study participants were asked to complete the Global Physical Activity Questionnaire (GPAQ) to measure overall physical activity levels. Previous research has shown this questionnaire to be reliable and valid compared to similar questionnaires often used in the literature [20]. Waist circumference was also measured on each participant’s torso directly above their iliac crest [21].

Figure 1.

Figure 1.

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Experimental setup of the instrumented staircase.

2.3. Motion capture and data analysis

Stair negotiation took place on an instrumented staircase with three force plates, measuring forces in 6 degrees of freedom (Bertec Corp, Columbus, OH, model number: FP3192-06-PT-2000). Force data were collected at 1000 Hz. A modified Helen Hayes marker set with thigh and shank clusters was used to record motion. A model with 6 degree of freedom segments in Visual3d was used analyze joint kinematics/kinetics [22]. Fifty-seven retro-reflective markers were placed on the participants’ upper and lower extremities and torso (Figure 2). Marker data were smoothed in Qualysis using a Butterworth filter with a cutoff frequency of 10 Hz. The marker model was made in Visual 3D where lower limb joint kinematics were used to calculate net internal knee extension and knee abduction moments. Marker data were collected using a 12-camera motion analysis system at 100 Hz (Oqus 500/100, Qualysis Ab, Göteborg, Sweden). Analog force plate data and lower limb joint kinematics were used to calculate net internal knee extension and knee abduction moments using Visual 3D (version #v2021.04.1, C-Motion, MD, USA). Joint moments were normalized to each participant’s body mass [23,24].

Figure 2.

Figure 2.

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Marker set used in this study. The marker set consisted of markers placed on the xyphoid process, sternum, C7 vertebrae, T8 vertebrae, anterior and posterior shoulder, acromion process, upper arm, medial and lateral elbow, forearm, hand, distal segments of the radius and ulna, anterior superior iliac spines, posterior superior iliac spines, sacrum, greater trochanters, thigh clusters, lateral epicondyle of the knees, lower leg shank clusters, between the first and second phalanges of the foot, the side of the fifth phalanx at the metatarsal phalangeal joint, the lateral side of the calcaneus, and the posterior side of the calcaneus bone.

2.4. Knee extensor strength

Each study participant’s knee extensor strength was obtained through maximal voluntary isometric contractions using a dynamometer (System 4 Pro, Biodex Medical Systems Inc., Shirley, NY, USA). Straps were used to secure the trunk and pelvis to better ensure strength outputs came from the knee extensors alone. The lateral epicondyle of the femur was aligned with the input axis of the dynamometer. The distal cuff was placed just above the participant’s ankle joint. All isometric testing was performed at a knee angle of 90° with the horizontal [25]. Study participants performed three sets of 10-second isometric contractions for each leg, with peak isometric knee extensor torque being the outcome measure of interest. Verbal encouragement was provided during testing. Torque data were normalized to each participant’s body mass.

2.5. Statistical analysis

Independent samples t-tests were used to determine differences between the individuals who are living with obesity and individuals with a healthy weight for anthropometric data, knee extensor strength, and gait speed. An independent samples Mann-Whitney U test was used to determine differences in physical activity data between groups. An ANCOVA (covariate: gait speed) design was used to test for significant differences between groups (obese and healthy weight) for peak knee extension and abduction moments. Gait speed was included in the model as a covariate if it was considered to be significant in the model. Gait speed was calculated using the mean of the first derivative of the sacral marker position from initial foot contact on the first step to toe-off from the final step of each trial. An a priori alpha value of p < 0.05 was used to determine statistical significance.

3. Results

The group of individuals who are living with obesity had significantly greater body mass (p < 0.001), BMI (p < 0.001), and waist circumference (p < 0.001) values compared to the persons with a healthy weight (Table 1). The group of individuals who are living with obesity also walked significantly slower than the individuals with a healthy weight during stair descent (obese: 0.63 ± 0.07 m/s, healthy: 0.71 ± 0.11 m/s, p = 0.036), but speed was not a significant factor in any of the ANCOVA statistical tests. No significant differences between groups were found for physical activity (p = 0.412) or knee extensor strength normalized to body mass (right leg: p = 0.462, left leg: 0.307). Additionally, no differences were found for either stair ascent (p = 0.496, d = 0.017) or stair descent (p = 0.362, d = 0.031) regarding the normalized peak knee extension moment between the individuals who are living with obesity and individuals with a healthy weight (Figure 3).

Figure 3.

Figure 3.

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Normalized peak internal knee extension moment during both stair ascent and stair descent for the individuals with a healthy weight and individuals who are living with obesity. No significant differences were found.

No significant differences were found for either stair ascent (p = 0.119, d = 0.087) or stair descent (p = 0.057, d = 0.128) regarding the normalized peak knee abduction moment between the obese and healthy weight groups (Figure 4).

Figure 4.

Figure 4.

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Normalized peak internal knee abduction moment during both stair ascent and stair descent for the individuals with a healthy weight and individuals who are living with obesity. No significant differences were found.

The individuals who are living with obesity demonstrated significantly larger non-normalized peak knee extension moments during both stair ascent (p < 0.001; d = 1.83) and descent (p < 0.001; d = 2.21) (Figure 5), but no differences in the non-normalized peak knee abduction moments for stair ascent (p = 0.078, d = 0.101) or stair descent (p = 0.711, d = 0.005) (Figure 6).

Figure 5.

Figure 5.

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Non-normalized peak internal knee extension moment during both stair ascent and stair descent for the individuals with a healthy weight and individuals who are living with obesity. No significant differences were found.

Figure 6.

Figure 6.

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Non-normalized peak internal knee abduction moment during both stair ascent and stair descent for the individuals with a healthy weight and individuals who are living with obesity. No significant differences were found.

4. Discussion

The purpose of this study was to investigate differences in normalized peak internal knee extension and abduction moments during stair negotiation in individuals who are living with obesity (BMI between 30 and 40 kg/m2) and persons of a healthy weight (BMI between 18.5 and 25 kg/m2). We rejected our hypothesis that the group of individuals who are living with obesity would produce a significantly greater normalized peak knee extension moment and normalized peak knee abduction moment compared to the healthy control group for stair ascent and stair descent. Our results contradict the study by Strutzenberger et al. (2011), which demonstrated differences in normalized peak ankle, knee, and hip joint moments between children who are living with obesity and children with a healthy weight during stair ascent and stair descent [26]. This may be due to strength deficits relative to body mass commonly seen in children who are living with obesity [27]. A lack of strength may have resulted in the children who are living with obesity needing to compensate and adapt their body, which would produce altered lower limb joint kinematics compared to their healthy weight counterparts. The current study found no significant quadriceps strength differences between the obese and healthy control groups. Additionally, Strutzenberger et al. (2011) controlled walking speed by having all individuals walk at 110 steps per minute, which was considered to be comfortable for the study participants [26]. The current study found significant differences in walking speed during stair descent between groups, which may have masked differences between groups. However, this is unlikely, given that speed was not a significant factor in ANCOVA statistical tests (see Figure 6).

The results of the current study are also contrary to Egret et al., (2019), who investigated lower limb biomechanics of individuals who are living with obesity and individuals with a healthy weight on the transition step between the staircase and the floor at the bottom of the staircase [11]. In this study, a healthy control group presented a significantly larger normalized peak knee abduction moment compared to the group of individuals who are living with obesity during stair descent [11]. However, they utilized a group of individuals who are living with obesity (BMI: 34.5 ± 0.92 kg/m2) with an average age of 14.55 years older than their healthy weight control group and more than 9 years older than either of the groups in the current study. Similar to the current study that sought individuals with no knee joint pain for enrollment, participants in the study by Egret et al. (2019) also reported no joint pain. However, considering age and obesity are two of the biggest risk factors for knee OA development [28], we believe this age difference between study participant groups may be a reason why Egret et al. (2019) found differences between groups and the current study did not. Additionally, data in the Egret et al. (2019) study were taken from the step at the bottom of the staircase for both stair ascent and stair descent. This step is considered a transition step and requires higher neuromuscular recruitment from the supporting limb during stair descent compared to steady-state stair walking [29]. Lastly, Egret et al. (2019) did not consider walking speed. Slower walking speeds are common in the obese population [30] and can significantly decrease knee joint moments [31]. In the current study, significant differences were present between groups when investigating knee abduction moment normalized to body mass during stair descent without using speed as a covariate (p = 0.018, d = 0.241). The healthy group presented a significantly larger knee abduction moment than the group of individuals who are living with obesity, which is consistent with the findings of Egret et al (2019). Egret et al. (2019) believed the decrease in knee abduction moment resulted from individuals with a healthy weight having more confidence in their ability to descend the stairs. This would lead to higher muscle activation and overall greater knee joint loading. However, the current study looked at confidence levels of both groups when negotiating stairs using the Stair Self-Efficacy Questionnaire [32] and found no difference between the two groups (p = 0.696). Previous research indicates no differences in knee abduction moment during overground walking at standardized and self-selected speeds between these two groups [33], future research is needed to compare knee joint loading and muscle activation during stair negotiation.

4.1. Limitations

A limitation of this study is the relatively healthy young obese population in this study. This group is likely not a true representation of the obese population in the community given the inclusion and exclusion criteria of the study excluding many comorbidities found in this population [33]. However, by utilizing a relatively healthy young population of individuals who are living with obesity, we were able to eliminate many confounding factors that may have altered study results. Additionally, with only three stairs, the staircase used in this study did not accurately represent stair negotiation in the real world. Although previous literature has indicated the second step to be consistent stair walking [18], this does not account for fatigue that may occur at the top of a large staircase and how a handrail would be used in this situation, especially for an obese individual. Lastly, individuals were instructed not to use the handrails, a compensatory strategy used by many individuals to aid in stair negotiation. Future research should investigate how using a handrail affects knee biomechanics during stair negotiation in individuals who are living with obesity.

Contrary to previous research [34,35], the group of individuals who are living with obesity presented no differences in non-normalized peak knee abduction moments despite significantly larger non-normalized peak knee extension moments during both stair ascent and descent. Previous research has shown knee extension and knee abduction moments are good surrogate measures for knee joint loading [14]. The knee abduction moment can account for 63% of the variance in predicting peak medial knee loading, but the knee extension moment still accounts for 22% of the variance. In the current study, the overall load at the knee was likely greater in the obese group than in persons with a healthy weight due to an increased knee extension moment and higher body mass. Previous literature has found excessive medial knee joint loading is a major contributor to knee joint degradation [36,37]. However, the obese group in this study was in overall good health despite their elevated BMI, and the absence of increased medial knee joint loading may indicate more normal joint health. Nonetheless, these findings alone cannot fully describe the true nature of knee joint loading in this population, and future research examining knee joint loading is needed.

5. Conclusions

The results of this study indicate no differences in normalized knee joint moments between young individuals who are living with obesity and individuals of a healthy weight. The relatively young age and good health of the group of individuals who are living with obesity may have influenced the lack of differences present. Future research should determine how these findings differ in an older, more unhealthy, obese population and how using a handrail would affect these outcomes.

Funding

Equipment for this study was supported by the Center of Research in Human Movement Variability of the University of Nebraska at Omaha and the National Institutes of Health [P20GM109090].

Footnotes

CRediT authorship contribution statement

Todd J. Leutzinger: Writing – original draft, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. David C. Kingston: Writing – review & editing, Validation, Supervision, Software, Resources, Methodology, Conceptualization. Danae M. Dinkel: Writing – review & editing, Validation, Supervision, Methodology, Conceptualization. Elizabeth Wellsandt: Writing – review & editing, Validation, Methodology, Conceptualization. Brian A. Knarr: Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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