Effects of obesity on lower extremity muscle function during walking at two speeds

Highlights

• We used musculoskeletal models to quantify muscle function in obese adults.

• Kinematic differences exist at certain speeds in obese vs. nonobese adults.

• Differences in the function of certain muscles exist between groups.

• Gait adaptation may be due to abnormal muscle requirements in obese adults.

Abstract

Walking is a recommended form of physical activity for obese adults, yet the effects of obesity and walking speed on the biomechanics of walking are not well understood. The purpose of this study was to examine joint kinematics, muscle force requirements and individual muscle contributions to the walking ground reaction forces (GRFs) at two speeds (1.25 m s⁻¹ and 1.50 m s⁻¹) in obese and nonobese adults. Vasti (VAS), gluteus medius (GMED), gastrocnemius (GAST), and soleus (SOL) forces and their contributions to the GRFs were estimated using three-dimensional musculoskeletal models scaled to the anthropometrics of nine obese (35.0 (3.78 kg m⁻²)); body mass index mean (SD)) and 10 nonobese (22.1 (1.02 kg m⁻²)) subjects. The obese individuals walked with a straighter knee in early stance at the faster speed and greater pelvic obliquity during single limb support at both speeds. Absolute force requirements were generally greater in obese vs. nonobese adults, the main exception being VAS, which was similar between groups. At both speeds, lean mass (LM) normalized force output for GMED was greater in the obese group. Obese individuals appear to adopt a gait pattern that reduces VAS force output, especially at speeds greater than their preferred walking velocity. Greater relative GMED force requirements in obese individuals may contribute to altered kinematics and increased risk of musculoskeletal injury/pathology. Our results suggest that obese individuals may have relative weakness of the VAS and hip abductor muscles, specifically GMED, which may act to increase their risk of musculoskeletal injury/pathology during walking, and therefore may benefit from targeted muscle strengthening.

Introduction

Obesity is a worldwide public health concern and obese adults and children are advised to engage in daily physical activity. Walking is a recommended form of physical activity for obese adults because it is convenient and suitable to elicit a moderate-vigorous metabolic response [1]. However, obese individuals have lower relative muscle strength compared to nonobese individuals [2]. Weakness and susceptibility to fatigue of certain key muscles (e.g. vasti (VAS) and gluteus medius (GMED)) can result in an abnormal gait pattern due to their critical role in locomotor tasks [3], predisposing individuals to musculoskeletal injury or pathology (e.g. large joint osteoarthritis (OA) and low back pain) [4], [5]. In addition, muscle force requirements increase with walking speed [6], so at the faster walking speeds used during exercise, certain muscles, including those responsible for forward progression (e.g. the gastrocnemius (GAST) and soleus (SOL)), may be unable to effectively perform their respective functions, resulting in gait deviations that may increase the risk of musculoskeletal injury/pathology.

Surprisingly, the degree to which obesity affects gait kinematics and kinetics is not clear. Some studies report that kinematics are similar in obese and nonobese groups [7], [8], while others report that obese individuals walk with a more extended leg and similar knee extensor moments during stance and greater step width compared to their nonobese counterparts [9], particularly at faster walking speeds. Unfortunately, there is limited information regarding how investigators did or did not account for the peripheral adiposity that obscures the motion of the underlying skeleton. Thus, differences in methodology may explain these equivocal kinematic results. In addition, studies that have reported lower extremity gait biomechanics in obese individuals [8], [9] have not provided a quantitative assessment of individual muscle function, which may help explain the observed gait patterns.

Musculoskeletal simulations can provide us with an improved understanding of the force requirements and roles that individual muscles play during locomotor tasks [10]. Recent studies have estimated the contributions of individual muscles to the ground reaction force (GRF) during walking in nonobese adults [11], [12]. These studies have shown that during early stance, VAS and GMED muscles are significant contributors to the vertical GRF (GRF_V), and function to decelerate and support the body, while during mid-late stance, the gastrocnemius (GAST) and soleus (SOL) are the primary contributors to the GRF_V and the anterior-posterior GRF (GRF_AP). In the frontal plane, GMED acts to maintain mediolateral (ML) stability and balance, and has been shown to be the primary contributor to the ML GRF (GRF_ML) [13]. Unlike in the sagittal plane, where a more aligned skeleton would reduce knee extensor muscle requirements, support and stability of the body in the frontal plane is largely accomplished by the hip abductor muscles (e.g. GMED). The effect of GMED weakness may be altered frontal plane kinematics of the pelvis (e.g. increased pelvic obliquity, an increase in pelvic drop of the contralateral hip) resulting in pathological hip joint articulation [14]. For this study, we focused our investigation on the muscles that have large contributions to all three components of the GRF (VAS, GMED, GAST, and SOL) [12].

The purpose of this study was to quantify joint kinematics, estimate individual muscle forces (VAS, GMED, GAST, SOL), and the individual muscle contributions to the walking GRFs at two speeds (1.25 and 1.50 m s⁻¹) in obese and nonobese adults. It has been reported that obese adults walking with a more erect posture and similar knee extensor joint torques compared to nonobese adults [9], suggesting reduced knee extensor muscle forces. We hypothesized that (1) peak knee flexion during stance would be less, while pelvis obliquity would be greater in the obese vs. nonobese group, and the differences between the obese and nonobese groups would be greater at the faster walking speed; (2) absolute and lean mass normalized forces for all muscles, except VAS, which we predict to be similar, would be greater in the obese vs. nonobese adults at both speeds; and (3) VAS contribution to the GRF_V would be similar between the obese and nonobese individuals at a velocity of 1.25 m s⁻¹ but would be reduced at a velocity of 1.50 m s⁻¹ in the obese group.