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Study: The Effects of Obesity on Lifting Performance; Authors Call for New Obesity Measurement to Replace Body Mass Index (BMI) Method

Obese individuals, as defined by Body Mass Index (BMI), performed symmetrical and asymmetrical lifting tasks with greater rotational velocity, rotational acceleration, sagittal velocity and sagittal acceleration compared to normal sized individuals in a laboratory study involving 12 subjects.  These data, by Xu et al., were unexpected in that it was anticipated obese individuals would perform lifting tasks slower so as to avoid higher and potentially injurious forces to the low back. Complicating the picture was the finding that there were no significant back strength differences between the normal and obese classified volunteers.
To perform a lift, the weight of the upper body mass (trunk and upper extremities) and the load is countered by low back muscle contraction forces. As upper body mass increases, more low back forces are needed to perform the lift, causing increased strain to low back muscle, ligament and disc tissues.
Obese individuals, already straining low back tissues more than normal sized individuals due to excessive mass, would be demanding even more from low back tissues when performing as they did in this study – by moving with greater speed and acceleration, creating higher inertial forces.
Xu et al. felt the unexpected results may rest in the way BMI is calculated: whole body mass in kilograms divided by the square of stature in meters. BMI does not take into account individual conditioning. A person with a high percent body fat can have the same high BMI value as a person with a very muscular build. The authors suggest a more functional definition of obesity is needed based on strength to body mass. 
The authors further state this weakness in BMI qualification may explain the mixed literature findings associating low back pain with high BMI values.
Study Design
Twelve volunteers (six normal weight individuals with a BMI less than 25 kg/m2 and six obese weight individuals with a BMI greater than 30 kg/m2) were recruited from a college and adjacent area population.
Experiment Procedure
The maximum trunk extension force capability of each volunteer at 45° of trunk flexion was measured using a lumbar dynamometer. From this value, a load weight (box weight) was calculated that would reflect the subject’s 10% and 25% maximum voluntary contraction (MVC) strength. Varying box weight in accordance to the subject’s MVC was done to make the weight a relative constant across subjects. Differences in lifting manner/method would reflect subject obesity as opposed to strength capability.
A box (36 cm deep, 36 cm wide, and 30 cm tall with handles located 19 cm from the ground) was placed on a wooden platform level with the force plate upon which the subject stood. The box was weighted to reflect the volunteer’s 10% and 25% MVC.
All subjects initially stood relative to the box such that there was a 51 cm horizontal moment arm. The volunteer lifted the box from the lifting platform to mid-chest height using a free-style form without moving their feet. A series of lifts as outlined in Table 1 were performed.
Orientation of lift
Weight of box
Number of lifts in one trial
Number of trials
Center of weighted box was directly in front of subject in the mid-sagittal plane with feet pointing parallel to mid-sagittal plane (symmetrical lift).
10% MVC
25% MVC
Center of weighted box was 45° to the right of mid-sagittal plane with feet pointing parallel to mid-sagittal plane (asymmetrical lift).
10% MVC
25% MVC
Table 1: Description of lifts performed in this experiment.
The trials were performed in a random manner with a one-minute break between each trial.
Experiment Measurements
The Lumbar Motion Monitor (LMM) was attached to each subject to measure trunk angular position in the sagittal, coronal and transverse planes. Motion relative to time provided angular velocity and acceleration data. Key measurements of interest were:
  • Peak sagittal flexion angle
  • Peak sagittal extension velocity
  • Peak sagittal extension acceleration
  • Peak rotational position
  • Peak rotational velocity
  • Peak rotational acceleration
Both inter-subject and intra-subject data were recorded.
Two force plates (one for each foot) were used to evaluate ground reaction forces and movements. Only the right foot force plate was considered of value due to the right asymmetric position of the load. Key measurements of interest were:
  • Lateral shear
  • Anterior shear
  • Vertical ground reaction forces
Other Comments and Findings
Compared to the normal BMI individuals, the obese subjects demonstrated:
  • a 59.2% higher twisting velocity
  • a 57.6% higher twisting acceleration
  • a 30.4% higher sagittal velocity
  • a 50.5% higher sagittal acceleration
Neither peak sagittal flexion angle or peak transverse position were significantly affected by BMI. Also, none of the ground reaction forces were significantly affected by BMI.
BMI classifications are presented in Table 2.

Weight Category
18.5 to 25 kg/m2
25 to 30 kg/m2
30 to 40 kg/m2
Extremely obese
Greater than 40 kg/m2
Table 2: Weight categories for BMI calculations.
Study Concerns/Limitations
The authors point out:
  1. The study interpretations are compromised by the use of the BMI as the indicator of obesity.In the workplace, a worker is expected to lift items with a variety of mass sizes – not just those that require 10% to 25% maximum voluntary contraction.  
Other points:
  1. Starting all lifts with a 51cm moment arm may overlook anthropometric differences (i.e., reach) among subjects that can effect lifting characteristics. 
  2. This study had a relatively small sample size of 12 volunteers.
Article Title: The effects of obesity on lifting performance
Publication: Applied Ergonomics 39, 93-98, 2008

Authors: X Xu, G A Mirka and S M Hsiang

This article originally appeared in The Ergonomics Report™ on 2007-11-13.