Low back injuries are most prevalent among working population whose jobs involve manual material handling (MMH) tasks. To create a safe working environment, material handling recommendations and standards were developed. Even with the presence of lifting limits and guidelines, low back injuries remain the single largest category comprising of 27% of all the non fatal occupational injuries.

Previous studies measured threshold limits of the thoracolumbar spine through cadaveric testing. While typical MMH tasks have lifting rates ranging from 1 to 15 lifts/minute, fatigue failure was tested at 15 to 120 lifts/minute. Investigating frequency affects during repetitive lifting was further complicated by variation in specimen preparation, storage, compressive preload, and loading rates. Specimen positioning in accordance with the natural curvature of the spine during testing was missing among prior cadaveric studies.

A special loading fixture was designed for compressive failure tests (CFTs) to position motion segments (MSs) either at Harrison angles or without segment positioning to study the effect of orientation on threshold parameters. A displacement controlled quasi static rate of 4 mm/hr was employed for threshold measurement in terms of force, stress, strain and energy density (ED). Repetitive loading tests (RLTs) were performed at 0.1 or 0.2 Hz where MSs were cycled at an adjusted compressive force calculated using 50% of the failure stress. The behavior of the intervertebral disc was measured using the fatigue parameters: strain, stiffness, and ED.

Segment positioning had a significant (p<0.05) effect on threshold parameters during CFTs while frequency had a significant (p<0.05) effect on fatigue parameters during RLTs. All MSs demonstrated end plate fractures after dissection. Strain and stiffness traced a logarithmic path suggesting their susceptibility to saturation, and strain at the 8th hour of loading never reached its CFTs threshold values. ED includes the effects of force, deformation, and loading rate while demonstrating a linear increase over duration suggesting it might be a better damage measure.

This study helps in understanding disc behavior at two different frequencies while further research is required to quantify the effect of frequency on ED for use in predicting the risk of work related low back injuries.