The term "stability," as used in the field of biomechanics, remains undefined in many clinical cases. This fact can impede the design of therapies intended to enhance joint stability. In fact, Fritz et al, in a review on lumbar instability, concluded that, "At present, much controversy exists regarding the proper definition of the condition, the best diagnostic methods, and the most efficacious treatment approaches." Some progress has been made in the biomechanics field toward the formulation and implementation of stability in musculoskeletal linkages and joints. The purpose of this review is to synthesize and interpret the biomechanical foundation for stability while avoiding mathematical complexity, to demonstrate the notion of stability using specific musculoskeletal examples, and to propose the next logical steps to full utilization of the stability concept for optimal rehabilitation. This review is not intended as a scholarly treatise but rather as a short commentary aimed at providing clinicians with a vantage point for making clinical decisions. Finally, because we are spine biomechanists, and because the original work defining the mechanics of stability of musculoskeletal systems used the spine as an example, this article emphasizes the spine in its examples.

J Orthop Sports Phys Ther. 2001;31(2):96-100.

Key Words: biomechanics, stability

Study Design: Longitudinal, repeated-measures, factorial design. The trunk axial rotation repositioning error was the dependent variable, while the orthoses, test mode (passive versus active), and the testing session were the independent variables.

Objectives: To verify whether lumbosacral orthoses (LSOs) affect proprioception in the lumbar spine and whether these effects change over a 3-week period during which the LSO is consistently worn.

Background: To date, there is no compelling evidence that lumbar orthoses support the spine. One hypothesis advanced by several authors is that they may enhance position sense (proprioception) in the lumbar spine.

Methods and Measures: Fourteen subjects without low back pain wore lumbosacral orthoses 3 hours a day for 3 weeks. Spine proprioception was tested in a seated posture in 3 sessions (days 0, 7, and 21).

Results: A significant 3-way interaction was found between the effects of the orthoses, session, and test mode (P = .03). The ratio of passive to active average error indicated that after 3 weeks of wearing LSO, proprioception in the passive test worsened in relation to the active test with the LSO. In contrast, proprioception in the passive test improved in relation to the active test when performed without the LSO.

Conclusions: The LSO did affect proprioception in the lumbar spine. These effects most likely changed over time due to sensorimotor adaptation. However, no overall proprioceptive benefits could be ascertained from healthy subjects wearing the LSO.

J Orthop Sports Phys Ther. 2006;36(4):225-231, doi:10.2519/jospt.2006.2143.

Key Words: brace, low back pain, lumbar support, spinal orthoses

Objectives: To examine muscle response to sudden trunk loading in athletes with and without a recent history of acute low back injury (LBI).

Background: Impaired neuromuscular function is associated with chronic low back pain. This study examined whether such impairment persists after recovery from an acute LBI.

Methods and Measures: Seventeen athletes who had a recent history of acute LBI and 17 matched healthy controls were tested. At the time of testing (mean = 56 days postinjury, range = 7–120 days postinjury), all athletes were symptom free and had returned to regular competition. Subjects performed isometric exertions in trunk flexion, extension, and left and right lateral bending against a trunk restraining cable. Upon reaching the target isometric force, the cable was released to impose sudden loading on the lumbar spine. Surface EMG signals from 12 major trunk muscles were recorded. The shut-off and switch-on latencies and number of muscles responding to sudden loading were compared between the 2 groups.

Results: In all 4 testing directions, the athletes with a recent history of acute LBI shut off significantly fewer muscles and did so with delayed latency. On average, the injured subjects shut off 4.0 out of 6.0 (SD = 1.3) muscles compared to 4.6 out of 6.0 (SD = 1.3) muscles in the control group. The average muscle shut-off latency was 71 (SD = 31) milliseconds for the injured and 50 (SD = 21) milliseconds for the control subjects. No differences were found in number or latency of muscles switching on.

Conclusions: These objective measures of neuromuscular function indicated an altered muscle response pattern to sudden trunk loading in athletes following their clinical recovery from a recent acute LBI.

J Orthop Sports Phys Ther 2002;32(11):568–575.

Keywords: lumbar spine, motor control, sudden loading