Invited Commentary.

We are grateful for the opportunity to comment on the article by Neumann. The elderly subjects with impairments who participated in the study and the excellent research design suggest that this will be an important and oft-quoted contribution to the hip rehabilitation literature.

The hip abductor force model proposed by Dr Neumann and published numerous times in the physical therapy literature has provided an uncomplicated and conceptually palatable means of understanding coronal-plane hip biomechanics.[1] Indeed, Dr Neumann's model has facilitated physical therapy teaching by illustrating clearly the real reason we recommend using a cane contralateral to a painful hip.[2] Any concept that comes by us in a straightforward and concise manner, however, often does so at a cost. If interpreted verbatim, the benefits of Dr Neumann's static, free body diagram hip model seem to imperil the mechanical laws described by Isaac Newton in his famous 1687 treatise, Philosophiae Naturalis Principia Mathematica.[3]

In his hip model, Dr Neumann describes the hip abductor force (HAF) and joint reaction force (JRF) as being essentially parallel. When a rigid body is assumed to be acted on by 3 forces (a 3-force body), classical Newtonian static force analysis requires that the lines of action of the 3 forces concur. That is, their lines of action must intersect at a common point. This was correctly depicted by Pauwels[4] more than 6 decades ago and later published in a classic paper by Blount.[5] Even the mechanics of a teeter-totter follow this fundamental law; the 3 parallel forces intersect at infinity.

When modeling the pelvis as a 3-force body, we know of only 3 conditions that allow the JRF and HAF to be parallel: (1) if the body weight vector is equal to zero (the pelvis is then a 2-force body and subject to equal and opposite action and reaction forces), (2) if the hip joint center and abductor insertion are coincident, or (3) if all 3 forces are parallel (the case of the teeter-totter). Conditions 1 and 2 are anatomically and physiologically impossible on earth. Condition 3 is not a "generalized" scenario; only in extreme cases will the body weight vector be parallel to the line of action of muscle and hip reaction force (such as excessive leaning to one side) and will certainly be ephemeral.

On the one hand, it must be recognized that many more than 3 forces act at the hip, which allows any pair or group of forces to act parallel to one another (eg, the relatively parallel fibers of the gluteus maximus muscle). On the other hand, if the limitations of simple static models are acknowledged, they may aid students in conceptualizing the complex biomechanics of a joint such as the hip. Indeed, previous reports from our group using direct measurements from instrumented hip prostheses[6,7] support much of what Dr Neumann's model predicts. We were surprised that Dr Neumann did not cite his own work with our group,[8,9] which is also consistent with the data presented in this report. We believe, however, that the reader should be well informed about the limitations of 3-force body static force analysis when interpreting results derived from such models. An article in an upcoming issue of Gait & Posture[10] details the extent to which static and quasi-static free body diagrams err, in part because they ignore the nearly 2 dozen muscles[11] (eg, the gemellus and quadratus femoris muscles) that co-contract in complex ways during human movement. Electromyographic results from a single muscle, especially those explained by incorrect free body diagrams, may tell an important, but necessarily incomplete, story of hip biomechanics.

References

[1] Neumann DA. Hip abductor muscle activity in persons with a hip prosthesis while carrying loads in one hand. Phys Ther. 1996;76: 1320-1330.

[2] Neumann DA. Biomechanical analysis of selected principles of hip joint protection. Arthritis Care Res. 1989;2:146-155.

[3] Newton I. The Principia. Motte A, trans. Amherst, NY: Prometheus Books; 1995.

[4] Pauwels F. Der Schenkelsbruch: Ein Mechanisches Problem. Stuttgart, Germany: Ferdinand Enke; 1935.

[5] Blount WP. Don't throw away the cane. J Bone Joint Surg Am. 1956;38:695-708.

[6] Fagerson TL, Krebs DE, Harris BA, Mann RW. Examining shibboleths of hip rehabilitation using in vivo contact pressures from an instrumented hemiarthroplasty. Physiotherapy. 1995;81:533-540.

[7] McGibbon CA, Krebs DE, Mann RW. In vivo hip pressures during cane and load-carrying gait. Arthritis Care Res. 1997;10:300-307.

[8] Neumann DA, Krebs DE, Riley PO, Mann RW. In vivo hip joint pressures while using one or two canes. Phys Ther. 1994;74(suppl):S133.

[9] Neumann DA, Krebs DE, Riley PO, Mann RW. In vivo hip joint pressures while using a cane and carrying a single hand-held load. In: Proceedings of the 12th International Congress of the World Confederation for Physical Therapy; June 25-30, 1995; Washington, DC. Alexandria, Va: American Physical Therapy Association; 1995:731.

[10] Park SS, Krebs DE, Mann RW. Hip muscle co-contraction: evidence from concurrent in vivo pressure measurement and force estimation. Gait & Posture. In press.

[11] Fagerson TL. The Hip Handbook. Boston, Mass: Butterworth-Heinemann; 1998.

David E Krebs, PT, PhD Professor and Director Massachusetts General Hospital Biomotion Laboratory and MGH Institute of Health Professions Boston, MA 02114

Chris A McGibbon, PhD Assistant Professor and Assistant Director Massachusetts General Hospital Biomotion Laboratory and MGH Institute of Health Professions

Timothy L Fagerson, PT Orthopaedic PT Services Inc 332 Washington St Suite 10 Wellesley Hills, MA 02481