The Poetry of…Microprocessors

Home > Articles > The Poetry of…Microprocessors
By Phil Stevens, MEd, CPO, FAAOP

In the article, "The Poetry of...Feet?" (The O&P EDGE, October 2010), I introduced the concept of Patient-Oriented Evidence that Matters (POEMs). The premise behind the POEMs concept—as originally described in the British Medical Journal1—is that some research may better inform day-to-day clinical care than others. According to the original description, evidence may be considered a POEM if it satisfies the following three criteria: (1) it addresses a question that physicians encounter; (2) it measures outcomes that physicians and their patients care about; and (3) it has the potential to change the way that physicians practice.

In recent years, no technology has received greater scrutiny in O&P research than microprocessor-regulated prosthetic knee joints (MPKs). Within this mass of information, the findings of some studies appear to be more "poetic" than others. The purpose of this article is to draw upon these "patient-oriented" findings in an attempt to answer two fundamental questions that physicians and prosthetists routinely encounter using outcomes that both medical professionals and their patients care about:

  • Is day-to-day walking any safer with MPK technology?
  • Does MPK technology make walking any easier or less demanding?

Question 1: Is day-to-day walking any safer with MPK technology?

For most transfemoral amputees, the safety and stability of their prosthetic knee joint is a predominant concern. When looking for studies that might answer this question, the POEMs' criteria suggest that we look for research in which the outcomes are both relevant and meaningful to patients. Fortunately, two recent studies satisfy these requirements, both of which utilized a crossover study design that accurately mimics a patient's transition from one prosthetic device to another.

In the first study, researchers from the Prosthetic Research Study (PRS), Kingston, Washington, collected a data series from 21 patients with transfemoral amputations, all of whom were using conventional mechanical prosthetic knee joints at the time of enrollment and were being transitioned into MPK systems.2 The study required eight weeks of monitored use of the legacy prostheses prior to the collection of the first data sets relevant to the question of enhanced safety. Following these initial observations, the subjects received and used their MPKs until they reached a previously defined state of acclimation. This was followed by eight weeks of monitored MPK use followed by a second round of formal data collection.

While the authors collected a wealth of data, some of the outcomes are more relevant to immediate patient concerns than others. For example, the mean number of reported stumbles during the final four weeks of each of the eight-week observation periods decreased from 5.6 to 3.1 with the conventional knee joints and MPKs respectively. Similar patterns were observed in the decreases of both semi-controlled and uncontrolled falls in the two systems.2

In addition to reporting their numbers of stumbles and falls, subjects were asked to respond to a set of questions regarding their confidence, concentration, and stability. Among these, significant improvements for the MPK condition were observed in the response to the following questions (see figure 1):

  • Over the past four weeks, how difficult has it been to complete a task while walking, such as talking or reading?
  • Over the past four weeks, how frustrated have you been with the number of falls you have taken?

Figure 1

Realizing that metrics slated to measure levels of "difficulty" and "frustration" are somewhat subjective, the authors of the study had the foresight to require that subjects revert to their legacy conventional knee joints for two additional weeks, participate in a third round of data collection, and return to the MPK condition for eight more weeks prior to a fourth and final data collection. This A-B-A-B study design illuminated additional relevant findings. As examples, we'll consider two additional questions that were asked at the conclusion of each of the four data collection periods:

  • Over the past four weeks, how confident have you been with your prosthesis?
  • Over the past four weeks, how often has your fear of falling kept you from performing activities that you would normally do?

Responses to both questions illustrate the roles of experience and comparison in the assessment of prosthetic performance. In both instances, the initial differences between the conventional knee joints and MPKs in the first and second observations were relatively small. However, once subjects were able to appraise the performance of their conventional knee joints against their experiences with the MPKs, the reported differences were far more striking. For example, keeping in mind that higher scores represent improvements in function, the reported confidence associated with conventional knee joints fell from 75/100 to 66/100 once subjects had experienced the MPK condition. Similarly, the return to the MPK condition in the last of the four test conditions elicited an even more favorable response, with MPK confidence values increasing from 79/100 to 88/100. A similar pattern was observed in response to the question involving fear of falling as a hindrance to activity and participation. The A-B-A-B study design yielded an 80-82-77-91 response, with the relative influence of the non-MPK joints on fear of falling becoming more apparent with experience and comparison (see figure 1).

The question of safety received further scrutiny in a second, somewhat similar publication by Kahle et al.3 Once again, various outcome measures were assessed across a cohort of 19 transfemoral amputees following 90 days of monitored use of their legacy, non-MPK joints. At the conclusion of this pretest phase, subjects were fitted and aligned with MPKs and given 90 days of acclimation before undergoing a second round of observation and data collection. Relative to the original question of safety, as part of the data collection, subjects were asked to recall the number of stumbles and falls they experienced during the preceding 60 days in each of the two knee conditions. As with the earlier study, there were fewer reported stumbles and falls in the MPK condition, with decreases of 59 percent and 64 percent respectively.

There are, of course, other studies that have examined the effects of MPKs on balance. Kaufment et al. performed a rather complex analysis utilizing computerized dynamic posturography, which permits individual assessments of the visual, somatosensory, and vestibular inputs on balance across a series of altered visual and surface conditions.4 Bellmann et al. took patients into the gait lab and performed comparative analyses on their responses to simulated fall and stumble events with several different MPKs.5 While these studies and others like them advance the current understanding of how MPKs might influence questions of safety and stability, the outcomes they measured are less immediately identifiable to patients than those used in the first two publications. Things like stumble and fall rates, the comparative difficulty of multitasking, confidence in the prosthesis, and the relative fear of falling with a given knee technology not only address the foundational question, "Will I be safer in this technology?" but they also answer it in a way that patients can immediately understand and relate to. In the parlance of the POEMs criteria, "they measure outcomes that patients care about."1 What's more, they provide combined evidence to strongly suggest that patient ambulation is safer when walking with an MPK.

Question 2: Does MPK technology make walking any easier or less demanding?

If safety represents one fundamental concern of the patient with a transfemoral amputation, the relative ease of walking represents another. While the analyses of this technology on concerns of balance and stability have yielded fairly consistent and uniform results, the same cannot be said of the existing data on MPKs and energy efficiency. Currently, the most thorough and objective analysis of this literature is found in the recent work of Highsmith et al.6 For our purposes, a brief synopsis will suffice. In general, most authors have reported a trend toward increased energy efficiency with MPKs, though the differences are generally so small that they fail to reach statistical significance.7-10 Two studies have reported statistically significant improvements in gait speed. Oddly, however, these were reported at conflicting gait speeds, with Seymour et al. reporting increased energy efficiency at faster than normal speeds11 and Schmaltz et al. suggesting increased efficiency at slower than normal speeds.12 Thus, to the extent that MPKs decrease the metabolic costs of ambulation, they do so with some inconsistency, and the differences tend to be rather small.

A challenge to the use of these studies to answer the original question regarding ease of ambulation lies in the types of outcomes assessed. All of the studies cited above examined metabolic efficiency based on rather rigorous laboratory analyses of expired gases. While these techniques allow for objective quantification, they are somewhat removed from the immediate concerns of amputee patients. Simply put, are patients likely to care about the oxygen and carbon dioxide differentials experienced while test subjects walked in carefully controlled environments while breathing into a laboratory mouthpiece?

A more immediately relevant answer to questions regarding relative ease of ambulation might be derived from the subjective reports of study subjects. In the one existing study that records such results,8 subjects walked on a treadmill at three previously identified gait speeds in two prosthetic conditions. The first condition was that of their legacy, conventional prosthetic knee mechanism. Following an average acclimation period of 18 weeks with an MPK, subjects returned for identical testing with the replacement MPK. As with earlier studies, the research involved expired gas analysis in each knee condition and across three different gait speeds. In addition, however, researchers also administered the Borg Rating of Perceived Exertion Scale (Borg).11

Figure 2

Borg Rating of Perceived Exertion Scale

Instructions: While doing physical activity, we want you to rate your perception of exertion. This feeling should reflect how heavy and strenuous the exercise feels to you, combining all sensations and feelings of physical stress, effort, and fatigue. Do not concern yourself with any one factor such as leg pain or shortness of breath, but try to focus on your total feeling of exertion.

Look at the rating scale below while you are engaging in an activity; it ranges from 6 to 20, where 6 means "no exertion at all" and 20 means "maximal exertion...."

Try to appraise your feeling of exertion as honestly as possible, without thinking about what the actual physical load is. Your own feeling of effort and exertion is important, not how it compares to other people's. Look at the scales and the expressions and then give a number.

6 No exertion at all
7.5 Extremely light
9 Very light
11 Light
13 Somewhat hard
15 Hard (heavy)
17 Very hard
19 Extremely hard
20 Maximal exertion

9 corresponds to "very light" exercise. For a healthy person, it is like walking slowly at his or her own pace for some minutes. 13 on the scale is "somewhat hard" exercise, but it still feels okay to continue. 17 "very hard" is very strenuous. A healthy person can still go on, but he or she really has to push him or herself. It feels very heavy, and the person is very tired. 19 on the scale is an extremely strenuous exercise level.

For most people this is the most strenuous exercise they have ever experienced.

Borg RPE Scale © Gunnar Borg, 1970, 1985, 1994, 1998. Source: Centers for Disease Control and Prevention (

An in-depth discussion of the Borg is beyond the scope of this article. For our purposes, the scale represents an aggressively validated means of determining a subject's perception of how hard they are working (see figure 2). Subjects are asked to rate their perceived exertion during physical activity using a numerical scale ranging from six (no exertion at all) to 20 (maximal exertion). Additional descriptions are identified with intervening numbers such as nine (very light), 13 (somewhat hard), and 17 (very hard). By administering the Borg as part of their protocol, Kaufman et al. were able to corroborate their objective metabolic findings with the subjective experiences of their individual patients. Consistent with the data cited earlier, the mean reported Borg values at each of the three test speeds confirmed that subjects experienced less perceived physical exertion in the MPK condition than with their conventional prosthetic knee joints. However, as with formal metabolic findings, the differences, though consistently in favor of the MPK condition, were slight.8

As with the earlier question of safety and stability, the published findings regarding comparative ease of ambulation are not limited to those reviewed above. Johansson et al. examined such biomechanic variables as hip-work production throughout the gait cycle and peak hip-power-generation events.10 Bellmann et al. compared the swing-phase knee kinematics and velocities observed with several MPKs across several walking velocities.5 While these studies may serve to augment an understanding of why subtle differences in energy expenditure are experienced with the use of MPKs, they are somewhat removed from the more immediate patient concerns. More generalized metabolic data and to an even greater extent, the subjective reports of perceived exertion in various prosthetic conditions, provide a more immediate answer to the question of whether using MPK technology makes walking any easier.


The intent of this article is not to imply that some research is better than others. There are valuable insights to be gained across the spectrum of published research on MPKs. However, it is reasonable to suggest that some research findings are more "patient oriented" than others. These findings are identified by their ability to answer fundamental questions encountered in clinical practice, and do so using outcomes that patients can immediately identify with, and, accordingly, have the potential to affect the way that clinical decisions are reached.

Within the domain of prosthetic knee joint selection and the value of MPKs, two fundamental questions are commonly encountered. Fortunately, both are reasonably addressed in the published literature. Are patients safer in MPK technologies? By virtue of consistent reports of decreased stumble and fall rates, as well as improved confidence and reduced fear of falling, existing literature suggests that patients are indeed safer when they use MPKs. Is walking easier with the use of an MPK? Based on a number of metabolic studies and preliminary subjective reports on perceived exertion, walking probably is easier when using an MPK. However, the differences are far short of overwhelming. As the available research on prosthetic technologies continues to expand, a focus on patient-oriented evidence may help identify those findings that will best inform and refine clinical decision-making.

Phil Stevens, MEd, CPO, FAAOP, is in clinical practice with Hanger Prosthetics & Orthotics, Salt Lake City, Utah. He can be reached at

Editor's note: For a comprehensive summary of the current evidence related to microprocessor controlled knees, read the Evidence Note, "Outcomes Associated with the Use of Microprocessor- and Non-Microprocessor-Controlled Prosthetic Knees after Unilateral Transfemoral Limb Loss" in the March 2011 edition of The Academy TODAY.


  1. Smith R. A POEM a week for the BMJ. BMJ. 2002;325 (7371):963.
  2. Hafner BJ, Willingham LL, Buell NC, Allyn KJ, Smith DG. Evaluation of function, performance, and preference as transfemoral amputees transition from mechanical to microprocessor control of the prosthetic knee. Arch Phys Med Rehabil. 2007;88:207-217.
  3. Kahle JT, Highsmith MJ, Hubbard SL. Comparison of nonmicroprocessor knee mechanism versus C-Leg on Prosthesis Evaluation Questionnaire, stumbles, falls, walking tests, stair descent and knee preference. J Rehabil Res Dev. 2008;45(1):1-14.
  4. Kaufman KR, Levine JA, Brey RH, et al. Gait and balance of transfemoral amputees using passive mechanical and microprocessor-controlled prosthetic knees. Gait Post. 2007;26:489-493.
  5. Bellmann M, Schmalz T, Blumentritt S. Comparative biomechanical analysis of current microprocessor-controlled prosthetic knee joints. Arch Phys Med Rehabil. 2010;91:644-652.
  6. Highsmith MJ, Kahle JT, Bongiorni DR, Sutton BS, Groer S, Kaufman KR. Safety, energy efficiency, and cost efficacy of the C-leg for transfemoral amputees: A review of the literature. Prosthet Orthot Int. 2010;43(4):362-377.
  7. Chin T, Machida K, Sawamura S, et al. Comparison of different microprocessor controlled knee joints on the energy comsumption during walking in trans-femoral amputees: Intelligent knee prosthesis (IP) versus C-Leg. Prosthet Orthot Int. 2006;30(1):73-80.
  8. Kaufman KR, Levine JA, Brey RH, McCrady SK, Padgett DJ, Joyner MJ. Energy expenditure and activity of transfemoral amputees using mechanical and microprocessor-controlled prosthetic knees. Arch Phys Med Rehabil. 2008;89(7):1380-1385.
  9. Orendurff MS, Segal AD, Klute GK, McDowell ML, Pecoraro JA, Czerniecki JM. Gait efficiency using the C-Leg. J Rehabil Res Dev. 2006;43(2):239-246.
  10. Johansson JL, Sherrill DM, Riley PO, Bonato P, Herr H. A clinical comparison of variable-dampening and mechanically passive prosthetic knee devices. Am J Phys Med Rehabil. 2005;84(8):563-575.
  11. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exerc. 1982;14:377-381.