Different Method, Different Results?

To the Editor:

I read with great interest the article by Daubney and Culham entitled "Lower-Extremity Muscle Force and Balance Performance in Adults Aged 65 Years and Older" (December 1999).

After reading the article and reviewing the data presented, I found several sections of the article that lead me to question the authors' results and conclusions. According to their own data, the 3 measures of balance--the Berg Balance Scale (BBS), the Timed Get Up & Go Test (GUG), and the Functional Reach Test (FRT)--failed to yield significant differences between subjects with a history of falling and subjects without a history of falling (Tab. 1). Intuitively, that result would suggest either that (1) the measures were not valid for the population being tested or (2) the authors' assumption that there is a relationship between balance and the incidence of falling is not valid. In either case, these measures should not have been used, because they failed to detect significant differences between the groups. If the muscle testing methods showed a difference between groups, what would that mean?

From the initial question, I went on to investigate the methodology and findings for the measurements of muscle force, which provoked several more questions. Why were of all the muscle groups tested in the midrange of joint motion? The principles of biomechanics and exercise physiology clearly dictate that the maximal force of the muscles in question would have been best tested in the position of function, standing, or at 90 degrees to the joint axis. In addition, and perhaps most perplexing, is why the authors performed all the tests with the subjects positioned supine. Logically, if you want to determine the relationship of muscle force to the 3 tests of standing balance, then the muscle force measurements should be performed in a standing position. Each muscle force test performed must be questioned as to its validity for measuring force in relationship to the function of standing, walking, or balance.

The force of the hip flexors and hip extensors was measured at 90 degrees of hip flexion, yet the functional joint position is 30 degrees of hip flexion for the extensors (initial contact during gait) and 10 to 15 degrees of hip extension for the hip flexors (terminal stance and preswing). I also could not determine from the Figure, which is supposed to demonstrate 90 degrees of hip flexion, that the thigh was in 90 degrees of flexion. The Figure seems to indicate significantly less flexion.

Ankle plantar-flexor muscles were tested in the correct range, but the results of a handheld test are difficult to accept. Kendall et al[1] described the test for these muscles in the standing position in which the subject undertakes a series of lifts that require elevation of the subject's entire body weight. In my clinical experience, a "Good" to "Normal" manual muscle test grade for the plantar-flexor group cannot be broken by a tester using only his or her individual strength. At this place in the article, the authors' findings break down. In Table 2, the authors report a maximum mean plantar-flexion force of 28.04 kg (range=14.48-38.68 kg) for day 1 and of 26.21 kg (range=17.17-40.51 kg) for day 2. Table 1 provides the demographics of the subjects and shows a mean body weight of 79.36 kg (range=66-103 kg) in the subjects with falls and of 74.28 kg (range=38-115 kg) in the subjects without falls.

These 2 sets of data would appear to indicate that none of the participants could even begin to complete a single lift of body weight using the plantar flexors. Thus, all of the participants would have had, at best, "Fair" or "Fairplus" plantar-flexor strength and would have had obvious gait deviations. Yet, none of this is reported. When the authors normalized their force measurements for body weight (in kilograms of force per kilogram of body weight), this should have been apparent. At this point, it is not clear whether the data presented in Table 2 are normalized or absolute; however, either way, because of the methods used for testing the hip flexors, extensors, and plantar flexors, the measurements of force generation are not relevant to standing or walking.

Finally, it is not clear what the term "push-off" means in the statement "[t]he ankle plantar flexors contribute to the support moment in the stance phase of gait and the plantar-flexor moment of the push-off phase of the gait cycle." According to Perry,[2] the plantar flexors act eccentrically to slow the transition of the tibia over the talus during mid-stance and do not participate in push-off of the body.

According to the authors, "[o]nly ankle dorsiflexion force contributed to the prediction of fall status." I suggest that an accurate measurement of plantar-flexor, hip extensor, and hip flexor strength in the upright posture at the correct functional angles--using tests that discriminate between subjects with a history of falling and subjects without a history of falling--might have produced entirely different results:

Scot Irwin, DPT ([e-mail])

References

[1] Kendall FP, McCreary EK, Provence PG. Muscle Testing and Function. 4th ed. Baltimore, Md: Williams & Wilkins;1993: 205-207.

[2] Perry J. Gait Analysis. Thorofare, NJ: Slack Inc; 1992:153-155.

Author Response:

Dr Irwin raised a number of interesting questions. In our opinion, the Berg Balance Scale (BBS) has been shown to yield valid measurements[1,2] for the population investigated in our study. However, in order to minimize any "ceiling effect" that could occur with the BBS in subjects with high levels of functioning, we quantified some of the measures of the BBS, such as a measure of functional reach, and added the Timed Get Up & Go Test (GUG). Measurements obtain-ed with all 3 tests were lower in those subjects whom we considered to be at risk for falling. The fact that the balance measurements were not different between the 2 groups in this study was surprising, as we have previously found differences in similar populations.[3] All subjects with a history of falling were recruited from the community, and we believe that balance impairment, though present, was not sufficient to be detected using the tests chosen. The assumption that balance and falling are related is a valid one and is well supported in the literature.[4-8]

Whether lower-extremity weakness precedes falling or is a secondary effect of a fall due to a decrease in mobility is not clear from retrospective investigations. However, prospective studies that have evaluated lower-extremity muscle force in subjects who have fallen and in subjects who have not fallen have shown lower-extremity weakness to be a risk factor for falls[9] and a predictor of falls.[10]

Dr Irwin questioned the reasons behind testing the muscle groups in midrange versus positions of function (eg, standing). Bohannon[11] suggested that testing muscles at a position midway between fully lengthened and fully contracted would give a more reasonable estimate of muscle group capacity. In the middle range, muscle groups are neither highly advantaged nor disadvantaged. Additionally, given the population of older adults that was being investigated, it would have been difficult to position subjects and adequately stabilize them in standing. The subjects were primarily tested in a supine position, and the hip rotators were tested with the subjects in a sitting position. We chose these positions because of the stability of the subjects in these positions, because they are previously documented positions for testing these muscle groups,[11] and because testing in only 2 positions minimized moving the subjects frequently between methods of muscle group testing. The test-retest reliability of all force measurements was determined prior to conducting the study. In our opinion, reliability would have been compromised had we tested subjects in a less stable position such as standing. Dr Irwin is correct in that our Figure unfortunately does not accurately reflect the test position for measurement of hip flexor muscle force.

Muscle force measures are dependent on position and muscle length. All of the force data for subjects in this study were similar to data reported for similar populations.[12-15] Data in Table 2 were not normalized so that values could be compared with those previously reported.[12-15]

Plantar-flexor muscle forces, in our opinion, can easily overpower testers (as can hip extensor muscle forces) during manual muscle testing. For this reason, we used the stabilization frame and guide ropes that were attached to the handheld dynamometer and strung through adjustable cam cleats on the frame in an attempt to provide additional support and stabilization to the researcher so that an isometric test could be performed.

The ankle plantar flexors are the power generators in the late stance phase of the gait cycle,[16,17] and the term "push off" is a commonly accepted label for the propulsive phase of gait. The eccentric contraction of the plantar flexors, as described by Dr Irwin, is followed by a concentric contraction as the ankle plantar flexes between heel-off and toe-off, generating the propulsive force for forward momentum.

Dr Irwin may be correct in assuming that results would have been different had forces been measured with the subjects in upright stance. Accurate and reliable methods of measuring force in an upright functional mode (preferably applicable in the clinical setting) are needed in order to test his hypotheses. However, the finding that the force-producing capability of the ankle dorsiflexors is related to postural control is consistent with previous literature.[18-19]

Elsie G Culham, PT, PhD Associate Professor Physical Therapy Program School of Rehabilitation Therapy Queen's University Kingston, Ontario, Canada K7L 3N6 ([e-mail])

Marguerite Elizabeth Daubney, PT, MSc

References

[1] Berg KO, Maki BE, Williams JI, et al. Clinical and laboratory measures of postural balance in an elderly population. Arch Phys Med Rehabil. 1992;73:1073-1080.

[2] Berg KO, Wood-Dauphinee SL, Williams JI, Maki B. Measuring balance in the elderly: validation of an instrument. Can J Public Health. 1992;83(suppl 2):S7-S11.

[3] O'Brien K, Culham EG, Pickles B. Balance and skeletal alignment in a group of elderly female fallers and nonfallers. J Gerontol A Biol Sci Med Sci. 1997;52:B221-B226.

[4] Wolfson LI, Whipple R, Amerman P, Kleinberg A. Stressing the postural response: a quantitative method for testing balance. J Am Geriatr Soc. 1986;34:845-850.

[5] Heitmann DK, Gossman MR, Shaddeau SA, Jackson JR. Balance performance and step width in noninstitutionalized, elderly, female fallers and nonfallers. Phys Ther. 1989;69:921-931.

[6] Robbins AS, Rubenstein LZ, Josephson KR, et al. Predictors of falls among elderly people. Arch Intern Med. 1989;149:1628-1633.

[7] MacRae PG, Lacourse M, Moldavon R. Physical performance measures that predict faller status in community-dwelling older adults. J Orthop Sports Phys Ther. 1992;16: 123-128.

[8] Maki BE, Holliday PJ, Topper AK. A prospective study of postural balance and risk of tailing in an ambulatory and independent elderly population. J Gerontol. 1994;49:M72-M84.

[9] Campbell AJ, Borrie MJ, Spears GF. Risk factors for falls in a community-based prospective study of people 70 years and older. J Gerontol. 1989;44:M112-M117.

[10] Tinetti ME, Speechley M, Ginter SF. Risk factors for falls among elderly persons living in the community. N Engl J Med. 1988; 319:1701-1707.

[11] Bohannon RW. Muscle strength testing with hand-held dynamometry. In: Amundsen L, ed. Muscle Strength Testing: Instrumented and Non-instrumented Systems. New York, NY: Churchill Livingstone Inc; 1990:69-112.

[12] Andrews AW, Thomas MW, Bohannon RW. Normative values for isometric muscle force measurements obtained with hand-held dynamometers. Phys Ther. 1996;76:248-259.

[13] Cahalan TD, Johnson ME, Liu S, Chao EYS. Quantitative measurements of hip strength in different age groups. Clin Orthop. 1989;246:136-145.

[14] Kramer JF, Vaz MD, Vandervoort AA. Reliability of isometric hip abductor torques during examiner- and belt-resisted tests. J Gerontol. 1991;46:M47-M51.

[15] Lord SR, Ward JA, Williams P, Strudwick M. The effect of a 12-month exercise trial on balance, strength, and falls in older women: a randomized controlled trial. J Am Geriatr Soc. 1995;43:1198-1206.

[16] Judge JO, Ounpuu S, Davis RB III. Effects of age on the biomechanics and physiology of gait. Clin Geriatr Med. 1996; 12:659-676.

[17] Prince F, Corriveau H, Hebert R, Winter DA. Gait in the elderly. Gait and Posture. 1997;5:128-135.

[18] Whipple RH, Wolfson LI, Amerman PM. The relationship of knee and ankle weakness to falls in nursing home residents: an isokinetic study. J Am Geriatr Soc. 1987;35:13-20.

[19] Studenski S, Duncan PW, Chandler J. Postural responses and effector factors in persons with unexplained falls: results and methodological issues. J Am Geriatr Soc. 1991;39:229-234.

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