Effects of age and sex on the results of an ankle plantar-flexor manual muscle test.

Background and Purpose. The ability to perform 20 or more one-leg heel-rises is considered a "normal" grade for muscle strength (force-generating capacity of muscle) of the ankle plantar flexors, regardless of age and sex. Because muscle strength is closely related to age and sex, the "normal" test criterion was re-evaluated in different groups categorized by age and sex. Subjects and Methods. One hundred eighty sedentary volunteers (21-80 years of age) without lower-limb lesions performed as many repetitions of one-leg heel-rise as possible. Lunsford and Perry criteria were used to determine completion of the test. Results. The age and sex of the participants influenced the maximal repetitions of heel-rise, and the repetitions decreased with age and in female subjects. Discussion and Conclusion. The muscle strength of the ankle plantar flexors, as measured by manual muscle testing, varied with age and sex. Clinicians should consider the variances of age and sex when they perform manual muscle testing of the ankle plantar flexors. [Jan MH, Chai HM, Lin YF, et al. Effects of age and sex on the results of an ankle plantar-flexor manual muscle test. Phys Ther. 2005;85:1078-1084.]

Key Words: Aging, Heel-rise, Manual muscle test, Plantar flexor.

Ankle plantar-flexor strength (force-generating capacity of muscle) has an important role in standing balance, walking ability, and most activities of daily living. (1-3) During walking, the plantar flexors slow down the forward motion of the tibia over the foot during the stance phase and propel the body forward during push-off. People with weak plantar flexors usually present a crouch gait pattern during walking, difficulty in performing activities involving lower limbs or even inability to perform such activities, or inadequate standing balance. (2-4) For patients displaying these signs, examination of plantar-flexor strength is prudent. (2,4)

Numerous methods have been developed to assess muscle strength, such as manual muscle testing (MMT), (5,6) tests using handheld dynamometers,(7,8) and isokinetic testing. (9-11) Clinically, MMT is more frequently used than the other methods by virtue of its simplicity and convenience. No expensive instruments are required during testing, and it can be applied virtually instantaneously. (5,6) Although MMT might yield subjective data or can be onerous to perform, depending on the examiner's strength, in comparison with the other methods, it remains the most popular clinical method to identify muscle weakness.

According to the grading system of traditional MMT, muscle strength is judged by the muscle's ability to resist either gravity alone or gravity plus the examiner's strength. (5,6) The strength of the ankle plantar flexors graded in the prone position and against the examiner's strength is insufficient for the use required of the flexors in daily activities. Therefore, the challenge in designing MMT for the plantar flexors has been to find a technique that is able to exert a force to counter the force that supports body weight rather than to counter the examiner's strength.

A modified method, therefore, has been proposed using the repeated one-leg heel-rise test. (5,6) The ability of a person to perform 20 or more repetitions of one-leg heel-rise in a standing position in good form (without loss of balance or showing fatigue) is considered to indicate "normal" strength of the plantar flexors, regardless of the individual's age or sex or the activity level. (5,12) However, it is well known that muscle strength is closely related to a person's age and sex. (13-18) Indeed, our clinical experience has shown that most women without known pathology or impairments and older men with normal function in their lower extremities are nonetheless incapable of achieving 20 repetitions of one-leg heel-rise in good form. In an effort to approach a more realistic assessment, the purpose of our study was to investigate the number of repetitions of the one-leg heel-rise test required for normal plantar-flexor strength in different groups of subjects categorized by age and sex.

Method

Participants

Participants were recruited using advertisement postings on public bulletin boards both on the National Taiwan University campus and at the Ever-Green Association in Taipei City, Taiwan. Participants needed to be adults aged 21 to 80 years without any lower-limb lesions or injuries involving the hip, knee, or ankle and free of diseases of the musculoskeletal or cardiopulmonary system. The exclusion criteria were inability to participate in outdoor activities, displaying an abnormal or limping gait pattern, inability to walk continuously more than 30 minutes without joint pain, or being engaged in some regular exercise (defined as physical activity at least 3 times per week and exercise of more than 15 minutes after warm-up exercise (19)). Our decision to exclude the individuals who receive regular exercise was based on observations that exercise programs affect muscle strength of the ankle plantar flexors. (20,21) Thus, only sedentary individuals were included in this study.

The strategy to recruit participants was designed based on participants' willingness and equal numbers of both sexes for each decade group from 21 to 80 years of age. Initially, a sample of convenience of 207 people (105 men, 102 women) volunteered to participate in the study. Subsequent exclusions were based on an inability to walk continuously for 30 minutes without joint pain (n=8) and presence of a regular exercise regimen (n= 12). In addition, 7 people chose to drop out after an explanation of the details of the experiment. Thus, a total of 180 volunteers (90 men, 90 women) participated in the study. All participants signed an informed consent statement approved by the Ethics Committee of the National Taiwan University Hospital prior to taking part in the study.

Although participants from 6 decades of age were initially recruited, the data were collapsed for adjacent decades and re-grouped into 3 age groups during statistical analysis. Because no statistical differences were evident in the achieved number of heel-rise repetitions between the age groups of 21 to 30 years and 31 to 40 years, the age group of 41 to 50 years and 51 to 60 years, and the age groups of 61 to 70 years and 71 to 80 years (all P>.05), the participants were regrouped into 3 age groups: 21 to 40 years, 41 to 60 years, and 61 to 80 years. For an analysis of power=.8 and [alpha]=.05, the sample size is around 29 for making the group differences of 6.6 (which was the least group mean difference in this study) between the means of any 2 age groups with a pooled standard deviation of 10. Therefore, for each age X sex group, the sample size of 30 is enough to obtain a power greater than .8.

One-Leg Heel-Rise Test

The testing procedure and criteria for test completion were based on Lunsford and Perry's suggestions (12) and are described briefly as follows. Each one-leg heel-rise action was completed with the first second spent raising the heel up and the following second lowering the heel down. A metronome was used to set a beat correspond ing to the rhythm of the heel-rise. The participant was encouraged to try his or her best to repeat heel-rise actions until one of the following situations occurred: the plantar-flexion angle recorded was less than 50% of the maximum angle; the participant rocked back and forth, left and right, or lost balance; the participant's fingers pressed down deeply on the examiner's shoulder; the straight knee joint of the tested lower extremity (LE) bent; or the participant was unable to raise his or her heel again. The maximum angle was defined as the first repetition of the one-leg heel-rise test. If the angle monitored during the test was less than 50% of the maximum angle, then the test was terminated. During the whole experimental procedure, the knee of the tested LE was maintained straight and only finger-touch support was allowed (ie, participants were permitted to gently put 1 or 2 fingers on the examiner's shoulder for balance). The test also was terminated if the knee of the tested LE bent a bit or the examiner felt the pressure applied by the participant become greater or deeper.

The angle of ankle plantar flexion was monitored using a Biometrics K100 electrogoniometer * as participants performed the one-leg heel-rise test. This electrogoniometer consisted of a 12-bit analog-to-digital converter box and 2 sensors attached to the participant that were capable of measuring their relative position in space. The system has been reliably used for a similar purpose. (22) One sensor was placed at the dorsum of the tested foot between the second and third metatarsal heads, and the other sensor was fastened along the midline of the middle third of the anterior aspect of lower leg (Fig. 1). (12) Signals acquired with use of the analog-to-digital converter were transmitted to a personal computer, which stored and displayed the real-time data of angular motion of ankle plantar flexion.

[FIGURE 1 OMITTED]

Experimental Procedure

Each participant was tested on his or her skill-dominant LE. Prior to the experiment, this LE was determined to be the limb that strode on the first step when the individual climbed up a couple of stairs. Participants were asked to stand upright and barefoot on their skill-dominant LE with their eyes looking straight ahead. The tested LE kept straight during the whole experimental procedure, whereas the contralateral LE was naturally bent and never touched the floor.

The one-leg heel-rise test was performed by asking the participants to raise their heel repetitively with maximum effort during the one-leg stance. The number of repetitions of heel-rise at the test termination was regarded as an indication of the strength of the plantar flexors. Before the test started, participants were permitted to practice several times to be familiar with the test process. There were 3 examiners for each participant during the test. One examiner provided the finger-touch support and counted the total number heel rises accomplished. Another examiner observed the participant laterally for any extraneous trunk lean or knee flexion. The third examiner read the electrogoniometer output on the monitor and terminated the test if the plantar-flexion angle became less than 50% of the maximum angle. It took approximately 20 minutes to complete the whole experimental process.

To assess the intrarater reliability of data for the heel-rise repetitions measurement protocol, 20 participants were repeatedly measured 1 week apart. The participants and the 3 examiners were asked not to memorize or recall the previous record. The resulting intraclass correlation coefficient was .89, suggesting high intrarater reliability of data for the one-leg heel-rise test.

Data Analysis

We used SPSS 10.0 software [daggrer] for the statistical analyses. Descriptive statistics were used to depict the participants' demographic characteristics, including age, body height, body weight, and body mass index. A 2-way analysis of variance (ANOVA) was used to compare the differences in those characteristics among age X sex groups. A multiple regression of heel-rise repetition was performed to analyze the association of heel-rise repetitions with age, sex, body weight, and height.

Means and standard deviations of the heel-rise repetitions were calculated to examine the variability of the results. The differences in heel-rise repetitions among different age groups and sexes were determined by 2-way ANOVA. A P value of .05 or below was considered significant. If any significance was found, post hoc comparisons were further tested with Bonferroni adjustment (P<.017). The number of individuals who achieved more than 20 repetitions and the repetitions that the 80th percentile of participants achieved also were recorded to be compared with Lunsford and Perry's suggestions. (12)

Results

Table 1 presents the demographic information of the participants. There were no significant differences in body height, body weight, and body mass index among the various age groups for each sex (P>.05). Most participants were right-leg skill-dominant (98.9%). Among all 180 participants, 172 participants (95.6%) terminated their one-leg heel-rise test due to loss of balance, whereas the remaining 8 participants (4.4%) could not continue the test due to lower-extremity fatigue.

Multiple regression analysis showed that the ability to repeat heel rises was closely related to age and sex (P<.001), but that there was no relationship to body height and weight. The regression equation was: number of repetitions=28.5 - 7.8 x (Age) - 3.4 x (Sex), [R.sup.2]=0.57 (Age: 1-3, representing age groups of 20-40, 41-60, and 61-80 years, respectively; Sex: male=0, female = 1).

Descriptive data of heel-rise repetition for both sexes within each age group are presented in Table 2, including means, standard deviations, ranges, and the repetitions that the 80th percentile of participants achieved. For each age group, women accomplished significantly less than men did at the corresponding age group (P<.001). For either sex, significant differences in heel-rise repetitions were noted among age groups (P<.001 for both sexes). Bonferroni multiple-comparison tests revealed that the heel-rise repetitions were significantly different for each 2 age groups (all P<.002, Fig. 2), indicating that the number of heel-rise repetitions decreased significantly as the participant's age increased every 20 years (P<.001). In addition, the 80th percentiles of the male participants were 17, 7, and 2 repetitions for the age groups of 21 to 40, 41 to 60, and 61 to 80 years, respectively. Those of the female participants were 10, 5, and 1 repetitions, respectively.

[FIGURE 2 OMITTED]

The maximum values of heel-rise repetitions for the male and female participants were 46 and 30 repetitions, respectively, but 1 male participant and 3 female participants were unable to perform even 1 repetition. Only 24 male participants (26.7%) and 9 female participants (10.0%) carried out the 20 or more repetitions that constitute "normal" strength in the conventional MMT. Most of the participants who could achieve this standard were under 40 years of age. Only 5 male participants between 41 and 60 years of age could achieve the standard.

Discussion

While recognizing the clinical value of one-leg heel-rise test, the performance of heel-rise repetitions in individuals of different age groups and both sexes allows for further comparisons with any population with weakness of the ankle plantar flexors. The present study revealed that, in adults without known pathology or impairments, there are significant differences between sexes and among different age groups in the strength of the plantar flexors as assessed by the one-leg heel-rise test. The results of this study were shown to be inconsistent with those of Lunsford and Perry, (12) who used the same method. The possible mechanisms for such age and sex variances as well as the inconsistencies with findings reported in previous literature are explained as follows.

Muscle Strength and Balance Decreases With Age

Muscle strength weakens with increasing age. (1, 15-18) Consistent with these findings, no participant more than 60 years of age in the present study attained 20 repetitions of one-leg heel rise, with the majority of them achieving only 2 or fewer repetitions. Decreasing performance with age is attributed to age-related changes in the structure and function of muscle fibers within the muscle belly. (23,24) These changes include shrinkage of muscle fiber, decrease in the number of muscle fibers, reduction of functional motoneurons, and slowdown of reaction rates. (23) Therefore, older adults did not perform as well as the young and middle-aged adults.

In addition to decreasing muscle strength, balance control is of immense importance in the one-leg heel-rise test. Older adults display a deterioration in proprioception, vestibular system function, and other mechanisms that control body balance. (25-28) According to the effect of these age-related changes of muscle and proprioception, older adults are destined to have poor performance in a repeated heel-rise test.

Influences of Sex on Heel-Rise Repetitions

In general, muscle strength differs between men and women. (15,25,29) Muscle strength of women is generally less than that of men due to the smaller physiological cross-sectional area of their involved muscles. (15,25,30) Our study results support this point. However, our results were not consistent with those of another study of the one-leg heel-rise test, (12) where, using the same testing method, no differences in maximum heel-rise repetitions due to sex were apparent. The dichotomy between our results and the previous results may be due to the participants. Only sedentary participants were included in our study, while Lunsford and Perry (12) made no attempt to monitor participants' sports activities. It has been well documented that exercise workouts or sports participation affect ankle plantar-flexor strength. (20,21) Furthermore, the resistance applied during the one-leg heel-rise test is the performer's body weight. The body weight of the male participants in the study by Lunsford and Perry(12) was approximately 20 kg greater than that of the female participants, whereas the body weight of the male participants in our study was only about 10 kg more than that of the female participants. The difference in the male participants' body weight might explain the discrepancy in the results for sex difference.

Different Criteria for Normal Grade of Plantar-Flexor Muscles Should Be Considered

The present results support the contention that the different standards used for "normal" grade of ankle plantar-flexor strength for different ages and for both sexes needs to be considered. Lunsford and Perry (12) increased the proposed grade of normality to 25 repetitions of heel rise. In 1995, Hislop and Montgomery(5) proposed 20 repetitions of heel rise to be the normal criterion for the plantar flexors. Neither Lunsford and Perry nor Hislop and Montgomery, however, considered the affects of age-related decline or sex difference on muscle strength. Using the same experimental method as that of the study by Lunsford and Perry, the average performance of heel-rise repetition was quite different for both sexes and the age groups in our study. We cannot discount the possibility that the differences in regard to age and sex might be predisposed by racial or cultural differences between Westerners and Easterners. It has been reported that 37.0% to 62.7% of Western people (19,31-33) but only 20.5% of the Taiwanese population (34) exercise regularly. In addition, previous researchers (35-37) have indicated definite ethnic differences in muscle strength and physical fitness. The sharp contrast to Lunsford and Perry's findings, as reflected our findings that none of the men greater than 60 years of age and no woman greater than 40 years of age achieved 20 repetitions of one-leg heel-rise, warrants further efforts to establish normative data for various races or people of various lifestyles.

Conclusion

Findings from this study suggest that the standards of the normal grade for the ankle plantar-flexor muscles vary with age and sex. The ability to perform heel-rise repetitions decreases with advancing age, and women do not perform as well as men. People of different races or cultures probably have different performance of heel-rise in the one-leg standing test. Among people who do not exercise regularly, few can meet the reported requirement of 20 repetitions of heel-rise for "normal" strength of the ankle plantar flexors. Clinicians are advised to consider the age and sex variation when they perform MMT for the ankle plantar flexors.

* Biometrics Ltd, Unit 25, Nine Mile Point Industrial Estate, Newport, Gwent, NP11 7HZ, United Kingdom.

[dagger] SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.

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MH Jan, PT, MS, is Associate Professor, School and Graduate Institute of Physical Therapy, College of Medicine, National Taiwan University, Taipei, Taiwan.

HM Chai, PT, PhD, is Lecturer, School and Graduate Institute of Physical Therapy, College of Medicine, National Taiwan University, Taipei, Taiwan.

YF Lin, MD, is Orthopaedic Surgeon, Taipei City Hospital, and a doctoral student, Institute of Biornedical Engineering, National Yang-Ming University, Taipei, Taiwan.

JCH Lin, PT, MS, is a doctoral student, Department of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, Calif. LY Tsai, PT, MS, and YC Ou, PT, MS, are Senior Physical Therapists, Department of Physical Therapy, Mackay Memorial Hospital, Taipei, Taiwan.

DH Lin, MD, is Orthopaedic Surgeon, Department of Orthopaedics, En Chu Kong Hospital, Taiwan ([e-mail]). Address all correspondence to Dr Lin.

Mrs Jan and Dr DH Lin provided concept/idea/research design. Mrs Jan, Dr Chai, and Dr YF Lin provided writing. Mrs Tsai and Mr Ou provided subjects and data collection. Dr Chai and Dr YF Lin provided data analysis. Mrs Jan provided project management, facilities/equipment, and institutional liaisons. Ms JC Lin provided consultation (including review of manuscript before submission).

The protocol was approved by the Ethics Committee of National Taiwan University Hospital.

The results of this study were presented orally at the Annual Research Conference of Physical Therapy Association of the Republic of China; September 21, 2003; Taipei, Taiwan.

This article was received October 25, 2004, and was accepted March 15, 2005.

Table 1.
Demographic Data for Study Participants (a)

                       Male Age Groups (y)

                    21-40      41-60      61-80
                   (n = 30)   (n = 30)   (n = 30)

Age (y)              29.0       50.2       69.0
                    (4.8)      (4.9)      (4.0)
Body height (cm)    169.7      167.2      166.3
                    (6.1)      (5.4)      (5.4)
Body weight (kg)     69.7       67.0       66.5
                    (8.0)      (8.0)      (6.5)
Body mass index      24.2       24.0       24.1
  (kg/[m.sup.2])    (3.0)      (2.5)      (2.3)

                       Female Age Groups (y)

                    21-40      41-60      61-80
                   (n = 30)   (n = 30)   (n = 30)

Age (y)              30.3       49.9       69.1
                    (4.9)      (1.0)      (4.1)
Body height (cm)    160.5      157.0      154.9
                    (3.9)      (6.0)      (5.2)
Body weight (kg)     52.4       57.9       58.8
                    (5.5)      (9.2)      (5.5)
Body mass index      20.4       23.6       24.5
  (kg/[m.sup.2])    (2.1)      (4.2)      (2.2)

(a) Data presented are means with standard deviations shown in
parentheses.

Table 2.
Comparison of Heel-Rise Repetitions Among Men and Women the 3 Age
Groups

                  Male Age Groups (y)

                  21-40          41-60       61-80
                  (n = 30)       (n = 30)    (n = 39)

[bar.X]           22.1 (a,b)     12.1 (c)    4.1
SD                 9.8            6.6        1.9
Range              9-46           4-30       0-7
80th percentile   17              7          2

                  Female Age Groups (y)

                  21-40          41-60       61-80
                  (n = 30)       (n = 30)    (n = 30)

[bar.X]           16.1 (a,b,d)   9.3 (c,d)   2.7 (d)
SD                 6.7           3.6         1.5
Range              6-30          5-19        0-5
80th percentile   10             5           1

(a) Significant difference between age groups 21-40 and 41-60 years
(P<.001).

(b) Significant difference between age groups 21-40 and 61-80 years
(P<.001).

(c) Significant difference between age groups 41-60 and 61-80 years
(P<.001).

(d) Significant difference between male and female participants
(P<.001).