Edited by: Rodrigo Orlando Kuljiš, Zdrav Mozak Limitada, Chile
Reviewed by: Elliot J. Roth, Rehabilitation Institute of Chicago, USA; Jan Hoff, Norwegian University of Science and Technology, Norway
*Correspondence: Thomas Cattagni, Faculté des sciences du sport—UFR STAPS, Université de Bourgogne, Campus universitaire Montmuzard, BP 27 877 – 21078 Dijon, Cedex, France e-mail:
This article was submitted to the journal Frontiers in Aging Neuroscience.
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It is well known that center of pressure (CoP) displacement correlates negatively with the maximal isometric torque (MIT) of ankle muscles. This relationship has never been investigated in elderly fallers (EF). The purpose of this study was thus to analyze the relationship between the MIT of ankle muscles and CoP displacement in upright stance in a sample aged between 18 and 90 years old that included EF. The aim was to identify a threshold of torque below which balance is compromised. The MIT of Plantar flexors (PFs) and dorsal flexors (DFs) and CoP were measured in 90 volunteers: 21 healthy young adults (YA) (age: 24.1 ± 5.0), 12 healthy middle-aged adults (MAA) (age: 50.2 ± 4.5), 27 healthy elderly non-fallers (ENF) (age: 75.5 ± 7.0) and 30 EF (age: 78.8 ± 6.7). The MIT of PF and DF were summed to obtain the overall maximal ankle muscle strength. Body weight and height were used to normalize MIT (nMIT) and CoP (nCoP), respectively. nCoP correlated negatively with nMIT. 90% of EF generated an nMIT <3.1 N·m·kg−1, whereas 85% of non-fallers generated an nMIT >3.1 N·m·kg−1. The relationship between nMIT and nCoP implies that ankle muscle weakness contributes to increased postural instability and the risk of falling. We observed that below the threshold of 3.1 N·m·kg−1, postural stability was dramatically diminished and balance was compromised. Our results suggest that measuring ankle torque could be used in routine clinical practice to identify potential fallers.
Falls are a major concern among older adults. Indeed, approximately 30% of people over 65 years old and 50% of those over 80 years old fall each year (Tinetti et al.,
The increase in body sway, generally observed with aging (Sheldon,
To control body sway while standing upright, humans need to generate appropriate torques at the ankle joint (Horak and Nashner,
Recently, Billot et al. (
In view of these considerations, the purpose of this study was to analyze the relationship between MIT of ankle muscles and CoP displacement in a sample aged between 18 and 90 years old that included EF, in order to identify a threshold of torque below which balance is compromised.
Ninety volunteers, aged between 18 and 90 years old, participated in this experiment. The sample was divided into four groups according to age and history of falls: 21 healthy YA (age: 24.1 ± 5.0), 12 healthy middle-aged adults (MAA) (age: 50.2 ± 4.5), 27 healthy ENF (age: 75.5 ± 7.0) and 30 EF (age: 79.8 ± 6.7). For subjects aged between 60 and 90 years old, the fall history of the previous 6 months was recorded by interview. People who had fallen unexpectedly at least once in the previous 6 months were included in the EF group. The YA and MAA were students or employees at the University of Burgundy. Exclusion criteria for all subjects were muscular disorders, neurological disorders (stroke, multiple sclerosis, Parkinson’s disease), serious visual impairment; body mass index >35, and impaired cognitive status (score of less than 23 on the Mini Mental State Examination). Informed written consent, approved by the local ethics committee, was obtained for all participants after they had been fully informed of all potential risks, discomfort and benefits of the study.
The protocol of the current investigation was approved by the French National Drugs and Health Administration and by the National Ethics Committee section Dijon Est I and was carried out in accordance with the Declaration of Helsinki.
Body weight was measured using an electronic scale (SOEHNLE Fitness 7850). Body height was obtained with a measuring rod, with a horizontal slider brought into contact with the vertex while the subject was standing with his back laying against a wall.
Participants were examined in the seated position with the trunk inclined forwards at 40° to the vertical, the knee joint angle at 180° and the ankle joint angle at 90°. Strength was measured with the foot secured by two straps to the footplate of a custom-made ergometer developed by the mechanical workshop of a local engineering school (I.U.T. Génie Mécanique, Dijon, France). The center of rotation of the ergometer shaft was aligned with the anatomical ankle flexion-extension axis. The footplate was connected to a strain-gauge transducer (Société Doerler-Vandoeuvre, France), placed on the axis of the device, and an amplifier (PM Instrumentation, model 1300B, amplification: 8 V = 200 Nm). This apparatus has already been used in previous published studies carried out in our laboratory (Scaglioni et al.,
Center of pressure displacement during the static postural task was assessed via a force-platform (Stabilotest, TechnoConcept, Cereste, France). This platform was composed of three force sensors which instantaneously measured the coordinates of the point of application of the resultant of the ground reaction forces or CoP. Data from the force-platform were sampled at a frequency of 40 Hz and were synchronized by triggering with the Biopac acquisition system.
All measurements were made in one experimental session lasting approximately 1.5 h. All subjects first carried out the orthostatic postural task and then the MVC task to avoid interference due to fatigue during the postural task.
During the postural task, subjects stood barefoot on the force-platform for 30 s, with their feet axes forming an angle of 30° (distance between the heels = 2 cm) and with their arms alongside their body. The subjects were asked to remain as still as possible, looking straight ahead at a target point located at eye height and placed at a distance of 150 cm in front of them. This postural task was repeated three times, and a rest period of 1 min was given between trials.
The MIT for PF and DF was obtained during maximal voluntary isometric contraction tests performed for each leg separately. A 3-min warm-up, which included several submaximal plantar and dorsal flexions, was carried out for each leg. Subjects then performed two maximal contractions lasting 5 s for each leg. A 1-min rest period was systematically given between trials to avoid any fatiguing effect on measurements. If there was a variation of more than 5% between the first and the second MVC, participants were asked to perform an additional MVC. Standardized verbal encouragement was given during attempts to produce the maximal effort.
Only the highest PF and DF MIT were considered for analysis. For each contraction type, the torques achieved by both legs were summed to obtain the overall force production capacity at the ankle joint. The MIT was then normalized for the body weight (nMIT).
During the postural task, the 95% elliptical area (EA) containing 95% of the CoP sampled positions, the antero-posterior (YCoP), the mediolateral (XCoP) and the total length of CoP displacement (TCoP) were recorded for 30 s. These parameters were expressed as an average of three measurements. Each parameter of the CoP displacement was normalized for the height (nEA, nYCoP, nXCoP and nTCoP).
All statistical tests were performed with SPSS software (SPSS, Inc., Chicago, IL, USA). Data are presented as means ± standard deviations (SD). The critical level for statistical significance was set at 5%. A multivariate analysis of variance (MANOVA) was performed in all outcome variables with group (YA, MAA, ENF and EF) as factor. When a main effect was found, Tukey’s
The main characteristics of participants are described in Table
Young adults |
Middle-aged adults |
Elderly non-fallers |
Elderly fallers |
|
---|---|---|---|---|
Age, year | 24.1 ± 5.0 | 50.2 ± 4.5* | 75.5 ± 7.0*$ | 79.8 ± 6.7*$ |
Body weight, kg | 72.5 ± 7.8 | 74.8 ± 16.3 | 63.6 ± 11.6$ | 63.7 ± 12.4$ |
Height, cm | 180.4 ± 5.9 | 174.7 ± 8.8 | 164.0 ± 7.7*$ | 160.4 ± 8.6*$ |
Body mass index (kg/m2) | 22.3 ± 2.2 | 24.6 ± 5.4 | 23.5 ± 3.4 | 24.7 ± 4.1 |
PF MIT, N·m | 320.4 ± 64.7 | 235.4 ± 65.8* | 164.6 ± 63.2*$ | 105.5 ± 34.8*$† |
PF nMIT, N·m·kg−1 | 4.4 ± 0.7 | 3.2 ± 0.7* | 2.5 ± 0.7*$ | 1.7 ± 0.5*$† |
DF MIT, N·m | 89.6 ± 13.8 | 75.9 ± 20.2 | 57.7 ± 19.2 | 42.7 ± 9.9* |
DF nMIT, N·m·kg−1 | 1.2 ± 0.1 | 1.0 ± 0.2 | 0.9 ± 0.2 | 0.7 ± 0.2* |
PF + DF MIT, N·m | 410.0 ± 74.3 | 311.3 ± 72.7* | 222.2 ± 78.2*$ | 148.2 ± 40.3*$† |
PF + DF nMIT, N·m·kg−1 | 5.6 ± 0.8 | 4.2 ± 0.7* | 3.4 ± 0.8*$ | 2.4 ± 0.6*$† |
TCoP displacement, mm | 297.9 ± 80.8 | 344.3 ± 89.5 | 410.7 ± 137.5 | 744.0 ± 405.6*$† |
nTCoP | 1.7 ± 0.4 | 2.0 ± 0.5 | 2.5 ± 0.8 | 4.7 ± 2.4*$† |
XCoP displacement, mm | 167.0 ± 56.9 | 159.7 ± 34.8 | 227.2 ± 103.7 | 309.8 ± 152.3*$† |
nXCoP | 0.9 ± 0.3 | 0.9 ± 0.2 | 1.4 ± 0.6 | 2.0 ± 0.9*$† |
YCoP displacement, mm | 208.8 ± 61.0 | 271.5 ± 78.3 | 290.4 ± 91.5 | 576.5 ± 324.5*$† |
nYCoP | 1.2 ± 0.3 | 1.6 ± 0.5 | 1.8 ± 0.5 | 3.6 ± 1.9*$† |
EA, mm2 | 113.0 ± 50.8 | 123.4 ± 76.3 | 185.1 ± 100.9 | 279.6 ± 152.5*$† |
nEA (EA/size), mm | 0.6 ± 0.3 | 0.7 ± 0.4 | 1.1 ± 0.6 | 1.7 ± 0.9*$† |
As shown in Table
The effect of age was more marked when the sum of PF and DF was considered. Indeed, among the three non-faller groups, age gradually affected PF + DF MIT (i.e., YA > MMA,
As shown in Table
As depicted in Figure
Figure
The purpose of this study was to analyze the relationship between MIT of ankle muscles and CoP displacement in a sample aged between 18 and 90 years old that included EF. The aim was to identify a torque threshold that discriminated between fallers and non-fallers. We found a negative curvilinear correlation between these parameters, which suggests that the decrease in ankle muscle strength is one of the factors responsible for impaired postural stability. In addition, our analysis indicated that a value of 3.1 N·m·kg−1 discriminated between fallers and non-fallers with a
Because statistical analysis showed similar changes in MIT and nMIT, to simplify matters in this paragraph we only used the abbreviation MIT. The present results showed that age gradually affected PF + DF MIT, thus corroborating previous findings (Vandervoort and McComas,
Because statistical analysis showed similar changes in CoP and normalize CoP (nCoP), to simplify matters in this paragraph we only used the abbreviation CoP. CoP displacement, whatever the parameter used to quantify it (i.e., EA, YCoP, XCoP, TCoP), was statistically similar among the three age groups of non-fallers (YA, MAA and ENF). Although this observation is in keeping with the results of some earlier studies (Shumway-Cook et al.,
Furthermore, our results showed that in EF, CoP displacement was greater than that in ENF, whatever the parameter considered (i.e., EA, YCoP, XCoP, TCoP). This finding confirms the general observation of the literature, that is to say greater postural sway in EF than in ENF (Fernie et al.,
The finding that elderly subjects have weaker ankle muscles than younger subjects, and that this weakness is more pronounced in EF, suggests that a deficit in strength may cause an increase in postural sway and consequently compromise the ability to maintain postural stability. Our results showed that, whatever the group of subjects (YA, MAA, ENF and EF), there was a significant negative correlation between nMIT of ankle muscles and nTCoP displacement. This confirms the findings of Kouzaki and Masani (
More specifically, the present analysis showed that, for the sample taken as a whole, the comprehensive relationship between nMIT and nTCoP is curvilinear. In contrast, if EF are separated from non-fallers (i.e., YA, MAA and ENF), the data are better fitted by two linear regression lines. The regression line for EF presented a higher correlation coefficient and a steeper slope than that for non-fallers. This suggests that diminished muscle strength has a greater functional impact in EF than in non-fallers because it induces a greater increase in nTCoP displacement. The present result supports our initial hypothesis of a slope failure in the general nMIT-nTCoP relationship provoked by the addition of data from EF which again leads us to the conclusion that the greater the instability of an individual (i.e., elderly faller) the more his balance depends on ankle muscle strength. Previous studies showed that the age-related changes in postural control induce an increased level of muscle activity across lower-limb joints in older adults (Laughton et al.,
We used the ROC analysis to identify a critical MIT value to discriminate between fallers and non-fallers, that is to say a threshold below which, the risk of falling is markedly increased. The analysis revealed that subjects who did not report any previous falls had a PF + DF nMIT >4.0 N·m·kg−1. For the clinician, this value could represent a first alert threshold, indicating the necessity to be particularly attentive to the progressive muscle weakening of patients who fall below this level of strength.
The highest Youden index was found for a 3.1 N·m·kg−1 value, corresponding to a sensitivity of 90% and a specificity of 85%, meaning that 90% of the subjects with a PF + DF nMIT under 3.1 N·m·kg−1 were fallers and 85% of subjects with a PF + DF nMIT above this value were non-fallers. The value of 3.1 N·m·kg−1 was close to the intersection point of the regression lines found for fallers and non-fallers (
In conclusion, the current investigation demonstrated that the maximal torque of ankle muscles is an easy and effective indicator of the risk of falling. A particularly interesting aspect that emerges from the present analysis is the threshold of alert that corresponds to a critical strength value (i.e., 3.1–3.4 N·m·kg−1), below which postural stability is severely impaired. The identification of a threshold that makes it possible to discriminate between fallers and non-fallers clearly indicates that weakness of the ankle plantar flexor and dorsal flexor muscles markedly aggravates postural instability. Including the assessment of ankle muscle strength in routine clinical practice therefore seems to be crucial so as to detect the risk of falling, which is an extremely disabling event for older adults. Finally, following the diagnosis of muscular weakness, patients should be steered towards an appropriate fall prevention program specifically designed to enhance the strength of postural muscles.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors thank Mrs. Delphine Besson-Bretin for her contribution to the experiments and M. Ballay for technical assistance.