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Maximum voluntary isometric contraction: Reference values and clinical application. DARA MELDRUM1, EIBHLIS CAHALANE2, RONAN CONROY3, DEIRDRE ...

Amyotrophic Lateral Sclerosis. 2007; 8: 47–55

ORIGINAL ARTICLE

Maximum voluntary isometric contraction: Reference values and clinical application

DARA MELDRUM1, EIBHLIS CAHALANE2, RONAN CONROY3, DEIRDRE FITZGERALD2 & ORLA HARDIMAN2 1

Royal College of Surgeons, Dublin, 2Beaumont Hospital, Dublin, and 3Royal College of Surgeons in Ireland, Ireland

Abstract Maximum voluntary isometric contraction (MVIC) is a standardized method for measurement of muscle strength in patients with neuromuscular disease. Values obtained from MVIC testing are difficult to interpret at present as normative data are limited. The objective of this study was to generate reference values for MVIC. A convenience sample of 494 healthy men and women aged 20–76 years was recruited. MVIC testing was performed on nine muscle groups bilaterally: neck flexors, shoulder abductors, shoulder adductors, elbow and knee flexors and extensors, and hip and ankle flexors. MVIC was performed using the Quantitative Muscle Assessment system. Age and sex related reference values were calculated for each muscle group using quantile regression. A clinical reporting system was developed to facilitate interpretation of patient values with reference to normal percentiles. Reference values generated from this study can be used to determine the presence and extent of muscle weakness in a given population and to evaluate the effectiveness of treatment interventions.

Key words: muscle, MVIC, strength measurement, quantile regression, ALS, post-polio

Introduction Quantification of muscle strength is an essential component of the assessment and treatment of neuromuscular patients. There is a comprehensive evidence base that supports objective and reliable assessment of muscle strength in monitoring disease progression and evaluating treatment interventions (1–8). MVIC (maximum voluntary isometric contraction), measured using strain gauge tensiometers and MMT (manual muscle testing) are the most common measurement techniques used in the clinical and research setting (9–11). Both have advantages and disadvantages. MMT is a quick, inexpensive method of testing strength that requires little equipment and personnel training but some studies have shown MMT lacks the sensitivity to detect small but potentially important changes in muscle strength (11–14). In a recent study of acute rehabilitation patients, it was found that reliance on manual muscle testing missed strength deficits 25% of the time (11). In contrast to this, one clinical trial found MMT to be more sensitive in detecting rates of muscle change than MVIC, although a smaller number of muscles was tested with MVIC than

MMT (15). There are wide ranges of forces unsuitable for quantification of normal and slightly decreased muscle strength within one grade of the Medical Research Council manual muscle testing scale (13). MVIC provides interval data (typically in units of kilograms or Newtons of force) that are more objective than manual muscle testing and is a safe and simple method of assessing muscle strength. MVIC has been used extensively in studies of many neuromuscular conditions including amyotrophic lateral sclerosis (1,2,8,14–17), post-polio syndrome (6), chronic inflammatory demyelinating polyneuropathy (7), fascioscapulohumeral dystrophy (5,18,19), and inclusion body myositis (20). The presence or significance of weakness measured with MVIC is unknown without reference values. Longitudinal studies of individual patients can measure changes in strength over time and with treatment; however, whether change is meaningful is difficult to establish without reference values. For instance, if MVIC strength values increase by 50% with an intervention, the question can still be posed as to whether this brings the individual to within normal performance values. Although efforts have

Correspondence: D. Meldrum, School of Physiotherapy, Royal College of Surgeons in Ireland, 123 St. Stephen’s Green, Dublin 2, Ireland. Fax: 00 353 1 4022471. E-mail: [email protected] (Received 23 March 2006; accepted 9 September 2006) ISSN 1748-2968 print/ISSN 1471-180X online # 2007 Taylor & Francis DOI: 10.1080/17482960601012491

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D. Meldrum et al.

Table I. Procedure for measuring MVIC. Movement

Position

Strap Position

Stabilization by tester

Instruction

Neck flexion

Supine with head on pillow

Large loop placed around head under chin with loops at top of head.

None (hold strap on chin if necessary)

Tuck your chin to your chest. Try not to lift your head up

Shoulder abduction

Supine, shoulder in 90˚ abduction

Proximal to elbow

Axilla and lateral trunk

Bring your arm away from your body without lifting your arm off the bed

Shoulder adduction

Supine, shoulder in 90˚ abduction

Proximal to elbow

Superior surface of shoulder

Bring your arm towards your body without lifting your arm off the bed

Elbow flexion

Supine shoulder in neutral elbow in 90˚ flexion

Proximal to wrist

Anterior surface of shoulder and volar surface of elbow

Bend your elbow

Elbow extension

Supine shoulder in neutral elbow in 90˚ flexion

Proximal to wrist

Anterior surface of shoulder and lateral elbow

Straighten your elbow

Hip flexion

Supine; hip and knee in 90˚ flexion. Opposite leg extended. Arms relaxed by the side

Lower 1/3 of thigh

Superior to subject with pressure applied down through femur. Lower leg supported with knee in 90˚

Pull your knee towards your chest

Knee extension

Sitting hips and knees at 90˚

Proximal to ankle joint

Pressure through shoulder and pelvis

Straighten your knee. Do not hold the couch

Knee flexion

Sitting hips and knees at 90˚

Proximal to ankle joint

None

Bend your knee. Do not hold the couch

Ankle dorsiflexion

Supine, legs on cushion with ankles free; ankle in 100˚ plantarflexion

Around metatarsal heads

Antero-medial tibia and antero-lateral distal thigh

Pull your foot up

been made to develop normative values, the last published database of normative MVIC values for multiple muscle groups using a standardized and reproducible method was 10 years ago and in a North American population (21). No normative values are available for the current adult European population. In our centre, MVIC is a standard assessment performed on neuromuscular patients. The objective of this study was to generate reference values using a large number of healthy controls drawn from the Irish population, permitting normative interpretations for patients undergoing neuromuscular assessment. We aimed to estimate predicted reference percentile values for nine muscle groups that would allow patient performance values to be quickly visually located on a continuum of ‘normal’. The relationship between normative MVIC values and intrinsic factors such as sex, height, weight and BMI was also investigated. Material and methods

local papers, retirement group centres, and from broadcasts on local radio stations. Subjects were included if they were aged between 20 and 80 years of age, were in good health, capable of providing informed consent and of understanding the testing procedure. Subjects were excluded if they had inflammatory muscle or joint pathology, reports of pain prior to or during testing, cardiac or pulmonary problems and if they were pregnant or elite athletes. Written consent was obtained and the study had ethical approval from the hospital ethics board. Age and sex were recorded and height (m) and weight (kg) were measured using standard scales. Dominance of upper and lower limbs was also recorded by questioning the subject as to which hand they normally wrote with and which foot they would kick a ball with. A weekly level of activity was estimated on the following scale: 1) highly active; if subject pursued aerobic exercise for 30 min at least three times a week; 2) moderately active; if subject pursued aerobic exercise for 30 min less than three times per week; and 3) sedentary; if subject did not pursue aerobic exercise.

Subjects A convenience sample of subjects was recruited from hospital staff, family members of staff and patients, and from advertisements placed in the

Investigators There were two investigators in the study, both of whom had extensive experience (w3 years) in

MVIC: reference values

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Table II. Demographic data on subjects. Males (n5235) Healthy Subjects (n5494)

Mean (SD)

Age (years) Weight (Kgs) Height (m) Limb dominance Right hand dominant Left hand dominant Ambidextrous Numbers in each age decade 20–29 30–39 40–49 50–59 60–69 70–79

44.3 (15.2) 82.5 (11.7) 1.76 (0.08)

Min 19 60 1.5 Males (n5235) 213 (90.6%) 20 (8.5%) 2 (0.9%)

Females (n5259) Max

Mean (SD)

76 127 1.93

44.14 (14.6) 66.3 (11.19) 1.62 (0.06)

Min 19 43 1.43 Females (n5259) 237 (91.5%) 22 (8.5%) 0 (0%)

53 51 40 41 46 4

MVIC measurement. Inter- and intra-rater reliability had previously been established and reported (22). Procedure Testing took place in the Physiotherapy Department. The quantitative muscle assessment system (QMA, described elsewhere (22))* was used to quantify (in kg of force) MVIC of nine muscle groups: neck flexors, shoulder abductors and adductors, elbow flexors and extensors, hip flexors, knee flexors and extensors and ankle dorsiflexors, The QMA is a computerized MVIC system consisting of a strain gauge, data acquisition pad and a muscle testing frame (21). The strain gauge is attached with straps at one end to the muscle testing frame and at the other to the patient. Graphical representation of contraction force is visible on a monitor in real time. All muscle testing was performed in a gravity eliminated position. The testing procedure was strictly standardized and followed a protocol previously validated and reported by our centre (Table I) (22). This included standardization of order of testing, rest time between each test, number of tests per muscle group (n52), and verbal instruction and encouragement. The left side was tested first in the limb tests, and for each muscle group the maximum of two values was used for analysis. Subjects were verbally encouraged by the investigator during each muscle contraction and instructed to continue the contraction until the examiner saw a maximum value, which was typically after 3–5 s. Statistical analysis The variables of height and weight were converted into body mass index (BMI) using the equation

Max 73 103 1.83

54 51 53 50 46 5

(weight/(height)2. Exploratory analysis was carried out in JMPH. We used scatterplot smoothing splines to examine the effect of age on each parameter. Regression models were calculated using Stata Release 7’s quantile regression and fractional polynomial regression procedures (23). Where there was no evidence of a non-linear age effect, regression models were calculated using linear regression. Where there was a significant non-linearity, models were calculated using fractional polynomials which allow the fitting of complex curvilinear relationships using a small number of transformed variables. It was decided to allow weight and height to follow their natural distribution with age, as increasing the number of variables in the model decreased the precision of predicted values. Reference values for ages (in multiples of five) were calculated for men and women and for right- and left-sided measurements using quantile regression. Previous studies have used least squares regression to estimate predicted mean values (21,24). Quantile regression differs from least squares regression by estimating

Table III. Table showing frequency distribution of body mass indices and levels of activity of subjects. Males (n5235) Body Mass Index

n (%)

v18.5 18.5–24.9 25–29.9 30–39.9

0 74 114 47

(0) (31.5) (48.5) (20)

Level of activity Highly active Moderately active Sedentary

28 (12) 141 (60) 66 (28)

Females (n5259) n (%) 1 140 83 35

(0.004) (54.1) (32.1) (13.5)

57 (22) 171 (66) 31 (12)

*The Computer Source, QMA systems, P.O. Box 170 Gainesville, Georgia, 30506. Ross and Krieger, assembly and positioning systems, Potsdamer strabe 9 D 32423, Minden, Germany. Qbitus products, Unit 12, Victoria Park, Lightowler Road, Halifax, West Yorkshire UK, HX1 5ND.

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(a) Shoulder abduction Age

Neck Flexion (kg)

20 25 30 35 40 45 50 55 60 65 70

15.1 14.6 14.2 13.7 13.2 12.8 12.3 11.8 11.4 10.9 10.4

[10.1] [9.6] [9.9] [9.3] [9.6] [9.0] [9.4] [8.8] [9.1] [8.5] [8.8] [8.2] [8.6] [7.9] [8.3] [7.7] [8.1] [7.4] [7.8] [7.1] [7.5] [6.8]

Right (kg) 19.9 20.2 20.0 19.6 19.0 18.4 17.7 17.0 16.2 15.5 14.7

Shoulder adduction

Left (kg)

[12.8] [11.8] [11.9] [10.6] [11.4] [9.8] [11.0] [9.3] [10.6] [8.9] [10.2] [8.6] [9.8] [8.2] [9.4] [7.8] [8.9] [7.4] [8.3] [6.9] [7.6] [6.4]

18.6 19.0 18.8 18.4 17.8 17.1 16.5 15.7 15.0 14.2 13.4

[10.1] [10.1] [10.1] [10.1] [10.1] [10.1] [10.1] [10.1] [10.1] [10.1] [10.1]

Right (kg)

[9.6] [9.6] [9.6] [9.6] [9.6] [9.6] [9.6] [9.6] [9.6] [9.6] [9.6]

33.0 32.4 31.9 31.3 30.7 30.2 29.6 29.0 28.5 27.9 27.4

[21.6] [21.3] [21.1] [20.8] [20.5] [20.3] [20.0] [19.7] [19.4] [19.2] [18.9]

Elbow flexion Left (kg)

[19.8] [19.5] [19.2] [18.9] [18.6] [18.3] [18.0] [17.7] [17.4] [17.1] [16.8]

31.8 31.2 30.7 30.1 29.5 29.0 28.4 27.9 27.3 26.7 26.2

[21.4] [21.1] [20.8] [20.5] [20.3] [20.0] [19.7] [19.5] [19.2] [18.9] [18.6]

Right (kg)

[19.4] [19.1] [18.8] [18.5] [18.2] [17.5] [17.6] [17.3] [17.0] [16.7] [16.4]

29.5 29.8 29.7 29.5 29.2 28.7 28.2 27.5 26.9 26.1 25.3

[23.5] [23.5] [23.4] [23.3] [23.1] [22.8] [22.3] [21.6] [20.7] [19.5] [18.1]

Elbow extension Left (kg)

[22.7] [22.4] [22.0] [21.5] [21.0] [20.5] [19.9] [19.4] [18.8] [18.3] [17.7]

29.0 29.2 29.1 28.9 28.6 28.1 27.6 27.0 26.3 25.5 24.7

[23.5] [23.5] [23.4] [23.3] [23.1] [22.8] [22.3] [21.6] [20.7] [19.5] [18.1]

Right (kg)

[21.8] [21.5] [21.1] [20.6] [20.1] [19.5] [19.0] [18.4] [17.9] [17.3] [16.8]

18.3 18.4 18.5 18.6 18.6 18.6 18.5 18.2 17.9 17.3 16.6

[14.6] [14.6] [14.6] [14.6] [14.6] [14.5] [14.2] [13.9] [13.4] [12.7] [11.8]

[13.8] [13.8] [13.8] [13.7] [13.6] [13.4] [13.1] [12.8] [12.3] [11.8] [11.1]

Left (kg) 17.5 17.6 17.7 17.8 17.8 17.8 17.7 17.5 17.1 16.6 15.8

[13.8] [13.9] [13.9] [13.9] [13.9] [13.7] [13.5] [13.2] [12.6] [12.0] [11.1]

[13.3] [13.3] [13.3] [13.2] [13.0] [12.8] [12.6] [12.3] [11.8] [11.3] [10.6]

(b) Hip flexion Age 20 25 30 35 40 45 50 55 60 65 70

Right (kg) 20.9 20.3 19.7 19.1 18.5 17.9 17.3 16.7 16.1 15.4 14.8

[14.8] [14.4] [14.1] [13.8] [13.4] [13.1] [12.8] [12.4] [12.1] [11.8] [11.5]

[13.7] [13.4] [13.1] [12.8] [12.4] [12.1] [11.8] [11.5] [11.2] [10.9] [10.6]

Knee extension Left (kg)

20.2 19.6 19.0 18.4 17.7 17.1 16.5 15.9 15.3 14.7 14.1

[13.7] [13.4] [13.1] [12.7] [12.4] [12.1] [11.7] [11.4] [11.1] [10.7] [10.4]

[12.4] [12.1] [11.8] [11.5] [11.2] [10.9] [10.6] [10.3] [10.0] [9.6] [9.3]

Right (kg) 33.5 31.2 29.8 28.8 27.8 26.8 25.8 24.6 23.3 21.8 20.0

[23.6] [22.9] [22.4] [21.8] [21.1] [20.3] [19.3] [18.1] [16.7] [15.0] [13.1]

[22.9] [21.4] [20.3] [19.5] [18.6] [17.7] [16.7] [15.5] [14.1] [12.4] [10.5]

Knee flexion Left (kg)

31.4 29.1 27.7 26.6 25.7 24.7 23.7 22.5 21.2 19.7 17.9

[21.9] [21.2] [20.7] [20.1] [19.4] [18.6] [17.6] [16.4] [15.0] [13.4] [11.4]

[21.7] [20.2] [19.1] [18.3] [17.4] [16.5] [15.5] [14.3] [12.9] [11.2] [9.3]

Right (kg) 17.4 16.2 15.5 14.9 14.4 13.9 13.4 12.8 12.1 11.4 10.5

[13.0][11.3] [11.5][10.1] [10.6] [9.4] [10.0] [8.8] [9.4] [8.4] [8.8] [7.9] [8.3] [7.4] [7.6] [6.9] [6.9] [6.3] [6.1] [5.6] [5.2] [4.8]

Ankle dorsiflexion Left (kg)

16.2 15.0 14.2 13.7 13.2 12.7 12.2 11.6 10.9 10.2 9.3

[12.5] [10.8] [11.0] [9.6] [10.1] [8.9] [9.5] [8.4] [8.9] [7.9] [8.4] [7.5] [7.8] [7.0] [7.2] [6.4] [6.4] [5.8] [5.6] [5.2] [4.7] [4.4]

Right (kg) 20.8 21.2 21.2 21.0 20.6 20.1 19.5 18.9 18.1 17.3 16.4

[16.0] [16.2] [16.3] [16.4] [16.3] [16.1] [15.6] [15.0] [14.0] [12.7] [11.1]

[15.3] [15.3] [15.3] [15.3] [15.1] [14.8] [14.3] [13.7] [12.9] [11.9] [10.6]

Left (kg) 20.1 20.5 20.5 20.3 19.9 19.4 18.9 18.2 17.4 16.6 15.7

[15.0] [15.2] [15.3] [15.4] [15.3] [15.0] [14.6] [13.9] [13.0] [11.7] [10.1]

[14.0] [14.0] [14.0] [14.0] [13.8] [13.5] [13.0] [12.4] [11.6] [10.6] [9.3]

D. Meldrum et al.

Table IV. Males: Median, [10th percentile] and [5th percentile] predicted MVIC normative values in kg of force for upper limb (a) and lower limb (b) muscle groups.

Table V. Females: Median, [10th percentile] and [5th percentile] predicted MVIC normative values in kg of force for upper limb (a) and lower limb (b) muscle groups. (a) Shoulder abduction Age 20 25 30 35 40 45 50 55 60 65 70

Neck Flexion (kg)

Right (kg)

[8.1] [7.8] [7.6] [7.3] [7.1] [6.8] [6.5] [6.3] [6.0] [5.8] [5.5]

11.2 [7.8] [7.3] 11.6 [6.9] [6.1] 11.4 [6.4] [5.4] 10.9[5.9] [4.9] 10.4 [5.6] [4.5] 9.7 [5.2] [4.1] 9.0 [4.8] [3.8] 8.3 [4.3] [3.4] 7.6 [3.8] [2.9] 6.8 [3.2] [2.5] 6.0 [2.6] [1.9]

12.0 11.5 11.1 10.6 10.1 9.7 9.2 8.7 8.3 7.8 7.4

[7.1] [6.8] [6.5] [6.2] [6.0] [5.7] [5.4] [5.1] [4.9] [4.6] [4.3]

Shoulder adduction Left (kg)

10.0 10.3 10.1 9.7 9.1 8.5 7.8 7.1 6.3 5.6 4.8

[7.1] [6.3] [5.7] [5.3] [4.9] [4.6] [4.2] [3.7] [3.2] [2.6] [1.9]

Right (kg)

[7.0] [5.8] [5.1] [4.6] [4.2] [3.8] [3.5] [3.1] [2.7] [2.2] [1.6]

19.5 18.9 18.3 17.8 17.2 16.6 16.1 15.5 15.0 14.4 13.8

Elbow flexion Left (kg)

[12.1] [10.4] [11.8] [10.1] [11.5] [9.8] [11.2] [9.5] [11.0] [9.2] [10.7] [8.9] [10.4] [8.6] [10.2] [8.3] [9.9] [8.0] [9.6] [7.7] [9.3] [7.4]

18.3 17.7 17.1 16.6 16.0 15.5 14.9 14.3 13.8 13.2 12.6

Right (kg)

[11.8] [10.0] [11.5] [9.7] [11.3] [9.4] [11.0] [9..1] [10.7] [8.8] [10.5] [8.5] [10.2] [8.2] [9.9] [7.9] [9.6] [7.7] [9.4] [7.4] [9.1] [7.1]

18.5 18.8 18.7 18.5 18.2 17.7 17.2 16.5 15.9 15.1 14.3

Elbow extension Left (kg)

[14.8] [14.6] [14.8] [14.4] [14.8] [14.0] [14.6] [13.5] [14.4] [13.0] [14.1] [12.4] [13.6] [11.9] [12.9] [11.3] [12.0] [10.7] [10.9] [10.2] [9.4] [9.7]

18.0 18.2 18.1 17.9 17.6 17.1 16.6 16.0 15.3 14.5 13.7

Right (kg)

[14.8] [13.7] [14.8] [13.4] [14.8] [13.0] [14.6] [12.5] [14.4] [12.0] [14.1] [11.5] [13.6] [10.9] [12.9] [10.4] [12.0] [9.8] [10.9] [9.3] [9.4] [8.7]

10.4 10.5 10.6 10.7 10.7 10.7 10.6 10.3 10.0 9.4 8.7

[8.3] [8.3] [8.4] [8.4] [8.3] [8.2] [8.0] [7.6] [7.1] [6.4] [5.5]

Left (kg)

[7.6] [7.6] [7.6] [7.5] [7.3] [7.1] [6.9] [6.6] [6.1] [5.6] [4.9]

9.6 9.7 9.8 9.9 9.9 9.9 9.8 9.6 9.2 8.6 7.9

[7.6] [7.6] [7.7] [7.7] [7.6] [7.5] [7.2] [6.9] [6.4] [5.7] [4.8]

[7.1] [7.1] [7.0] [7.0] [6.8] [6.6] [6.4] [6.0] [5.6] [5.1] [4.4]

(b) Hip flexion Age

28.2 27.6 27.0 26.4 25.8 25.2 24.6 24.0 23.4 22.8 22.2

[20.3] [20.0] [19.7] [19.4] [19.0] [18.7] [18.4] [18.0] [17.7] [17.4] [17.0]

[19.2] [18.9] [18.6] [18.3] [18.0] [17.7] [17.4] [17.1] [16.7] [16.4] [16.1]

Left (kg) 27.5 26.9 26.3 25.7 25.1 24.5 23.9 23.3 22.7 22.1 21.5

[19.3] [19.0] [18.6] [18.3] [18.0] [17.6] [17.3] [17.0] [16.6] [16.3] [16.0]

[18.0] [17.7] [17.4] [17.0] [16.7] [16.4] [16.1] [15.8] [15.5] [15.2] [14.9]

Right (kg) 49.6 47.4 46.0 44.9 43.9 43.0 41.9 40.8 39.4 37.9 36.2

[33.5] [32.9] [32.4] [31.8] [31.1] [30.3] [29.3] [28.1] [26.7] [25.0] [23.1]

[32.3] [30.8] [29.7] [28.9] [28.0] [27.1] [26.1] [24.9] [23.4] [21.8] [19.9]

Knee flexion Left (kg)

47.5 45.3 43.9 42.8 41.8 40.9 39.8 38.7 37.3 35.8 34.1

[31.9] [31.2] [30.7] [30.1] [29.4] [28.6] [27.6] [26.4] [25.0] [23.3] [21.4]

[31.1] [29.6] [28.5] [27.7] [26.8] [25.9] [24.9] [23.7] [22.2] [20.6] [18.7]

Right (kg) 25.7 24.5 23.8 23.2 22.7 22.2 21.7 21.1 20.5 19.7 18.8

[17.8] [16.4] [15.5] [14.8] [14.2] [13.7] [13.1] [12.5] [11.8] [11.0] [10.0]

[15.1] [13.9] [13.1] [12.6] [12.1] [11.7] [11.2] [10.7] [10.1] [9.4] [8.6]

Ankle dorsiflexion Left (kg)

24.5 23.3 22.6 22.0 21.5 21.0 20.5 19.9 19.2 18.5 17.6

[17.3] [14.6] [15.9] [13.4] [15.0] [12.7] [14.3] [12.1] [13.8] [11.7] [13.2] [11.2] [12.6] [10.7] [12.0] [10.2] [11.3] [9.6] [10.5] [8.9] [9.6] [8.1]

Right (kg) 29.7 30.0 30.0 29.8 29.4 29.0 28.4 27.7 27.0 26.1 25.2

[22.1] [22.3] [22.4] [22.5] [22.4] [22.2] [21.7] [21.0] [20.1] [18.8] [17.2]

[20.2] [20.3] [20.3] [20.2] [20.0] [19.7] [19.3] [18.7] [17.9] [16.9] [15.6]

Left (kg) 29.0 29.4 29.4 29.1 28.8 28.3 27.7 27.0 26.3 25.5 24.6

[21.0] [21.2] [21.4] [21.4] [21.4] [21.1] [20.7] [20.0] [19.0] [17.8] [16.2]

[19.0] [19.0] [19.0] [18.9] [18.8] [18.5] [18.0] [17.4] [16.6] [15.6] [14.3]

MVIC: reference values

20 25 30 35 40 45 50 55 60 65 70

Right (kg)

Knee extension

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Figure 1. 46-year-old male MND patient performance on MVIC testing showing location of patient values in relation to 50th and 5th predicted percentile values obtained from normative data.

specified quantiles using the method of minimum absolute deviation. This has two advantages over the more usual method of least squares regression. The first is that percentiles are inherently easier to understand than averages. The second is that expressing a result as ‘percent below average’ does not tell one how ‘abnormal’ a value is. Percentiles allow location of the score in terms of the rest of the population and easy identification of when a subject’s values have dropped below typical values. Median, 10th and 5th percentile values were calculated for each muscle group. Higher percentile values were not calculated as lower strength values are more commonly encountered in neuromuscular patient groups.

Results Four hundred and ninety-four subjects – 235 males and 259 females – were recruited and tested. Demographic data including numbers of patients per age decade stratified by sex, and upper limb

dominance data are shown in Table II and body mass index data and subjects’ level of activity are shown in Table III. Normative values 50th (median), 10th and 5th predicted percentile MVIC values for muscle tests stratified by side and sex are shown in Tables IV and V. Values decreased with age with strongest muscle groups showing the greatest decline. As expected, males had significantly higher values than females in all tests (pv0.05 using an independent t-test). For the majority of tests, highest values were obtained by males in the 20–30 years of age categories. Right side values were also significantly higher than left side values (pv0.05 using a dependent t-test). Pearson’s correlation showed a significant inverse correlation of values with age (pv0.05; all muscle tests). BMI values were positively correlated (pv0.05) with all tests with the exception of hip flexion and neck flexion in both males and females, shoulder adduction (males only) and ankle dorsiflexion (females only).

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Figure 2. 60-year-old female post-polio patient performance on MVIC testing showing location of patient values in relation to 50th and 5th predicted percentile values obtained from normative data. Lower limbs were affected more by polio virus than upper limbs.

Subjects’ level of activity was also positively correlated with all tests except shoulder adduction and abduction. The 95% confidence intervals for each of the predicted values were also calculated (not shown) and these can be downloaded from http://www.rcsi. ie/school_physiotherapy/QMA_Normative_Data/. Case study No. 1. Patient with motor neuron disease This patient was a 46-year-old male who was admitted with a 2.5 year history of fasiculation of his upper and lower limbs. Subjectively, he complained of no muscle weakness. Nerve conduction studies were normal and electromyography showed diffuse fasciculation with decreased recruitment in the majority of muscles sampled. Pulmonary function tests showed decreased lung volumes. MVIC testing (Figure 1) revealed markedly reduced values on all muscles tested, particularly of the elbow. No muscle group reached the 50th predicted percentile for his age group. The patient underwent sequential

MVIC testing over a period of a year and a diagnosis of MND was made. Case study No. 2. Patient with post-polio syndrome A 60-year-old female with a history of contracting the polio virus at age 3 years presented complaining of deteriorating mobility and low back pain. Initially the virus affected her lower limbs with the right being more severely affected, but she had recovered to the point of ambulation independently with a caliper on her right lower limb and two crutches. On MVIC testing (Figure 2) all lower limb muscle groups were below the 5th percentile for her age group. Upper limb values were generally above the 50th percentile except for her elbow flexors and left shoulder abductors. Interestingly, she reported the initial polio virus did not affect her upper limbs but she had a history of carpal tunnel syndrome bilaterally. Sequential MVIC over five years showed lower values in most muscle groups confirming a diagnosis of post-polio syndrome.

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Discussion Patients presenting with muscle weakness due to neurological or other disorders should undergo strength testing as part of a standard clinical assessment. When strength testing is quantitative, as it is with the QMA system in our centre, patients usually enquire as to whether their values are ‘normal’ or not. The development of this database permits the definition of a continuum of normative predicted percentile value (Figures 1, 2). This is useful in diagnosis, estimation of progression, plateau or amelioration of disease. It also assists in treatment planning, as the relevance of any beneficial effect can be determined with respect to the normal population. Although normative values have previously been established in a North American population (21), this study provides European normative values. The statistical analysis we performed differed from previous studies as it generated predicted percentile values rather than predicted average values (21,24). Percentile values are easier to interpret clinically, as it can be seen when a patient is performing within normal ranges, and if not, the extent of weakness compared to age-adjusted norms (Figures 1, 2). Thus the clinical application of this method is readily apparent and was the predominant aim of this study. However, for research studies investigating therapeutic interventions or longitudinal studies the data are equally utilizable and will easily compute differences. There are some important limitations to the values of the database. First, although the sample is drawn from a control population, it was not a random sample, leading to possible bias. Secondly, the database represents a cross-section of individuals and does not track strength in individuals longitudinally. Thirdly, numbers in the older age category of 70+ years were small and there are no normative data for the over 80 years age group. It is commonly accepted that a more rapid decrease in strength occurs the 70–90 years age group (25). Finally, the values are not transferable to strain gauge equipment or methods of testing other than described and standardized at our site. However, this system is used in many other centres across the world and if the testing protocol is followed, and reliability established, the values identified in our centre may be used as European population normative values against which other populations can be calibrated. Conclusion In conclusion, we have developed a normative reference database of percentile values of muscle strength for use in the clinical and research setting. We have also developed a clinical reporting system that is inherently easy for all end-users to interpret.

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