Normalizing Lower-Extremity Strength Data for Children Without Disability Using Allometric Scaling

Normalization alters the interpretation of hip strength in established unilateral lower limb prosthesis users

BACKGROUND

Valid comparisons of muscle strength between individuals or legs that differ in size requires normalization, often by simple anthropometric variables. Few studies of muscle strength in lower-limb prosthesis users have normalized strength data by any anthropometric variable, potentially confounding our understanding of strength deficits in lower-limb prosthesis users. The objective of this pilot study was to determine the need for as well as effectiveness and impact of normalizing hip strength in lower-limb prosthesis users.

METHODS

Peak isometric hip extension and abduction torques were collected from 28 lower-limb prosthesis users. Allometric scaling was used to determine if hip torque values were significantly associated with, and therefore needed to be adjusted for, body mass, thigh length, or body mass x thigh length, and whether normalization was effective in reducing any associations. Between limb differences in peak hip torque, and correlations with balance ability, were inspected pre- and post-normalization.

FINDINGS

Hip torques were consistently and significantly associated with body-mass x thigh length. Associations between peak hip torque and body-mass x thigh length were reduced by normalization. After normalization by body-mass x thigh length, between limb differences in hip extension torque, as well as the correlation between hip abduction torque and balance ability, changed from non-significant to significant.

INTERPRETATION

In the absence of normalization, hip strength (i.e., peak torque) in lower-limb prosthesis users remains dependent on basic anthropometric variables, masking relationships between hip strength and balance ability, as well as between limb differences.

Establishing Reference Values for Isometric Knee Extension and Flexion Strength

Single-joint isometric and isokinetic knee strength assessment plays an important role in strength and conditioning, physical therapy, and rehabilitation. The literature, however, lacks absolute reference values. We systematically reviewed the available studies that assessed isometric knee strength. Two scientific databases (PubMed and PEDro) were searched for the papers that are published from the inception of the field to the end of 2019. We included studies that involved participants of both genders and different age groups, regardless of the study design, that involved isometric knee extension and/or flexion measurement. The extracted data were converted to body-mass-normalized values. Moreover, the data were grouped according to the knee angle condition (extended, mid-range, and flexed). A meta-analysis was performed on 13,893 participants from 411 studies. In adult healthy males, the pooled 95% confidence intervals (CI) for knee extension were 1.34–2.23Nm/kg for extended knee angle, 2.92–3.45Nm/kg for mid-range knee angle, and 2.50–3.06Nm/kg for flexed knee angle, while the CIs for flexion were 0.85–1.20, 1.15–1.62, and 0.96–1.54Nm/kg, respectively. Adult females consistently showed lower strength than adult male subgroups (e.g., the CIs for knee extension were 1.01–1.50, 2.08–2.74, and 2.04–2.71Nm/kg for extended, mid-range, and flexed knee angle condition). Older adults consistently showed lower values than adults (e.g., pooled CIs for mid-range knee angle were 1.74–2.16Nm/kg (male) and 1.40–1.64Nm/kg (female) for extension, and 0.69–0.89Nm/kg (male) and 0.46–0.81Nm/kg (female) for flexion). Reliable normative for athletes could not be calculated due to limited number of studies for individual sports.

Effect of Plantar Flexor Muscle Strengthening on the Gait of Children With Idiopathic Toe Walking: A Study Protocol

PURPOSE

To evaluate the effect of plantar flexor strengthening associated with conventional physical therapy treatment in participants with idiopathic toe walking.

METHODS

Thirty participants, of both sexes diagnosed with idiopathic toe walking, aged 5 and 11 years, will be recruited and randomized into 2 groups: the control group, who will undergo gait training, triceps surae muscle stretching, anterior tibial muscle strengthening, and motor sensory training, and the intervention group, who will undergo the same training as the control group and, additionally, triceps surae muscle strengthening. The intervention will be performed twice a week for 8 weeks. The participants will undergo a 3-dimensional gait kinematic analysis, passive amplitude of dorsiflexion movement, isometric dynamometry of the anterior tibial and triceps surae muscles, motor coordination, quality of life, and perception of the parents regarding the equinus gait at baseline and at the end of treatment. Quality of life will be reevaluated during a 24-week follow-up.

Hand-Held Dynamometry Isometric Torque Reference Values for Children and Adolescents

PURPOSE

To establish hand-held dynamometry (HHD) maximal isometric muscle torque (MIT) reference values for children and adolescents who are developing typically.

METHODS

The MIT of 10 upper and lower limb muscle groups was assessed in 351 Caucasian youth (4 years 2 months to 17 years) using a standardized HHD protocol, previously shown to be feasible, valid, and reliable.

RESULTS

The mean MIT and 95% confidence interval of the mean for all muscle groups, for each of the 14 age groups (1 year age span for each group), and for each sex, were reported in both absolute (Nm) and normalized (Nm/kg) values.

CONCLUSION

These HHD reference values may be helpful in the identification of muscle strength impairments in several pediatric populations, especially when bilateral impairments are present.

Knee extensor strength differences in obese and healthy-weight 10-to 13-year-olds

The purpose of this study was to investigate if obese children have reduced knee extensor (KE) strength and to explore the relationship between adiposity and KE strength. An observational case-control study was conducted in three Australian states, recruiting obese [N = 107 (51 female, 56 male)] and healthy-weight [N = 132 (56 female, 76 male)] 10- to 13-year-old children. Body mass index, body composition (dual energy X-ray absorptiometry), isokinetic/isometric peak KE torques (dynamometry) and physical activity (accelerometry) were assessed. Results revealed that compared with their healthy-weight peers, obese children had higher absolute KE torques (P ≤ 0.005), equivocal KE torques when allometrically normalized for fat-free mass (FFM) (P ≥ 0.448) but lower relative KE torques when allometrically normalized for body mass (P ≤ 0.008). Adjustments for maternal education, income and accelerometry had little impact on group differences, except for isometric KE torques relative to body mass which were no longer significantly lower in obese children (P ≥ 0.013, not significant after controlling for multiple comparisons). Percent body fat was inversely related to KE torques relative to body mass (r = -0.22 to -0.35, P ≤ 0.002), irrespective of maternal education, income or accelerometry. In conclusion, while obese children have higher absolute KE strength and FFM, they have less functional KE strength (relative to mass) available for weight-bearing activities than healthy-weight children. The finding that FFM-normalized KE torques did not differ suggests that the intrinsic contractile properties of the KE muscles are unaffected by obesity. Future research is needed to see if deficits in KE strength relative to mass translate into functional limitations in weight-bearing activities.

Knee extension strength in obese and nonobese male adolescents

The aim of the present study was to compare "absolute" and "relative" knee extension strength between obese and nonobese adolescents. Ten nonobese and 12 severely obese adolescent boys of similar chronological age, maturity status, and height were compared. Total body and regional soft tissue composition were determined using dual-energy X-ray absorptiometry (DXA). Knee extensors maximum voluntary contraction (MVC) torque was measured using an isometric dynamometer at a knee angle of 60° (0° is full extension). Absolute MVC torque was significantly higher in obese adolescents than in controls. However, although MVC torque expressed per unit of body mass was found to be significantly lower in obese adolescent boys, no significant difference in MVC torque was found between groups when normalized to fat-free mass. Conversely, when correcting for thigh lean mass and estimated thigh muscle mass, MVC torque was significantly higher in the obese group (17.9% and 22.2%, respectively; P <0.05). To conclude, our sample of obese adolescent boys had higher absolute and relative knee extension strength than our nonobese controls. However, further studies are required to ascertain whether or not relative strength, measured with more accurate in vivo methods such as magnetic resonance imaging, is higher in obese adolescents than in nonobese controls.

The normalization of explosive functional movements in a diverse population of elite American football players

The objective of this study was to investigate the need to normalize, for body mass, explosive functional tasks in a population exhibiting diverse body masses. Measures investigated in elite college American football players attending the National Football League's annual combine (n = 1,136) were the 9.1-, 18.3-, and 36.6-m sprints, vertical and horizontal jumps, 18.3-m shuttle, and 3-cone drill. To determine the relationship between body mass and performance outcomes, Pearson's correlation coefficients (r) were generated using log-transformed data. Task-specific allometric exponents, accounting for body mass, were also determined. The strength of the correlations suggests that sprint and jump abilities are associated with body mass, whereas change-of-direction ability is not. The determined allometric exponents range between 0.296 and -0.463 for the sprint and jump tasks and are -0.022 and -0.006 for the 18.3-m shuttle and the 3-cone drill, respectively. In populations exhibiting relatively large variations in body mass, normalization of sprint and jump abilities is recommended, whereas normalization of change-of-direction ability is unwarranted. Novel suggestions derived from the present research are that sprint and jump abilities in diverse populations warrant normalization and that physical attributes associated with explosive functional movements deserve attribute-specific consideration when contemplating normalization.

Normalizing hip muscle strength: establishing body-size-independent measurements

OBJECTIVE

To investigate the effectiveness of computing body-size-independent hip strength measures using muscle-specific allometric scaling and ratio standard normalization methods.

DESIGN

Cross-sectional study.

SETTING

University laboratory.

PARTICIPANTS

A convenience sample of healthy participants (N=113; 42 men, 71 women).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Anthropometric measurements of the leg and thigh were obtained, and maximal hip strength was tested (medial and lateral rotation, abduction, adduction, flexion, extension). Strength was measured isometrically as force (kg) and then converted to torque (Nm).

RESULTS

The allometric scaling analysis resulted in exponents for normalizing body mass (BM) in each muscle group assessed. In addition, a 6-muscle average exponent was also computed (bavg) for force (men, .554; women, .335) and torque (men, .792; women, .482). The nonsignificant results of the linear regression analysis revealed that normalizing hip strength to BM(bavg) (hip strength/BM(bavg)) effectively removed the influence of BM on force and torque. However, sex should be factored into analyses of allometric scaling because men have higher b-values than women for both force and torque. The linear regression analyses also demonstrated that force normalized to BM (P=.162-.895) and torque normalized to BM × Height (P=.146-.889) were body-size-independent measures. Force normalized to BM⁰·⁶⁷ (P=.001-.191) and body mass index (BMI) (P=<.001-.066), and torque normalized to BM (P=.004-.415) and BMI (P<.001) were significantly related to BM and therefore were not body-size independent.

CONCLUSIONS

Normalizing force and torque to BM(bavg) is the most effective method of removing body-size dependence and allowing comparisons of persons with differing body sizes.

Obesity: the new childhood disability?

This review addresses the impact of obesity on paediatric physical functioning utilizing the World Health Organization International Classification of Functioning, Disability and Health Framework (ICF). The ICF encompasses functioning (as it relates to all body functions and structures), activities (undertaking a particular task) and participation (in a life situation) with disability referring to impairments in body functions/structures, activity restrictions or participation limitations. Electronic databases were searched for peer-reviewed studies published in English prior to May 2009 that examined aspects of physical functioning in children (≤18 years). Eligible studies (N = 104) were ranked by design and synthesized descriptively. Childhood obesity was found to be associated with deficits in function, including impaired cardiorespiratory fitness and performance of motor tasks; and there was some limited evidence of increased musculoskeletal pain and decrements in muscle strength, gait and balance. Health-related quality of life and the subset of physical functioning was inversely related to weight status. However, studies investigating impacts of obesity on wider activity and participation were lacking. Further research utilizing the ICF is required to identify and better characterize the effects of paediatric obesity on physical function, activity and participation, thereby improving targets for intervention to reduce disability in this population.

A regression method for strength comparisons in children

A method to estimate means and variance of strength based on anthropometric data is presented. The method is applied using a database of 10 lower extremity strength measures recorded from 48 typically developing children with a handheld dynamometer. Seven anthropometric variables are considered, and the combination of height and BMI is determined as a set of variables best suited to model these muscle groups. This regression scheme accounts for 45-58% of the observed variance. A clinical example illustrating the utility of the method is presented.

Is grip strength a predictor for total muscle strength in healthy children, adolescents, and young adults?

The primary purpose of this study was to examine whether grip strength is related to total muscle strength in children, adolescents, and young adults. The second purpose was to provide reference charts for grip strength, which could be used in the clinical and research setting. This cross-sectional study was performed at primary and secondary schools and the University of Applied Sciences. Three hundred and eighty-four healthy Dutch children, adolescents, and young adults at the age of 8 to 20 years participated. Isometric muscle strength was measured with a handheld dynamometer of four muscle groups (shoulder abductors, grip strength, hip flexors, and ankle dorsiflexors). Total muscle strength was a summing up of shoulder abductors, hip flexors, and ankle dorsiflexors. All physical therapists participated in a reliability study. The study was started when intratester and intertester reliability was high (Pearson correlation coefficient >0.8). Grip strength was strongly correlated with total muscle strength, with correlation coefficients between 0.736 and 0.890 (p < 0.01). However, the correlation was weaker when controlled for weight (0.485-0.564, p < 0.01). Grip strength is related to total muscle strength. This indicates, in the clinical setting, that grip strength can be used as a tool to have a rapid indication of someone's general muscle strength. The developed reference charts are suitable for evaluating muscle strength in children, adolescents, and young adults in clinical and research settings.

Normalizing lower extremity strength data for children, adolescents, and young adults with cerebral palsy

The traditional method for normalizing quantitative strength data is to divide force or torque by body mass. We have previously shown that this method is not appropriate for able-bodied children and young adults and that normalization using allometric scaling is more effective. The purpose of the current study was to evaluate the effectiveness of applying existing normalization equations for lower extremity strength to children, adolescents, and young adults with cerebral palsy (CP) and, if appropriate, to develop CP-specific normalization equations using allometric scaling. We measured the maximum torque generated during hip abduction/adduction, knee extension/flexion, and ankle dorsiflexion/plantar flexion in 96 subjects with spastic diplegia CP ages 4-23 years. Traditional mass normalization (Torque/Mass1.0) and allometric scaling equations from children without disability (Torque/Mass1.6 for hip and knee; Torque/Mass1.4 for ankle) were not effective in eliminating the influence of body mass. Normalization using CP-specific allometric scaling equations was effective using both muscle-specific and common (Torque/Mass0.8 for ankle plantar flexors; Torque/Mass1.4 for all others) scaling relationships. For the first time, normalization equations have been presented with demonstrated effectiveness in adjusting strength measures for body size in a group of children, adolescents, and young adults with CP.

Walking ability is related to muscle strength in children with cerebral palsy

The purpose of this study was to assess the relation between muscle strength and walking ability in children with bilateral spastic cerebral palsy at GMFCS levels I-III. 55 children (mean age 10.7, range 5-15) were tested for muscle strength in eight lower limb muscle groups with a handheld myometer. They were also tested with the Gross Motor Function Measure domains for standing and walking, running and jumping. Muscle strength in the legs was below normative predicted value in most of the children, with muscle weakness most pronounced around the ankle, followed by the hip muscles. There was a significant difference in muscle strength between GMFCS levels. There was also a moderate to high correlation between muscle strength and the GMFM, indicating that muscle weakness affects walking ability. Independently walking children had more than 50% of predicted muscle strength values.