Soleus muscle length, stretch reflex excitability, and the contractile properties of muscle in children and adults: a study of the functional joint angle

Calculating sample entropy from isometric torque signals: methodological considerations and recommendations

We investigated the effect of different sampling frequencies, input parameters and observation times for sample entropy (SaEn) calculated on torque data recorded from a submaximal isometric contraction. Forty-six participants performed sustained isometric knee flexion at 20% of their maximal contraction level and torque data was sampled at 1,000 Hz for 180 s. Power spectral analysis was used to determine the appropriate sampling frequency. The time series were downsampled to 750, 500, 250, 100, 50, and 25 Hz to investigate the effect of different sampling frequency. Relative parameter consistency was investigated using combinations of vector lengths of two and three and tolerance limits of 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, and 0.4, and data lengths between 500 and 18,000 data points. The effect of different observations times was evaluated using Bland-Altman plot for observations times between 5 and 90 s. SaEn increased at sampling frequencies below 100 Hz and was unaltered above 250 Hz. In agreement with the power spectral analysis, this advocates for a sampling frequency between 100 and 250 Hz. Relative consistency was observed across the tested parameters and at least 30 s of observation time was required for a valid calculation of SaEn from torque data.

Entropy Analysis in Gait Research: Methodological Considerations and Recommendations

The usage of entropy analysis in gait research has grown considerably the last two decades. The present paper reviews the application of different entropy analyses in gait research and provides recommendations for future studies. While single-scale entropy analysis such as approximate and sample entropy can be used to quantify regularity/predictability/probability, they do not capture the structural richness and component entanglement characterized by a complex system operating across multiple spatial and temporal scales. Thus, for quantification of complexity, either multiscale entropy or refined composite multiscale entropy is recommended. For both single- and multiscale-scale entropy analyses, care should be made when selecting the input parameters of tolerance window r, vector length m, time series length N and number of scales. This selection should be based on the proposed research question and the type of data collected and not copied from previous studies. Parameter consistency should be investigated and published along with the main results to ensure transparency and enable comparisons between studies. Furthermore, since the interpretation of the absolute size of both single- and multiscale entropy analyses outcomes is not straightforward, comparisons should always be made with a control condition or group.

Age-related differences in the neural regulation of stretch-shortening cycle activities in male youths during maximal and sub-maximal hopping

The aim of the current study was to investigate potential age-related differences in neural regulation strategies during maximal and sub-maximal hopping. Thirty-two boys from three different age groups (9-, 12- and 15-years), completed trials of both maximal and sub maximal hopping, and based on contact and flight times, measures of reactive strength index (RSI=jump height/contact time) and leg stiffness (peak ground reaction force/peak displacement of centre of mass) were collected respectively. During all trials, surface electromyograms (EMG) were recorded from four different muscle sites of the dominant lower limb, during 100ms pre-ground contact, and then four subsequent stretch reflex phases: background muscle activity (0-30ms), short-latency stretch reflex (31-60ms), intermediate15 latency stretch reflex 61-90ms and long-latency stretch reflex (91-120ms). Reactive strength index and leg stiffness were measured during the hopping trials. During maximal hopping, both 12- and 15-year olds produced significantly greater RSI (P<0.02) than 9-year olds, with 15-year olds utilising significantly greater soleus muscle activity during the 100ms prior to ground contact than the younger age groups (P<0.01). During sub-maximal hopping, 15-year olds produced significantly greater absolute leg stiffness than both 12- and 9-year olds (P<0.01), with 9-year olds producing significantly less soleus muscle activity during the 31-60ms time phase. For all age groups, sub-maximal hopping was associated with significantly greater background muscle activity and short-latency stretch reflex activity in the soleus and vastus lateralis, when compared to maximal hopping (P<0.001). Results suggest that as children mature, they become more reliant on supra-spinal feed forward input and short latency stretch reflexes to regulate greater levels of leg stiffness and RSI when hopping.

Reliability and validity of the standing heel-rise test

A standardized protocol for a pediatric heel-rise test was developed and reliability and validity are reported. Fifty-seven children developing typically (CDT) and 34 children with plantar flexion weakness performed three tests: unilateral heel rise, vertical jump, and force measurement using handheld dynamometry. Intraclass correlation coefficients (ICC) varied from 0.85-0.99 for reliability analyses in both participant groups. Construct validity analysis revealed a significant difference between groups (F = 44.57, p < .05) and age-related differences among CDT; 5- to 8-year olds (mean = 15.2, SD = 5.4) performed fewer repetitions compared to 9- to 12-year olds (mean = 27.7, SD = 11.7) (p < .05). Age explained 41% of the variance in the number of heel-rise repetitions. Correlations between the three tests (r = 0.56 to 0.66) provide evidence of convergent validity. The results indicate that the standardized protocol is both reliable and valid for use in 5- to 12-year-old children with and without plantar flexion weakness.

Tuning passive mechanics through differential splicing of titin during skeletal muscle development

During postnatal development, major changes in mechanical properties of skeletal muscle occur. We investigated passive properties of skeletal muscle in mice and rabbits that varied in age from 1 day to approximately 1 year. Neonatal skeletal muscle expressed large titin isoforms directly after birth, followed by a gradual switch toward progressively smaller isoforms that required weeks-to-months to be completed. This suggests an extremely high plasticity of titin splicing during skeletal muscle development. Titin exon microarray analysis showed increased expression of a large group of exons in neonatal muscle, when compared to adult muscle transcripts, with the majority of upregulated exons coding for the elastic proline-glutamate-valine-lysine (PEVK) region of titin. Protein analysis supported expression of a significantly larger PEVK segment in neonatal muscle. In line with these findings, we found >50% lower titin-based passive stiffness of neonatal muscle when compared to adult muscle. Inhibiting 3,5,3'-tri-iodo-L-thyronine and 3,5,3',5'-tetra-iodo-L-thyronine secretion did not alter isoform switching, suggesting no major role for thyroid hormones in regulating differential titin splicing during postnatal development. In summary, our work shows that stiffening of skeletal muscle during postnatal development occurs through a decrease in titin isoform size, due mainly to a marked restructuring of the PEVK region of titin.

Children sport shoes--a systematic review of current literature

OBJECTIVE

The child's foot is clearly distinct from the adult foot in its functional anatomy and ability to cope with pressure. This requires special considerations in the development of a children's sport shoe.

METHODS

Medical and sport science databases were thoroughly searched for studies pertaining to the anatomy and biomechanics of children's feet during their development. With the data found, a list of requirements for the children's shoe was compiled.

RESULTS

Small children should have a sports shoe, which is as flexible as their own foot. The small impact forces during their sports activities make extra cushioning superfluous. During school age the connective tissue gains stability. The growing amount of sports activities, much of which is performed on hard indoor surfaces, enhances the need for cushioning. At the same time there is a growing necessity for adequate mechanical stimuli to help the muscles and bones develop. The strength of the connective tissue and the flexibility of the joints reach adult levels by the age of 15. In small shoes, the displacement of proportions can lead to improper positioning of the flex zone and thereby causing harmful stress on the foot. Cushioning elements are often oversized. Considering the wide range of anatomy in the child's foot, it is advisable to produce children's shoes in different widths.

CONCLUSION

The child's foot differs in anatomy and function from the adult foot. Children sport shoes should meet the child specific requirement.

Hyperactive stretch reflexes, co-contraction, and muscle weakness in children with cerebral palsy

The aim of this study was to examine the repeatability of and relationships among spasticity, co-contraction of agonist-antagonist, and muscle strength in children with cerebral palsy (CP). Eight children with spastic diplegic CP (five males, three females; Gross Motor Function Classification System [GMFCS] Levels I-III; mean age 10y 2mo [SD 2y 9mo], range 6-13y) and nine children in a comparison group (six males, three females; mean age 8y 10mo [SD 2y 4mo], range 6y to 12y 6mo) were assessed twice to examine repeatability of Composite Spasticity Scale, soleus stretch reflexes, electromyography (EMG) co-contraction ratio, and torque recorded during maximal isometric voluntary contraction of ankle dorsiflexors and plantarflexors. Sixty-one children with spastic CP, (54 diplegic, seven hemiplegic; 32 males, 29 females; GMFCS levels I-III; mean age 10y 8mo [SD 2y 9mo], range 6-15y) were then assessed to delineate possible correlations among these measures. Intraclass correlation coefficients (0.78-0.97) showed high data repeatability in both groups. Children with spastic CP demonstrated significantly larger soleus stretch reflex/M-response areas smaller torques, but larger EMG co-contraction ratios during both voluntary dorsiflexion and plantarflexion (all p<0.05). Children with spastic CP who had larger soleus stretch reflex/M-response areas demonstrated larger plantarflexion co-contraction ratio (r = 0.28), and produced smaller plantarflexion and dorsiflexion torques (r = -0.48 and -0.27 respectively). However, no correlation was noted between soleus stretch reflex and clinical spasticity. Our findings demonstrated that hyperactive soleus stretch reflex affected torque production of ankle muscles. Moreover, the severity of spasticity may not be fully described by either stretch reflex or tone measure alone.

Influences of experimental factors on spinal stretch reflex latency and amplitude in the human triceps surae

The spinal stretch reflex (SSR) is commonly assessed via electromyographic (EMG) analysis of joint perturbations inducing changes in muscle length. Previous literature indicates that when large experimental changes in magnitude of agonist background EMG, perturbation velocity, and perturbation amplitude are employed, SSR latency and amplitude are significantly altered. The purpose of this investigation was to evaluate the relative dependence of SSR latency and amplitude on inherent variability in these experimental variables. Soleus SSR latency and amplitude were assessed in 40 healthy subjects following dorsiflexion perturbation under an active state ( approximately 14% MVC). Experimental variables displayed limited variability (means +/- SD): soleus background EMG (13.47 +/- 7.08% MVC), perturbation velocity (96.1 +/- 30 degrees /s), and perturbation amplitude (4 +/- 1 degrees ). SSR latency was not significantly related to soleus background EMG (r = 0.189), perturbation velocity (r = 0.213), or perturbation amplitude (r = 0.202). Similarly, SSR amplitude was not significantly related to soleus background EMG (r = 0.306), perturbation velocity (r = 0.053), or perturbation amplitude (r = 0.056). Variability in experimental variables was much smaller than what has been reported in the literature to significantly impact SSR characteristics. These results suggest that SSR latency and amplitude are independent of agonist background EMG, perturbation velocity, and perturbation amplitude when experimental variability is relatively limited.

Quantifying the passive extensibility of the flexor pollicis longus muscle in people with tetraplegia

STUDY DESIGN

Repeat measures design.

OBJECTIVE

The purpose of this study was firstly, to describe a simple clinical tool that can be used to measure the extensibility of the flexor pollicis longus (FPL) muscle; secondly, to test its reliability; and thirdly, to attain some 'normative' data of the extensibility of the FPL muscle in a representative sample of people with tetraplegia.

SETTING

A spinal cord injury unit in Sydney.

SUBJECTS

A total of 37 people (62 hands) with C4-C7 tetraplegia.

MAIN OUTCOME MEASURES

Angle of the carpometacarpal (CMC) joint of the thumb was measured in all subjects with the application of a series of thumb extensor torques. A device specifically designed for this purpose was used to standardize the torque and objectively quantify the CMC joint angle. In addition, repeat measurements were taken 3-5 days later in one subgroup of 13 subjects (one hand per subject) and 3 months later in another subgroup of 13 subjects (one hand per subject).

ANALYSIS

Intraclass correlation coefficients and percent close agreement scores were derived to quantify the 3-5 days and 3-month reliability between repeat measurements.

RESULTS

The median CMC angle of the thumb with the application of a 0.044 Nm torque was 63 degrees (range, 20-93 degrees). The intraclass correlation coefficients with the application of a 0.044 Nm torque were 0.88 (95% CI, 0.65-0.96) for measurements taken 3-5 days apart, and 0.90 (95% CI, 0.67-0.97) for measurements taken 3 months apart.

CONCLUSION

This study describes a simple and reliable way of measuring the extensibility of the FPL muscle in people with tetraplegia. This assessment tool and the 'normative' data provided in this study can be used to further investigate the contribution of the passive mechanical properties of the FPL muscle to hand function of people with C6 and C7 tetraplegia.

Acute passive stretching alters the mechanical properties of human plantar flexors and the optimal angle for maximal voluntary contraction

The purpose of this study was to investigate whether acute passive stretching (APS) reduced maximal isometric voluntary contraction (MVC) of the plantar flexors (PF) and if so, by what mechanisms. The PF in 15 female volunteers were stretched for 10 min (5 x 120 s) by a torque motor to within 2 degrees of maximum dorsiflexion (D) range of motion (ROM). MVC with twitch interpolation, maximal Hoffmann reflex (H(max)) and compound action potentials (M(max)) were recorded at 20 degrees D. Stretch reflexes (SR) were mechanically induced at 200 degrees s(-1) between 0 degrees and 10 degrees ( )D and SR torque and EMG amplitude were determined. All tests were assessed pre- (pre) and post-APS (post-test(1)). MVC, SR, and M(max) were again assessed after additional stretch was applied [mean 26 (1) degrees D; post-test(2)] to test if the optimal angle had been altered. EMG was recorded from soleus (SOL), medial gastrocnemius (MG) and tibialis anterior (TA) using bipolar surface electrodes. APS resulted in a 27% decrease in mean peak passive torque (P<0.05). MVC and SR torque were 7% (P<0.05) and 13% lower at post-test(1) (P<0.05), respectively. SR EMG amplitude of SOL and MG was reduced by 27% (P<0.05) and 22% (P<0.05), respectively. The H(max)/M(max) EMG and H(max)/M(max) torque ratios were unchanged at post-test(1). At post-test(2), MVC and SR EMG recovered to pre-APS values, while the SR and M(max) torque increased by 19% and 13%, respectively (P<0.05). The decrease in MVC during post-test(1) was attributed to changes in the mechanical properties of PF and not to reduced muscle activation.

Influence of muscle fibre shortening on estimates of conduction velocity and spectral frequencies from surface electromyographic signals

The study of surface electromyographic (EMG) signals under dynamic contractions is becoming increasingly important. However, knowledge of the methodological issues that may affect such analysis is still limited. The aim of the study was to analyse the effect of fibre shortening on estimates of conduction velocity (CV) and mean power spectral frequency (MNF) from surface EMG signals. Single fibre action potentials were simulated, as detected by commonly used spatial filters, for different fibre lengths. No physiological modifications were included with changes in fibre length, and thus only geometrical artifacts related to fibre shortening were investigated. The simulation results showed that the dependence of CV and MNF on fibre shortening is affected by the fibre location, electrode position and the spatial filter applied. With shortening of up to 50% for a fibre of 50 mm semi-length, the variations in CV and MNF estimates with shortening in bipolar recordings were 0.5% (CV) and 0.7% (MNF) for superficial fibres, and 3.6% and 5.1% for deeper fibres. Using the longitudinal double differential filter, under the same conditions, the percent variation was 0% and 0.2%, and 24.7% and 15.8%, respectively. The main conclusions were, first, muscle fibre shortening can significantly affect estimates of CV and MNF, especially for short fibre lengths. However, for long (semi-length >50 mm) and superficial fibres, this effect is limited for shortenings of up to 50% of the initial fibre length. Secondly, CV and MNF are almost equally affected by changes in muscle length; and, thirdly, sensitivity to fibre shortening depends on the spatial filter applied for signal detection.