The effects of regional quadriceps architecture on angle-specific rapid torque expression

doi:10.21203/rs.3.rs-2224121/v1

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The effects of regional quadriceps architecture on angle-specific rapid torque expression

https://doi.org/10.21203/rs.3.rs-2224121/v1

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Evaluating anatomical contribution to performance can build an understanding of muscle mechanics and guide physical preparation. While the impact of anatomy on muscular performance is well studied, the effects of regional quadriceps architecture on rapid torque expression are less clear. Regional (proximal, middle, distal) quadriceps (vastus lateralis, rectus femoris, lateral and anterior vastus intermedius) thickness (MT), pennation angle (PA), and fascicle length (FL) of 24 males (48 limbs) were assessed via ultrasonography. Participants performed isometric contractions at 40º, 70º, and 100º of knee flexion to evaluate rate of torque development from 0-200 ms (RTD_{0 − 200}). Measurements were repeated on three separate occasions with the greatest RTD_{0 − 200} and average muscle architecture measures used for analysis. Linear regression models predicting angle-specific RTD_{0 − 200} from regional anatomy provided adjusted simple and multiple correlations (√adjR²) with bootstrapped compatibility limits to assess magnitude. Mid-rectus femoris MT (√adjR² = 0.41–0.51) and mid-vastus lateralis FL (√adjR² = 0.41–0.45) were the best single predictors of RTD_{0 − 200}, and the only measures to reach acceptable precision with 99%CL. Small simple correlations were found across all regions and joint angles between RTD_{0 − 200} and vastus lateralis MT (√adjR² = 0.28 ± 0.13; mean ± SD), vastus lateralis FL (√adjR² = 0.33 ± 0.10), rectus femoris MT (√adjR² = 0.38 ± 0.10), and lateral vastus intermedius MT (√adjR² = 0.24 ± 0.10). Multiple correlations are reported within the article. Researchers should measure mid-region rectus femoris MT and vastus lateralis FL to efficiently and robustly evaluate potential anatomical contributions to changes in rapid knee extension torque expression.

Health sciences/Anatomy/Musculoskeletal system

Biological sciences/Physiology

Health sciences/Medical research/Translational research

architecture

correlation

fascicle length

force

length-tension

pennation angle

rectus femoris

regression

vastus intermedius

vastus lateralis

Measures of muscle size (e.g., muscle thickness (MT), cross-sectional area, volume) and architecture (e.g., pennation angle (PA), fascicle length (FL)) have become commonplace when assessing acute or chronic effects of exercise^1–3, baseline characteristics of performance, or clinical populations^4–6. Several research groups have also aimed to understand the effect of muscle architecture on function and performance^5,7−12, often with the intent of guiding exercise prescription relating to contraction type or other variables^1,3,13,14. For example, Oranchuk et al.¹ compared the acute effects of eccentric and eccentric quasi-isometric resistance exercise on quadriceps muscle architecture and reported differing changes in regional MT and echo intensity, which may relate to long-term adaptations.

While the relationships between muscle architecture and maximal force or torque expression are well studied^5,7−12, very few studies have utilized rapid force or torque expression as a variable, possibly due to larger measurement variabilities¹⁵. One such recent study by Coratella et al.¹⁶ examined the relationships between the rate of force development at several epochs and muscle architecture of the vastus lateralis, vastus medialis, rectus femoris, and vastus intermedius. The correlations logically increased with larger epochs, with the greatest correlations between the rate of force development and vastus intermedius MT (r = 0.27–0.69), vastus lateralis FL (r = 0.37–0.63), and vastus intermedius FL (r = 0.34–0.64)¹⁶. While Coratella and colleagues¹⁶ were, to our knowledge, the first to examine the relationship between quadriceps architecture and rapid force expression, only the mid-region of each muscle, and a single joint angle, were examined. Also several findings from Coratella et al. run contrary to a previous study⁹ that demonstrated smaller correlations between mid-region vastus intermedius MT and maximal isometric torque (√adjR² = 0.23–0.42).

While several researchers have examined the relationships between anatomy and isometric force or torque-time characteristics, none have determined the strength of association between regional architecture and rapid torque expression at multiple joint angles. Based on previous research, we hypothesized that mid and distal vastus lateralis and lateral vastus intermedius MT and distal vastus lateralis FL would be the strongest predictors of rapid torque expression. Additionally, we hypothesized that joint angle would not substantially affect the form-to-function relationship.

Experimental design

Using a repeated measures design, we examined measures of regional quadriceps anatomical parameters and RTD at a range of joint angles. Each participant was tested on three separate occasions, separated by 5–8 days. At each session, an ultrasonographic assessment of regional quadriceps MT, PA, and FL, followed by an isometric assessment of RTD at 40º, 70º, and 100º of knee flexion (0º=full extension), was completed. Inter-session reliability was determined for all measures. Average muscle architecture was used for analysis to minimize daily fluctuations in hydration and sonographer error, whereas the greatest RTD from all sessions were used for analysis to minimize issues of motivation and muscular activation.

Participants

Twenty-four healthy males (mean ± SD: 28.5 ± 4.7 years, 180.1 ± 7.7 cm, 81.6 ± 11.8 kg) volunteered for this study. All participants were required to have at least six months of resistance training experience (6.73 ± 4.83, range: 1–18 years), with at least two weekly sessions of lower-body resistance training (2.54 ± 0.75, range: 2–5 sessions.week^− 1), and be free of any musculoskeletal injuries for at least three months before data collection. Participants were instructed to maintain their current level of physical activity throughout the data collection period apart from refraining from strenuous physical activity, alcohol, caffeine, and other ergogenic aids for at least 48 hours before each session. The Auckland University of Technology Research Ethics Committee approved the study (18/232), and all subjects provided written informed consent. All ethical conciderations were performed inline with the World Medical Association and the Declaraton of Helsinki¹⁷.

Testing procedures

Ultrasonography

All ultrasound images were collected using the same transducer and built-in software (45 mm linear array, GE Healthcare, Vivid S5, Chicago, IL). Ultrasound settings (frequency, 12 MHz; brightness, maximum; gain, 60 dB; dynamic range, 70; depth, individualized) were recorded and kept consistent across all sessions. All assessments were performed by the same sonographer with ~ 3 years of experience. Upon arrival, participants were positioned in a supine position on a massage table for 15 minutes, to allow for fluid re-distribution¹⁸. The participants remained in a supine position with knees and hips fully extended with a foot strap used to prevent excessive external rotation¹⁹. During this period, the participants had each thigh measured and marked by an ISAK level-2 anthropometrist. The length of the lateral aspect of the thigh was determined by measuring the distance from the superior border of the greater trochanter to the inferior border of the lateral condyle of the femur. The anterior aspect of the thigh was determined by measuring the distance between the superior border of the patella and the inferior border of the anterior, superior iliac spine²⁰. Thigh lengths were recorded, and markings were made at 30% (proximal), 50% (middle), and 70% (distal) of the lateral and anterior distances, respectively¹⁹. The vastus lateralis was marked by determining the most lateral aspect of the thigh. Participants were instructed to briefly tense the quadriceps so that the researcher could mark the rectus femoris. Pilot testing with our resistance-trained male cohort determined that extended field-of-view scans of the vastus intermedius were not consistently clear. Thus, scanning depths were adjusted to only capture the vastus lateralis and rectus femoris muscles. The vastus medialis was excluded as it can be further broken down into the obliquus and longus portions, with deep and superficial fiber bundles²¹. Furthermore, MT of the vastus medialis oblique and longus have substantially smaller correlations with magnetic resonance imaging derived cross-sectional area when compared to the vastus lateralis or rectus femoris²². MT and PA of the vastus lateralis and lateral vastus intermedius, and the rectus femoris and anterior vastus intermedius were collected in the same snap-shot images, respectively. Typical proximal, middle, and distal lateral and anterior images are provided in Fig. 1.

Fascicle length was measured by the extended field-of-view function as extensively detailed previously^23,24. The ultrasound probe was moved across a muscle, while a texture mapping algorithm merged the sequence of images into a composite image^23,25. A water-soluble gel was applied to the scanning head of the ultrasound probe to achieve acoustic coupling, with care taken to avoid tissue deformation^23–25. The probe was oriented perpendicular to the skin and parallel to the estimated fascicle direction^23–25. Two images were captured for each region or extended field-of-view scan. Two measurements for MT, PA and FL were quantified from each image, and the mean calculated (i.e., image mean). The mean of the two image means was then calculated and used for subsequent analyses²⁴.

Isometric dynamometry

Following the ultrasonographic assessment, participants warmed up by cycling at low to moderate resistance using a self-selected pace for five minutes¹⁵. Subjects were seated on the isokinetic dynamometer (CSMi; Lumex, Ronkonkoma, NY) at a hip angle of 85º, with shoulder, waist, and thigh straps to reduce body movement during contractions^15,26. The shin-pad was positioned ~ 5 cm superior to the ankle joint malleoli¹⁵. Subjects were required to hold handles at the sides of the chair, while the non-working limb was positioned behind a restraining pad¹⁵. Knee alignment was determined by visual inspection and unloaded knee extensions. Dynamometer settings were recorded and matched for subsequent sessions.

Once fitted to the dynamometer, subjects underwent a series of extensions and flexions of the knee to determine safety stop positions and calibrate gravity correction¹⁵. Subjects then completed a standardized warmup of concentric contractions of 30%, 50%, 70%, 85%, and 100% of perceived maximal voluntary contraction¹⁵. One minute after the completion of the warmup contractions, the participants' knee was positioned at 40º of flexion, where one familiarization isometric knee extension at 50% of perceived maximum torque was performed ¹⁵. Sixty seconds later, two maximal contractions lasting four seconds were completed with 30 seconds separating each contraction¹⁵. Participants were instructed to contract "as fast and hard as possible," following a countdown of "3-2-1-go!"^15,27. All participants were given strong verbal encouragement along with visual feedback of the torque-time tracing during each trial^15,27. Participants were instructed to avoid any pre-tension and countermovement of the knee extensors, while the live torque-time trace was carefully inspected by the examiner leading up to each contraction^15,27. The cut-off for pre-tension was set at 10 N¹⁵. Any contractions with a countermovement or an unsteady baseline were rejected and repeated^15,27. The participants then completed the same series at 70º and 100º of knee flexion. The isometric contractions were always performed in series from 40º to 100º to avoid greater muscle damage and fatigue synonymous with long muscle length contractions²⁰. Ten minutes following the final isometric contraction, the concentric warm-up, and isometric assessments were repeated on the opposite limb. Limb order was randomized throughout the testing sessions and counterbalanced over the participant group. All contractions were collected, without filtering, via a custom-made software (LabVIEW; National Instruments, New Zealand) sampling at 2000 Hz^15,27.

Data processing and analysis

Ultrasonography

Images were stored and analyzed via digitizing software (ImageJ; National Institutes of Health). Muscle thickness (cm) was defined as the perpendicular distance between the deep and superficial aponeurosis, and PA was defined as the angle of the fascicles relative to the deep aponeurosis²⁵. Due to the depth of the muscle, the fascicles on the lateral and anterior vastus intermedius were not consistently visible²⁴. Therefore, PA and FL were not recorded for the lateral and anterior vastus intermedius. As the rectus femoris is complex, in that there are distinctly different directions of pennation²⁰, only 23/48 of the distal rectus femoris FL measurements were able to be determined. Representative vastus lateralis and rectus femoris extended field-of-view images are illustrated in Fig. 2A and B, respectively. Anatomical variables were averaged across all sessions to reduce errors arising from sonography or participant variability.

Isometric dynamometry

Data were analyzed via a customized MATLAB (MathWorks, Natick, MA) script. As the moment arm substantially affects isometric outputs^11,18, all torque data were divided by the length of the dynamometer arm, in meters, to normalize the difference in shank length between participants¹⁵. Thus, all torque data are reported in Newtons per second (N·s^− 1). Normalized torque over 200 N were identified to signify a full contraction and eliminate false contractions¹⁵. The contraction with the highest RTD was analyzed. The maximum values of RTD across the three sessions were utilized to reduce error arising from submaximal voluntary activation.

Statistical analysis

Reliability

Measures of RTD, MT, and FL were log-transformed before analysis to reduce non-uniformity of error^28,29 and analyzed with an Excel spreadsheet³⁰. To reduce bias in the correlations between variables, only measures with sufficiently high intersession reliability were included in the correlational analysis. Since the mean or maximum of the three sessions was used in the correlation analysis, the intraclass correlation coefficient (ICC) of the mean of the three sessions was used as the measure of interest²⁹. A value of ICC > 0.75 was considered sufficiently reliable³¹. For comparison with other studies, the between-session typical error of measurement (TEM) was also analyzed and expressed as a percentage for RTD, MT, and FL^28,29. TEM is expressed in degrees for PA, as there is an upper limit of 90º, and unlike other measures, the error would not be expected to increase with increasing values^28,29.

While not a primary aim of the study, the variability of intra-image measures (e.g., the variability of two proximal vastus lateralis FL measures in the same image) was performed to determine variations in quadriceps anatomy across regions and muscles using the same procedure and interpretation as for the intrasession reliability analysis. Intra-image variability statistics were not analyzed for RTD as the greatest value, regardless of contraction or session, was utilized for the correlational analysis.

Correlational analysis

As the primary goal was to examine the effects of regional quadriceps anatomy on RTD, a measure was included only if it was sufficiently reliable (ICC > 0.75) in all three regions (proximal, middle, distal) and knee joint angles (40º, 70º, 100º). Simple linear regressions were used to predict angle specific RTD with anatomical parameters in isolation (e.g., distal vastus lateralis MT). Multiple linear regressions (explained in detail below) were used to determine if the difference in predictive ability between regions (e.g., distal vastus lateralis FL vs middle vastus lateralis FL) was substantial.

All regression analyses were performed with Proc Reg in the Statistical Analysis Software (University Edition of SAS Studio, version 9.4, SAS Institute, Cary NC). Measures of the ability of the regression models to predict RTD were the multiple R (the correlation between observed and predicted values), √R² (equivalent to the absolute value of Pearson's r for a single predictor), and the adjusted multiple R (the square-root of the R-squared, √adjR², adjusted for degrees of freedom). The adjustment removes the upward bias in the R-squared that occurs when any predictor is added to a model with a finite sample size. Negative values of adjR² were expressed as negative correlations by changing the sign before taking the square root. Effects (adjusted multiple R, their means, and their differences) are presented with the qualitative magnitude of their observed (sample) value and interpreted as trivial, small, moderate, large, very large, and extremely large for values < 0.10, ≥ 0.10, ≥ 0.30, ≥ 0.50, ≥ 0.70, and ≥ 0.90 respectively²⁹.

Sampling uncertainty in the estimates of effects is presented as 90% compatibility limits (90%CL), which were estimated from percentiles of the effects in 10,000 bootstrapped samples. The bootstrapped effects were estimated initially by performing multiple linear regression with each bootstrapped sample, but the median values of the effects were substantially higher than those in the original sample. Therefore, values in the bootstrapped samples were derived as follows: the regression coefficients in the original sample were used to predict the dependent variable in each bootstrapped sample; the correlation between the predicted and observed values of the dependent variable in each sample was squared and adjusted for degrees of freedom; the square root of this adjusted R-squared then provided values of adjusted multiple R, which showed close agreement between the original and bootstrapped median values. As a further check on the adequacy of this bootstrap method, the analyses were performed on simulated data with a sample size of 24 and simple correlations between a dependent variable and two predictor variables in two limbs. One hundred analyses each of 10,000 bootstrap samples were performed for correlations ranging from 0.00 to 0.70. Observed and median adjusted multiple R showed a downward bias of 0.05 to 0.10 for true correlations of 0.00-0.30, 0.04 for true correlations of 0.50, and only 0.02 for true correlations of 0.70. Despite the bias, 90%CLs showed less than the 10% expected error rate in their coverage of low values of true correlations and a slightly higher error rate (~ 12%) for true correlations of 0.50 and 0.70. The bootstrapping was therefore judged to be trustworthy.

The non-clinical version of magnitude-based decisions with a minimally informative prior was used to interpret the uncertainty in effect magnitudes³². Chances that the true magnitude was a substantial negative value, a trivial value, and a substantially positive value were derived directly from the bootstrapped samples as the proportion of sample values with those magnitudes. Effects were deemed to be clear (to have adequate precision) if the chances of one or other substantial true values were < 5% (i.e., the 90%CLs did not include substantial positive and negative values). Clear effects are reported with a qualitative descriptor for the magnitude with chances > 25% using the following scale: >25%, possibly (*); >75%, likely (**); >95%, very likely (***); and > 99.5%, most likely (****)²⁹. When the chances of a substantial or trivial magnitude were > 95%, the magnitude itself is described as clear. Effects with inadequate precision are described as unclear. Effects with adequate precision defined by 99%CL are highlighted in bold in tables and figures; the overall error rate for coverage of 10 independent true values with such intervals is that of a single effect with a 90%CL (10%), and interpretation of outcomes is focused on these effects.

Reliability

Intersession, intrasession and intra-image (two fascicles from the same image) reliability of quadriceps architectural parameters have been previously reported in detail⁹. In brief, all MT and vastus lateralis PA and FL measures were sufficiently reliable (ICC = 0.77–0.98). However, rectus femoris PA and FL were either unreliable, or unable to be consistently evaluated in each participant, and therefore not included in the correlational analysis.

RTD_{0 − 100} was reliable across all limbs at 40º (ICC = 0.85, TEM = 15%), 70º (ICC = 0.79, TEM = 20%), and 100º (ICC = 0.80, TEM = 19%). RTD_{0 − 200} was slightly more reliable at 40º (ICC = 0.90, TEM = 13%), 70º (ICC = 0.81, TEM = 15%), and 100º (ICC = 0.87, TEM = 12%). Furthermore, correlations between muscle architecture and RTD were always trivially different between 0-100 and 0-200 epochs. Therefore, due to the very large number of correlations, all subsequent results are reported for RTD_{0 − 200} only.

Correlational analysis

Mean regional anatomical measures are summarized in Table 1. RTD_{0 − 200} for the dominant and non-dominant limbs were (mean ± SD) 2510 ± 657 N·s^− 1 and 2462 ± 599 N·s^− 1 at 40º, 2876 ± 873 N·s^− 1 and 2789 ± 725 N·s^− 1 at 70º, and 2388 ± 656 N·s^− 1 and 2319 ± 701 N·s^− 1 at 100º, respectively. All between-limb differences in predictive ability were unclear, or possibly small; therefore, all correlations are presented after pooling dominant and non-dominant limbs.

Table 1
Regional quadriceps architecture averaged over three sessions.
Muscle	Measurement	Limb	Proximal	Middle	Distal
Vastus lateralis	MT (cm)	Dominant	2.54 ± 0.36	2.81 ± 0.43	2.21 ± 0.34
		Non-dominant	2.44 ± 0.33	2.74 ± 0.41	2.20 ± 0.37
	PA (°)	Dominant	17.1 ± 3.2	20.6 ± 2.6	22.7 ± 2.7
		Non-dominant	16.3 ± 2.7	20.2 ± 2.1	22.5 ± 2.8
	FL (cm)	Dominant	7.73 ± 0.68	8.18 ± 0.83	7.16 ± 0.83
		Non-dominant	7.70 ± 0.73	8.16 ± 0.72	7.12 ± 0.90
Rectus femoris	MT (cm)	Dominant	2.80 ± 0.26	2.69 ± 0.33	1.96 ± 0.30
		Non-dominant	2.76 ± 0.33	2.69 ± 0.31	1.99 ± 0.31
Lateral vastus intermedius	MT (cm)	Dominant	2.02 ± 0.44	2.04 ± 0.54	1.93 ± 0.44
		Non-dominant	2.01 ± 0.51	2.03 ± 0.53	1.84 ± 0.42
Anterior vastus intermedius	MT (cm)	Dominant	2.94 ± 0.46	2.24 ± 0.47	1.88 ± 0.42
		Non-dominant	2.96 ± 0.47	2.36 ± 0.49	1.78 ± 0.37
MT = muscle thickness. PA = pennation angle. FL = fascicle length. Data are mean ± SD. Data are across 48 limbs.

Simple linear regressions

Correlations and bootstrapped 90%CLs of individual measures of regional muscle anatomy with RTD_{0 − 200} at 40º, 70º, and 100º of knee flexion are presented in Fig. 3. Regardless of region or joint angle, small correlations were found between RTD_{0 − 200} and vastus lateralis MT (mean ± SD √adjR² = 0.28 ± 0.13), vastus lateralis FL (√adjR² = 0.33 ± 0.10), rectus femoris MT (√adjR² = 0.38 ± 0.10), and lateral vastus intermedius MT (√adjR² = 0.24 ± 0.10). Neither vastus lateralis PA (√adjR² = 0.00 ± 0.18), nor anterior vastus intermedius (√adjR² = 0.06 ± 0.12) were substantially associated with RTD_{0 − 200}. The largest single correlation was between mid-rectus femoris MT and RTD_{0 − 200} at 100° (√adjR² = 0.51, 90%CL: 0.16–0.73) with mid-rectus femoris MT also having the greatest mean correlation with RTD_{0 − 200} regardless of joint angle (√adjR² = 0.47 ± 0.05). This finding was followed by mid-vastus lateralis FL(√adjR² = 0.43 ± 0.02) and distal rectus femoris MT (√adjR² = 0.42 ± 0.02) as the only measures consistently correlating with RTD_{0 − 200} (√adjR² ≥ 0.40). Additionally, middle rectus femoris MT and middle vastus lateral FL were the only measures to reach acceptable precision with 99%CLs at all three joint angles.

Regardless of model, correlations were greatest for RTD_{0 − 200} at 100º (√adjR² = 0.24 ± 0.15) and decreased at 70º (√adjR² = 0.21 ± 0.20) and 40º (√adjR² = 0.15 ± 19). However, the differences were either unclear, or possibly to likely trivial-small.

Multiple linear regressions

Between-region differences in simple correlations are presented in Supplementary file 1. Of the 54 between region comparisons, 51 were trivial or small, with 36 decisions being unclear. Distal vastus lateralis MT had likely higher correlations with RTD_{0 − 200} than proximal vastus lateralis MT at all joint angles (Δ√adjR² = 0.22–0.27). Likewise, mid vastus lateralis MT was likely better correlated to RTD_{0 − 200} than proximal vastus lateralis MT at the 70° joint angle (Δ√adjR² = 0.21). Distal vastus lateralis PA had likely higher correlations with RTD_{0 − 200} than proximal vastus lateralis PA at all joint angles (Δ√adjR² = 0.33–0.39). Distal vastus lateralis PA also had a possibly greater correlation to RTD_{0 − 200} than mid vastus lateralis PA at the 100° joint angle (Δ√adjR² = 0.27). Middle vastus lateralis FL was likely and possibly better correlated to RTD_{0 − 200} than distal vastus lateralis FL at 70° (Δ√adjR² = 0.17) and 100° (Δ√adjR² = 0.27), respectively. Middle vastus lateralis FL was also possibly better correlated with RTD than proximal vastus lateralis FL at 100° (Δ√adjR² = 0.17). Middle rectus femoris MT has possibly and likely higher correlations with RTD_{0 − 200} than proximal rectus femoris MT at 40° (Δ√adjR² = 0.14) and 100° (Δ√adjR² = 0.27), respectively. Distal lateral vastus intermedius MT had likely higher correlations with RTD_{0 − 200} proximal lateral vastus lateralis MT at 70° (Δ√adjR² = 0.23) and 100° (Δ√adjR² = 0.26), respectively. Distal lateral vastus lateralis MT was also possibly better correlated to RTD_{0 − 200} than mid lateral vastus lateralis MT at 70° (Δ√adjR² = 0.19).

While several studies have examined the effects of quadriceps anatomy on force or torque expression, none have investigated the relationship between regional architecture and rapid torque expression at multiple joint angles. Our primary findings were that mid-region vastus lateralis FL, and mid-region rectus femoris MT were the strongest and most consistent predictors of the rate of torque development. When paired with previous findings, researchers should assess distal or mid vastus lateralis, mid rectus femoris MT and mid vastus lateralis FL to estimate the knee extensors' potential for both maximal and rapid torque expression.

A key finding of this study was the relatively high and consistent correlations between rectus femoris MT and RTD_{0 − 200} (√adjR² = 0.27–0.51). This finding contrasts with previous work demonstrating that rectus femoris MT (√adjR²=-0.01-0.49) was a minor and inconsistent contributor to peak torque when compared to vastus lateralis MT (√adjR² = 0.18–0.64) or FL (√adjR² = 0.29–0.60)⁹. This discrepancy can most likely be explained by several factors. While all quadriceps muscles insert into the patella, the rectus femoris is oriented and attaches directly superior⁷. Therefore, the rectus femoris has the most direct line-of-pull of the superior quadriceps muscles⁷, potentially reducing the time required for measurable torque following contraction on-set. However, the rectus femoris is substantially smaller than the vastus lateralis, limiting its contribution to maximal knee extensor strength^5,20,33,34.

Vastus lateralis FL being a top predictor of rapid torque expression is logical given the implication of sarcomeres-in-series to contraction velocity³⁵, or more recently sarcomere resting length theories³⁶. Though our hypothesis of distal vastus lateralis FL being a top predictor of RTD was not confirmed as mid-region FL had higher correlations at all three joint angles. It is difficult to determine why this is the case, especially as distal vastus lateralis FL was consistently the worst predictor of RTD on the FL measures. However, it is plausible that regional FL is representative of other regions of the same muscle. The mid-region of the vastus lateralis is also thicker than the proximal or distal regions and, therefore, likely has the greatest potential for maximal force and torque expression²⁴. Given that maximal strength is a good predictor of rapid force and torque expression³⁷, our finding is logical. This finding is also interesting as distal vastus lateralis FL had the greatest inter-participant variability.

No meaningful relationships between vastus lateralis PA and RTD_{0 − 200} were found (√adjR²=-0.23–0.32), regardless of region or joint angle. This finding, similar to our previous work examining peak torque⁹, is contrary to several studies examining maximal or rapid contractile performance^8,10,33,38. However, our findings are similar to a recent study also examining knee extension kinetics that found small correlations between vastus lateralis PA and rate of force development over several epochs (r=-0.23–0.19)¹⁶. Though it is difficult to determine the reasons for these conflicting results, it has recently been suggested that assessing PA may lack functional significance due to a variety of factors, including its relationship to MT and FL, changes in PA based on joint position and muscle force, and concepts such as muscle gearing³⁹.

Contrary to Coratella et al.,¹⁶ this study did not find large or consistent correlations between the RTD measure and vastus intermedius MT. The discrepancy between studies could be due to several factors. Firstly, Coratella et al.,¹⁶ utilized a relatively small sample of 17, whereas we examined 48 limbs across 24 individuals. Secondly, and similarly to Ando et al.,¹² Coratella et al.¹⁶ was only concerned with knee extensor capabilities and muscle architecture on a single occasion, inviting additional uncertainty. Additionally, while our participants were well-trained (≥ 2 lower body resistance-training sessions per week for ≥ 6 months), the participants in the aforementioned studies were 'recreationally trained' and 'healthy', respectively. Therefore, training status may have affected the correlational results as the force or torque expression of the well-trained participants is likely more influenced by neural versus anatomical factors⁴⁰. These differences in neural control would partially explain the generally weaker correlations in this present study as compared to those of Coratella et al.,¹⁶ though this could be due to other factors including the fact that Coratella and colleagues utilized a strain-gauge, whereas an isokinetic dynamometer was used in this study. Finally, while we, and other researchers, have utilized several strategies to maximize reliability, determined good inter-rater variability, it cannot be assumed that sonographers, are without substantive error.

Other researchers have determined that cross-sectional areas and muscle volume are stronger predictors of muscle function than MT²⁵. Therefore, it could be recommended that the distal and middle quadriceps cross-sectional area be assessed via panoramic ultrasound to obtain a robust predictor of torque expression in only two scans. Additionally, a third scan examining mid-region vastus lateralis FL could be performed if predicting RTD potential is a priority.

Limitations and future research directions

While the aims of this study were achieved, there are several limitations to consider. Firstly, the use of surface or needle electromyography to determine individual muscle activation at different joint angles was not utilized; yet similar examinations have reported electromyography to have trivial to moderate correlations to isometric^10,38 and cycling performance³³, and/or contributed little to multiple regression models^10,11,38. Similarly, we could only evaluate our participants' voluntary efforts, not the maximal potential of the quadriceps musculature through peripheral or transcranial stimulation techniques^41,42. While the aforementioned strategy to maximize voluntary activation removes many limitations, the lack of joint angle order randomization may have led to the accumulation of non-trivial fatigue levels as contractions progressed from 40º to 100º of flexion. We used the greatest RTD outputs over the three collection sessions to partially address this.

Future research could include the knee joint moment arm, which can change through the range of motion^10,43. Likewise, compression of the dynamometer padding means that the joint angles reported were likely overestimated and RTD was delayed. Researchers may also wish to scan the quadriceps in the same position as the isometric strength assessment to represent joint angle-specific architecture better. More experienced sonographers or different hardware could have consistently produced extended field-of-view rectus femoris and vastus intermedius FLs, allowing for further elucidation of the relationship between regional quadriceps anatomy and angle-specific torque. Likewise, sonographer error is an ever-present consideration, and this study is no exception. Analyzing the mean fascicle length over multiple regions may also result in interesting findings while diminishing the effect of sonographer error.

While several researchers have examined inhomogeneous morphological and architectural adaptations, very few have evaluated more than two regions of an individual muscle⁴⁴. Therefore, we recommended that future investigations examine three or more regions to illuminate further the effect of training or disuse on region-specific muscle adaptations. Other advanced methods used as muscle volume assessments and high-density electromyography may unveil additional insights. The bi-articular structure and function of the rectus femoris should be further explored as it is the most commonly injured quadriceps muscle during accelerations and sprinting⁴⁵, and may play a critical role in clinical populations⁴⁶.

This study was the first to examine the effect of regional quadriceps architecture on RTD at multiple joint angles. We found that the relative contribution of regional anatomical parameters to RTD does not change at different joint angles. However, middle and distal rectus femoris MT and mid-region vastus lateralis FL were the strongest predictors of RTD, likely due to these respective muscles and structures having the most direct line-of-pull on the musculo-tendonous unit, greatest number serial sarcomeres, respectively. Combined with previous research, ultrasonic evaluations of the quadriceps should focus on mid-region MT and FL of the vastus lateralis and mid-region rectus femoris MT to obtain a thorough yet time-efficient model of quadriceps force expression.

Author contributions

DJO conceived the study. DJO, JBC, AGS and ARN obtained ethical approval. DJO and AGS recruited participants. DJO collected and processed the data. DJO and WGH performed the statistical analysis. DJO, WGH and ARN interpreted the results. DJO created the tables and figures. DJO wrote the manuscript. All authors reviewed and edited, and approved the manuscript before submission.

Conflict of interest

The authors have no conflicts of interest to disclose.

Ethics approval and consent to participate

The Auckland University of Technology Research Ethics Committee approved the study (18/232), and all subjects provided written informed consent.

Funding

Dustin J. Oranchuk was supported by the Auckland University of Technology's Vice-Chancellor's Doctoral Scholarship. There was no other funding provided for this study.

Data availability statement

Raw data is available upon reasonable request by contacting Dr Dustin J. Oranchuk: [email protected].

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No competing interests reported.

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The effects of regional quadriceps architecture on angle-specific rapid torque expression

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Abstract

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Introduction

Methods

Experimental design

Participants

Testing procedures

Ultrasonography

Isometric dynamometry

Data processing and analysis

Ultrasonography

Isometric dynamometry

Statistical analysis

Reliability

Correlational analysis

Results

Reliability

Correlational analysis

Simple linear regressions

Multiple linear regressions

Discussion

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Conclusion

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