Lower-extremity side-to-side strength asymmetry of professional soccer players according to playing position

Previous studies have utilized screening values of 10-15% of lower-extremity side-to-side strength asymmetry in soccer players with conflicting results. The purpose of this study was to determine differences in screening values for side-to-side asymmetry in soccer players according to their playing position, as well as to compare the differences in strength asymmetry between particular playing positions. Seventy-nine Brazilian male professional soccer players (age 26.1±5.3 years; body mass 79.8±14.4 kg; body height 180.4±12.9 cm) were grouped into playing positions of: goalkeepers, side backs, central backs, central defending midfielders, central attacking midfielders, and forwards. They performed maximal knee extension and flexion concentric and knee flexion eccentric actions on their preferred and non-preferred legs at 1.047 rad·s-1. Forwards and goalkeepers had hamstrings concentric peak torque asymmetry (18.0±9.9%) and eccentric peak torque asymmetry (20.1±10.7%) significantly greater than 10% (p<.05). All other playing positions had values less than 15%. Our results indicate that the use of either 10 or 15% asymmetry cut off may result in different conclusions, which may affect decision-making regarding strength ratios. Furthermore, if 10% is used as a screening value, hamstrings strengthening programs based on bilateral equivalency should be prescribed for goalkeepers and forwards to reduce asymmetry

Acute effects of proprioceptive neuromuscular facilitation on peak torque and muscle imbalance

Background: The effects of proprioceptive neuromuscular facilitation (PNF) stretching on muscle imbalance are not fully understood. The aim of this study was to examine the acute effects of PNF stretching on knee extension and flexion peak torque (PT), as well as the conventional and functional hamstrings to quadriceps (H:Q) ratios. Methods: Fifteen men (age = 22 ± 1 years; body mass = 76 ± 12 kg; height = 176 ± 7 cm) and fifteen women (age = 22 ± 2 years; body mass = 63 ± 8 kg; height = 161 ± 5 cm) performed concentric quadriceps and hamstrings, and eccentric hamstrings muscle actions at different angular velocities (60, 180, and 300 ◦ ·s −1 concentric; 60 and 180 ◦ ·s −1 eccentric) before and after a bout of PNF stretching, and a control condition. Results: Neither PNF or control conditions affected concentric PT or H:Q ratios (p \u3e 0.05), apart from knee extension at 60 ◦ ·s −1 in men (p = 0.001). However, there was a reduction in hamstrings eccentric PT in both control and PNF conditions for men and women (p = 0.003). Conclusions: PNF stretching of the hamstrings may not adversely affect the H:Q ratios, and consequently not negatively affect injury risk associated with muscular strength imbalances

Resistance training effects on pubertal children with a risk of developing pediatric dynapenia

Purpose: Many modern-day children are at risk of pediatric dynapenia (muscle weakness). We examined the effects of a 12-week resistance training (RT) program on neuromuscular function and body composition parameters in pubertal children with a risk of dynapenia. Methods: Twelve children (13.4 ± 0.9 y) with dynapenia performed a progressive RT program consisting of knee extension and flexion, bench press, abdominal crunch, back extension, lateral pull-down, elbow flexion, and upright row (1–2 sets of 10–15 repetitions/exercise) twice/week for 12 weeks. Outcome measures included one-repetition maximum (1-RM) strength, maximal voluntary isometric contraction (MVIC) torque, rate of torque development (RTD), electromyographic (EMG) activity, muscle thickness (MT), muscle quality (MQ) assessed by echo intensity (MQEI) of the knee extensors and specific tension of MVIC torque to thigh fat-free mass (MQST), and total and regional body and bone composition assessed by dual-energy X-ray absorptiometry. Changes in the measures before and after the 12-week RT and associations among the measures were analyzed by linear mixed models. Results: Significant (p \u3c 0.05) increases in 1-RM (63.9 ± 4.5%), MVIC torque (16.3 ± 17.8%), MT (18.8 ± 5.5%) and MQ (MQEI: -25.9 ± 15.2%; MQST: 15.1 ± 18.8%;) were evident from pre- to post-training. Total fat-free mass (FFM) increased by 2.3 ± 3.2% from baseline (p = 0.01), but no changes (p \u3e 0.05) in the other measures were observed. Significant (p \u3c 0.05) associations between the changes in 1-RM and/or MVIC torque and the changes in quadriceps MT, MQEI, MQST and total body FFM were evident. Conclusions: The 12-week RT was effective for improving neuromuscular and body composition parameters, and thereby reversed the risk of pediatric dynapenia

Effects of different combinations of concentric and eccentric resistance training programs on traditional and alternative hamstrings-to-quadriceps ratios

Resistance training is often recommended for combined increases in traditional and alternative hamstrings-to-quadriceps (H:Q) ratios in order to reduce knee strength imbalance and associated hamstrings and knee ligament injury risk. The aim of this study was to investigate the effect of different concentric and eccentric resistance training programs on traditional and alternative H:Q ratios. Forty male volunteers were assigned to one of 4 groups: concentric quadriceps and concentric hamstrings (CON/CON, n = 10), eccentric quadriceps and eccentric hamstrings (ECC/ECC, n = 10), concentric quadriceps and eccentric hamstrings (CON/ECC, n = 10), or no training (control (CNTRL), n = 10). Traditional conventional (CR) and functional (FR), alternative rate of torque development (RTD), muscle size (MS), and muscle activation (MA) H:Q ratios were measured before and after six weeks of unilateral nondominant knee extension–flexion resistance training performed on an isokinetic dynamometer. The ECC/ECC training significantly increased FR (pre = 0.75 ± 0.11; post = 0.85 ± 0.15), whereas the lack of training (CNTRL) decreased the RTD H:Q ratio (pre = 1.10 ± 0.67; post = 0.73 ± 0.33). There were no differences between groups for the other traditional and alternative ratios following resistance training protocols. These findings suggest eccentric exercise for quadriceps and hamstrings as the most beneficial training program for inducing increases in the traditional FR. However, different resistance training strategies may be needed to also elicit increases in the alternative RTD, MS, and MA H:Q ratios for fully restoring muscle balance and reducing potential hamstrings and knee ligament injury risk

Alternative methods of determining hamstrings-to-quadriceps ratios: A comprehensive review

The hamstrings-to-quadriceps muscle strength ratio calculated by peak torque has been used as an important tool to detect muscle imbalance, monitor knee joint stability, describe muscle strength properties and functionality, and for lower extremity injury prevention and rehabilitation. However, this ratio does not consider other neuromuscular variables that can also influence the antagonist to agonist muscle relationship, such as torque produced at multiple angles of range of motion, explosive strength, muscle size, muscle fatigue, or muscle activation. The aim of this study was to comprehensively review alternative methods of determining the hamstrings-to-quadriceps ratio. These include ratios calculated by angle-specific torque, rate of torque development, muscle size, fatigue index, and muscle activation (measured by electromyography). Collectively, the literature demonstrates that utilizing alternative methods of determining the hamstrings-to-quadriceps ratio can be functionally relevant for a better understanding of the neuromuscular mechanisms underpinning the interaction of strength between hamstrings and quadriceps. However, there is insufficient evidence to recommend any of the alternative methods as sensitive clinical tools for predicting injury risk and monitoring knee joint integrity. Future longitudinal studies, along with injury incidence, are needed to further investigate all alternative methods of determining the hamstrings-to-quadriceps ratio. These have potential to offer insight into how athletes and the general population should be trained for performance enhancement and injury reduction, and may be used along with traditional methods for a thorough assessment of an individual\u27s H:Q muscle balance

Neuromuscular characteristics of eccentric contractions of the knee extensors and their muscle damage profiles

Muscle can generate greater force with lower muscle activation during eccentric (ECC) than isometric (ISO) and concentric (CON) contractions (actions). Unaccustomed ECC exercise induces muscle damage represented by prolonged impairment of muscle function and delayed onset muscle soreness (DOMS), but also confers protection from muscle damage, known as the repeated bout effect (RBE). However, the neuromuscular characteristics of ECC contractions, and the mechanisms involved in the muscle damage and RBE induced by ECC are not fully understood. Therefore, the purpose of this PhD thesis was to examine the neuromuscular characteristics of ECC contractions of the knee extensors in relation to muscle damage and the RBE profiles. This thesis consisted of five studies as summarised below. Study 1: Short-interval intracortical inhibition (SICI) is often used to examine inhibitory responses in the primary motor cortex representation of the quadriceps, but appropriate pairedpulse transcranial magnetic stimulation (TMS) parameters to optimise SICI measurement were not clear. Using 9 men and 6 women (26.6 ± 4.4 y), responses to single and paired-pulse (3-ms interval) TMS were recorded from vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM). Test stimulus intensity was 140% of active motor threshold (AMT), and conditioning stimulus intensities (CSIs) ranged from 55-90% (eight intensities) of AMT (5 test and 5 paired responses for each intensity). With CSI of 55% AMT, SICI was minimal (conditioned:test motor evoked potential [MEP]; 1.00, 0.96 and 0.95 for VL, RF and VM, respectively, -1.00 indicates inhibition). Inhibition was greater at 70-90% AMT for VL (0.67- 0.85), at 75-90% AMT for RF (0.70-0.78) and at 80-90% AMT for VM (0.59-0.68) when compared to 55% AMT. The CSIs that elicited maximal and 50% maximal inhibition were ~84% and ~75% AMT, respectively. This method was shown to provide a practical approach to investigate quadriceps inhibitory networks. Study 2: Responses to TMS, twitch forces (TF) and voluntary drive were compared between ECC, ISO and CON contractions of the knee extensors. Sixteen participants (20-33 y) performed submaximal and maximal voluntary contractions (MVCs) for ISO and isokinetic (30˚/s) CON and ECC of knee extensors. EMG was recorded from VL. Supramaximal femoral nerve stimulation during and after MVCs evoked superimposed (ST) and resting TF to calculate voluntary activation (VA). Maximal M-waves (MMAX) were recorded. During 30% MVCs, single- and paired-pulse TMS elicited MEPs and assessed SICI. MVC torque was greater (P Study 3: ECC-only and coupled concentric-eccentric contractions (CON-ECC) of the knee extensors were compared for changes in neurophysiological indices before, immediately after and 1-3 days post-exercise. Twenty participants (19-36 y) were randomly assigned to ECC (n=10) or CON-ECC (n=10) group. ECC group performed 6 sets of 8 ECC-only contractions at 80% of ECC one-repetition maximum (1-RMecc), while CON-ECC group performed 6 sets of 8 alternating CON and ECC (i.e., 8 CON and 8 ECC) contractions at 80% of CON 1-RM and 1-RMecc, respectively. Maximal voluntary isometric contraction (MVIC) force, rate of force development (RFD), TF elicited by femoral nerve stimulation, VA, MEPs, corticospinal silent period (CSP) and SICI assessed by single- and paired-pulse TMS, and muscle soreness were measured before, immediately after, and 1-3 days post-exercise. No significant (p\u3e0.05) differences between ECC and CON-ECC were evident for the changes in any variables post3 exercise. MVIC force decreased immediately post-exercise compared to baseline (ECC: -20.7 ± 12.8%, CON-ECC: -23.6 ± 23.3%) and remained low at 3 days post-exercise (-13.6 ± 13.4%, -3.3 ± 21.2%), and changes in RFD were greater than those of MVIC force (immediately postexercise: ECC: -38.3 ± 33.9%, CON-ECC: -30.7 ± 38.3%). VA, TF and MEP/MMAX decreased and CSP increased post-exercise (p Study 4: Since the participants from Study 3 responded similarly for indirect indicators of muscle damage, the 20 participants were used to examine whether the magnitude of muscle damage indicated by changes in MVIC force 1-3 days after ECC could be predicted by changes in central and peripheral neuromuscular parameters immediately post-ECC. The criterion measures were the same as those in Study 2, and additional analyses of the rate of force development (RFDRT) and rate of relaxation (RRRT) of the TF were performed. Relationships between changes in the variables immediately post-ECC and changes in MVIC force at 1-3 days post-ECC were examined by Pearson product-moment (r) or Spearman correlations. MVIC force decreased (-22.2 ± 18.4%) immediately post-exercise, and remained below baseline at 1 (-16.3 ± 15.2%), 2 (-14.7 ± 13.2%) and 3 days post-ECC (-8.6 ± 15.7%). Immediately post-ECC, RFD (0-30-ms: -38.3 ± 31.4%), TF (-45.9 ± 22.4%), RFDRT (-32.5±40.7%), RRRT (-38.0±39.7%), VA (-21.4 ± 16.5%) and MEP/MMAX at rest (-42.5 ± 23.3%) also decreased, while CSP at 10%-MVIC increased by 26.0 ± 12.2% (p Study 5: Among the 20 participants described in Study 4, 10 participants (6 from the eccentriconly exercise group and 4 from the concentric-eccentric exercise group) returned two weeks after the first exercise bout to perform the second bout of the same exercise. Changes in MVIC force, RFD, muscle soreness, TF, VA, MMAX, MEPs, and SICI before, immediately after and 1–3 days post-exercise were compared between the first (ECC1) and the second bouts (ECC2). ECC2 induced less (P=0.01) muscle soreness (peak: 27.5 ± 26.6 mm) than ECC1 (50.7 ± 31.8 mm), and MVIC force decreased more immediately post-ECC1 (-21.6 ± 23.3%) than ECC2 (-11.0 ± 11.3%) and recovered faster to the baseline after ECC2 than ECC1. RFD decreased immediately after ECC1 (e.g. 0-50-ms: -50.5 ± 52.5%, P0.05). The non-significant differences in VA and MEPs between ECC1 and ECC2, and lack of changes in MMAX and SICI suggest that changes in neural factors after eccentric exercise do not explain the repeated bout effect. Collectively, these studies advance understanding of the characteristics of ECC muscle contractions and their relationship to muscle damage and the RBE, and can be used to inform future research directions

Contraction Velocity of the Elbow Flexors Assessed by Tensiomyography: A Comparison Between Formulas

Contraction velocity of the elbow flexors assessed by tensiomyography: A comparison between formulas. J Strength Cond Res 37(10): 1969-1977, 2023-Muscle contraction velocity ( Vc ) assessed by tensiomyography is a promising measure for athlete profiling. Multiple formulas are used to estimate Vc , but the most suitable method is yet to be established. Fifteen adults (2 female subjects) underwent tensiomyography assessment of biceps brachii muscle at 10, 45 and 90\ub0 of elbow flexion on 2 separate days. Vc was calculated using 6 formulas. Formulas 1 and 2 are measures of the early phase of the twitch; Formulas 3-5 are measures over a wider time-window, with Formula 5 normalizing Vc to maximal displacement ( D m); and we proposed Formula 6 as a measure of peak Vc . Test-retest reliability, the required minimum number of trials, proportional bias, and effects of joint angle were investigated. Higher reliability (coefficient of variation: 2.8-6.9%) was found for Formula 1 (0-2 mm of displacement) and Formula 5 (normalized 10-90% of D m). Overall, a minimum of 6-7 trials was required to obtain reliable estimates. For 10\ub0 only, significant positive proportional bias ( r = 0.563-0.670) was found for all formulas except Formula 5. Vc was faster ( p &lt; 0.001) at shorter muscle lengths for all formulas except Formula 5 ( p = 0.06). Vc in the early phase of the twitch was more reliable when calculated using absolute displacement (Formula 1) than a relative threshold (Formula 2). Over a larger time-window, Formulas 3 and 4 were similarly reliable. Because they are derived from different components of the twitch and different parameters, the different formulas should not be used interchangeably. Additionally, more precise nomenclature is required to describe the information obtained from each formula