Longer Electromechanical Delay Impairs Hamstrings Explosive Force versus Quadriceps

Introduction Explosive neuromuscular performance refers to the ability to rapidly increase force in response to neuromuscular activation. The lower explosive force production of the hamstrings relative to the quadriceps could compromise knee joint stability and increase the risk of anterior cruciate ligament injury. However, the time course of the rise in explosive force of the hamstrings and quadriceps from their initial activation, and thus the explosive hamstrings-to-quadriceps (H/Q) force ratio, has not been documented.Methods The neuromuscular performance of 20 untrained males was assessed during a series of isometric knee flexion and extension contractions, with force and surface EMG of the hamstrings and quadriceps recorded during explosive and maximum voluntary contractions. Hamstrings force was expressed relative to quadriceps force to produce hamstring-to-quadriceps ratios of explosive H/Q force and H/Q maximum voluntary force. For the explosive contractions, agonist electromechanical delay (EMD), agonist and antagonist neural activation were assessed.Results The quadriceps was 79% stronger than the hamstrings, but quadriceps explosive force was up to 480% greater than the hamstrings from 25 to 50 ms after first activation. Consequently, the explosive H/Q force ratio was very low at 25 and 50 ms (0%–17%) and significantly different from H/Q maximum voluntary force ratio (56%). Hamstrings EMD was 95% greater than quadriceps EMD (44.0 vs 22.6 ms), resulting in a 21-ms later onset of force in the hamstrings that appeared to explain the low explosive H/Q force ratio in the early phase of activation.Conclusions Prolonged hamstrings EMD appears to impair early phase (0–50 ms) explosive force production relative to the quadriceps and may render the knee unstable and prone to anterior cruciate ligament injury during this period

Sex differences in muscle morphology of the knee flexors and knee extensors

Introduction Females experience higher risk of anterior cruciate ligament (ACL) injuries; males experience higher risk of hamstring strain injuries. Differences in injury may be partially due to sex differences in knee flexor (KF) to knee extensor (KE) muscle size ratio and the proportional size of constituent muscles. Purpose To compare the absolute and proportional size, and mass distribution, of individual KE and KF muscles, as well as overall size and balance (size ratio) of these muscle groups between the sexes. Methods T1-weighted axial plane MR images (1.5T) of healthy untrained young males and females (32 vs 34) were acquired to determine thigh muscle anatomical cross-sectional area(ACSA). Maximal ACSA (ACSAmax) ofconstituent muscles, summated for KF and KE muscle groups, and the KF:KE ratio were calculated. Results Females had 25.3% smaller KE ACSAmax (70.9±12.1 vs 93.6±10.3 cm2; P<0.001) and 29.6% smaller KF ACSAmax than males (38.8±7.3cm2 vs 55.1±7.3cm2; P<0.001).Consequently, females had lower KF:KE ACSA ratio (P = 0.031). There were sex differences in the proportional size of 2/4 KE and 5/6 KF. In females, vastus lateralis (VL), biceps femoris long-head (BFlh) and semimembranosus (SM) were a greater proportion and sartorius(SA), gracilis (GR) and biceps femoris short-head (BFsh) a smaller proportion of their respective muscle groups compared to males (All P<0.05). Conclusion Sex differences in KF:KE ACSAmax ratio may contribute to increased risk of ACL injury in females. Sex discrepancies in absolute and proportional size of SA, GR, VL and BFlh may contribute further anatomical explanations for sex differences in injury incidence

Effect of high impact exercise on femoral neck bone mineral density and T2 relaxation times of articular cartilage in postmenopausal women [Abstract]

Effect of high impact exercise on femoral neck bone mineral density and T2 relaxation times of articular cartilage in postmenopausal women [Abstract

Adherence and feasibility of a high impact exercise intervention in post-menopausal women [Abstract]

Adherence and feasibility of a high impact exercise intervention in post-menopausal women [Abstract

Training-specific functional, neural, and hypertrophic adaptations to explosive- vs. sustained-contraction strength training

Training specificity is considered important for strength training, although the functional and underpinning physiological adaptations to different types of training, including brief explosive contractions, are poorly understood. This study compared the effects of 12 wk of explosive-contraction (ECT, n = 13) vs. sustained-contraction (SCT, n = 16) strength training vs. control (n = 14) on the functional, neural, hypertrophic, and intrinsic contractile characteristics of healthy young men. Training involved 40 isometric knee extension repetitions (3 times/wk): contracting as fast and hard as possible for ∼1 s (ECT) or gradually increasing to 75% of maximum voluntary torque (MVT) before holding for 3 s (SCT). Torque and electromyography during maximum and explosive contractions, torque during evoked octet contractions, and total quadriceps muscle volume (QUADSVOL) were quantified pre and post training. MVT increased more after SCT than ECT [23 vs. 17%; effect size (ES) = 0.69], with similar increases in neural drive, but greater QUADSVOL changes after SCT (8.1 vs. 2.6%; ES = 0.74). ECT improved explosive torque at all time points (17-34%; 0.54 ≤ ES ≤ 0.76) because of increased neural drive (17-28%), whereas only late-phase explosive torque (150 ms, 12%; ES = 1.48) and corresponding neural drive (18%) increased after SCT. Changes in evoked torque indicated slowing of the contractile properties of the muscle-tendon unit after both training interventions. These results showed training-specific functional changes that appeared to be due to distinct neural and hypertrophic adaptations. ECT produced a wider range of functional adaptations than SCT, and given the lesser demands of ECT, this type of training provides a highly efficient means of increasing function

The influence of patellar tendon and muscle-tendon unit stiffness on quadriceps explosive strength in man

What is the central question of this study? \ud Do tendon and/or muscle–tendon unit stiffness influence rate of torque development? What is the main finding and its importance? In our experimental conditions, some measures of relative (to maximal voluntary torque and tissue length) muscle–tendon unit stiffness had small correlations with voluntary/evoked rate of torque development over matching torque increments. However, absolute and relative tendon stiffness were unrelated to voluntary and evoked rate of torque development. Therefore, the muscle aponeurosis but not free tendon influences the relative rate of torque development. Factors other than tissue stiffness more strongly determine the absolute rate of torque development. The influence of musculotendinous tissue stiffness on contractile rate of torque development (RTD) remains opaque. In this study, we examined the relationships between both patellar tendon (PT) and vastus lateralis muscle–tendon unit (MTU) stiffness and the voluntary and evoked knee-extension RTD. Fifty-two healthy untrained men completed duplicate laboratory sessions. Absolute and relative RTD were measured at 50 N m or 25% maximal voluntary torque (MVT) increments from onset and sequentially during explosive voluntary and evoked octet isometric contractions (supramaximal stimulation; eight pulses at 300 Hz). Isometric MVT was also assessed. Patellar tendon and MTU stiffness were derived from simultaneous force and ultrasound recordings of the PT and vastus lateralis aponeurosis during constant RTD ramp contractions. Absolute and relative (to MVT and resting tissue length) stiffness (k) was measured over identical torque increments as RTD. Pearson's correlations tested relationships between stiffness and RTD measurements over matching absolute/relative torque increments. Absolute and relative PT k were unrelated to equivalent voluntary/evoked (r = 0.020–0.255, P = 0.069–0.891). Absolute MTU k was unrelated to voluntary or evoked RTD (r ≤ 0.191, P ≥ 0.184), but some measures of relative MTU k were related to relative voluntary/evoked RTD (e.g. RTD for 25–50% MVT, r = 0.374/0.353, P = 0.007/0.014). In conclusion, relative MTU k explained a small proportion of the variance in relative voluntary and evoked RTD (both ≤19%), despite no association of absolute MTU k or absolute/relative PT k with equivalent RTD measures. Therefore, the muscle-aponeurosis component but not free tendon was associated with relative RTD, although it seems that an overriding influence of MVT negated any relationship of absolute MTU k and absolute RTD

Neural adaptations after 4 years vs. 12 weeks of resistance training vs. untrained

The purpose of this study was to compare the effect of resistance training (RT) duration, including years of exposure, on agonist and antagonist neuromuscular activation throughout the knee extension voluntary torque range. Fifty‐seven healthy men (untrained [UNT] n=29, short‐term RT [12WK] n=14, and long‐term RT [4YR] n=14) performed maximum and sub‐maximum (20‐80% maximum voluntary torque [MVT]) unilateral isometric knee extension contractions with torque, agonist and antagonist surface EMG recorded. Agonist EMG, including at MVT, was corrected for the confounding effects of adiposity (i.e. muscle‐electrode distance; measured with ultrasonography). Quadriceps maximum anatomical cross‐sectional area (QACSAMAX; via MRI) was also assessed. MVT was distinct for all three groups (4YR +60/+39% vs. UNT/12WK; 12WK +15% vs. UNT; 0.001<P≤0.021), and QACSAMAX was greater for 4YR (+50/+42% vs. UNT/12WK; [both] P<0.001). Agonist EMG at MVT was +44/+33% greater for 4YR/12WK ([both] P<0.001) vs. UNT; but did not differ between RT groups. The torque‐agonist EMG relationship of 4YR displayed a right/down shift with lower agonist EMG at the highest common torque (196 Nm) compared to 12WK and UNT (0.005≤P≤0.013; Effect size [ES] 0.90≤ES≤1.28). The torque‐antagonist EMG relationship displayed a lower slope with increasing RT duration (4YR<12WK<UNT; 0.001<P≤0.094; 0.56≤ES≤1.31), and antagonist EMG at the highest common torque was also lower for 4YR than UNT (‐69%; P<0.001; ES=1.18). In conclusion, 4YR and 12WK had similar agonist activation at MVT and this adaptation may be maximised during early months of RT. In contrast, inter‐muscular coordination, specifically antagonist co‐activation was progressively lower, and likely continues to adapt, with prolonged RT

THE RELATIONSHIP OF ANTHROPOMETRY AND BODY COMPOSITION WITH RUNNING ECONOMY

The aim of this study was to investigate the relationships of anthropometry and body composition with running economy within a large heterogeneous cohort of runners. Locomotory energy cost was determined in ninety-four healthy male and female endurance runners across a range of performance standards. Various anthropometric and body composition measurements were taken manually and via DXA scans. The relationships between anthropometry and running economy were assessed using independent Pearson’s correlation and stepwise multiple linear regression. Three parameters, normalised neck and calf perimeters and normalised whole body bone mass explained 30% of the variance in locomotory energy cost. Low locomotory energy cost was related solely to parameters indicating relative slenderness of the body

The Human Muscle Size and Strength Relationship. Effects of Architecture, Muscle Force and Measurement Location.

Purpose This study aimed to determine the best muscle size index of muscle strength by establishing if incorporating muscle architecture measurements improved the human muscle size-strength relationship. The influence of calculating muscle force, and the location of anatomical cross-sectional area (ACSA) measurements on this relationship were also examined. Methods Fifty-two recreationally active males completed unilateral isometric knee extension strength assessments and MRI scans of the dominant thigh and knee to determine quadriceps femoris (QF) size variables (ACSA along the length of the femur, maximum ACSA [ACSAMAX] and volume [VOL]) and patellar tendon moment arm. Ultrasound images (2 sites per constituent muscle) were analyzed to quantify muscle architecture (fascicle length, pennation angle), and when combined with VOL (from MRI), facilitated calculation of QF effective PCSA (EFFPCSA) as potentially the best muscle size determinant of strength. Muscle force was calculated by dividing maximum voluntary torque (MVT) by the moment arm and addition of antagonist torque (derived from hamstring EMG). Results The associations of EFFPCSA (r=0.685), ACSAMAX (r=0.697), or VOL (r=0.773) with strength did not differ, although qualitatively VOL explained 59.8% of the variance in strength, ~11-13% greater than EFFPCSA or ACSAMAX. All muscle size variables had weaker associations with muscle force than MVT. The association of strength-ACSA at 65% of femur length (r=0.719) was greater than for ACSA measured between 10-55% and 75-90% (r=-0.042-0.633) of femur length. Conclusions In conclusion, using contemporary methods to assess muscle architecture and calculate EFFPCSA did not enhance the muscle strength-size association. For understanding/monitoring muscle size, the major determinant of strength, these findings support the assessment of muscle volume, that is independent of architecture measurements, and was most highly correlated to strength

What makes long-term resistance-trained individuals so strong? A comparison of skeletal muscle morphology, architecture, and joint mechanics.

The greater muscular strength of long-term resistance-trained (LTT) individuals is often attributed to hypertrophy but the role of other factors, notably maximum voluntary specific tension (ST), muscle architecture and any differences in joint mechanics (moment arm) have not been documented. The aim of the present study was to examine the musculoskeletal factors that might explain the greater Quadriceps strength and size of LTT vs untrained (UT) individuals. LTT (n = 16, age 21.6 ± 2.0 years) had 4.0 ± 0.8 years of systematic knee extensor heavy-resistance training experience, whereas UT (n = 52; age 25.1 ± 2.3 years) had no lower-body resistance training experience for > 18 months. Knee extension dynamometry, T1-weighted magnetic resonance images of the thigh and knee and ultrasonography of the Quadriceps muscle group at 10 locations were used to determine Quadriceps: isometric maximal voluntary torque (MVT), muscle volume (QVOL), patella tendon moment arm (PTMA), pennation angle (QΘP) and fascicle length (QFL), physiological cross-sectional area (QPCSA) and ST. LTT had substantially greater MVT (+60% vs UT, P<0.001) and QVOL (+56%, P<0.001) and QPCSA (+41%, P<0.001) but smaller differences in ST (+9%, P<0.05) and moment arm (+4%, P<0.05), and thus muscle size was the primary explanation for the greater strength of LTT. The greater muscle size (volume) of LTT was primarily attributable to the greater QPCSA (+41%; indicating more sarcomeres in parallel) rather than the more modest difference in FL (+11%; indicating more sarcomeres in series). There was no evidence for regional hypertrophy after LTT