Neuromuscular fatigue differs following unilateral vs bilateral sustained submaximal contractions

The purpose of the present study was to compare the mechanisms of fatigue induced by a unilateral vs a bilateral submaximal isometric knee extension. Ten physically active men completed two experimental sessions, randomly presented. They were asked to maintain an isometric knee extension force corresponding to 20% of the maximal voluntary contraction (MVC) until task failure with one leg (unilateral) vs two legs (bilateral). MVCs were performed before and after the sustained contraction. Transcutaneous electrical stimuli were used to examine central (voluntary activation) and peripheral (peak doublet force at rest) fatigue on the exercised leg. Time to task failure was significantly shorter (P<0.05) for the bilateral (245±76 s) compared with the unilateral task (295±85 s). Unilateral MVC force and maximal voluntary activation losses were significantly greater (P<0.05) after the unilateral task than after the bilateral task. Peak doublet force was significantly reduced (P<0.01) after the unilateral task, but not after the bilateral task. The present results demonstrated that time to task failure of a submaximal fatiguing contraction may depend on the number of limbs involved in the task. The greater time to task failure with one leg may have induced greater contractile alterations and a larger MVC loss following the unilateral task.     

Task failure during sustained low-intensity contraction is not associated with a critical amount of central fatigue

Fatigue-related mechanisms induced by low-intensity prolonged contraction in lower limb muscles are currently unknown. This study investigated central fatigue kinetics in the knee extensors during a low-intensity sustained isometric contraction. Eleven subjects sustained a 10% maximal voluntary contraction (MVC) until task failure (TF) with neuromuscular evaluation every 3 minutes. Testing encompassed transcranial magnetic stimulation to evaluate maximal voluntary activation (VA_TMS), motor evoked potential (MEP), and silent period (SP), and peripheral nerve stimulation to assess M-wave. Rating of perceived exertion (RPE) was also recorded. MVC progressively decreased up to 50% of the time to TF (ie, 50%_TTF) and then plateaued, reaching ~50% at TF (P < .001). VA_TMS progressively decreased up to 90%_TTF and then plateaued, the decrease reaching ~20% at TF (P < .001). SP was lengthened early (ie, from 20%_TTF) during the exercise and then plateaued (P < .01). No changes were reported for MEP evoked during MVC (P = .87), while MEP evoked during submaximal contractions decreased early (ie, from 20%_TTF) during the exercise and then plateaued (P < .01). RPE increased linearly during the exercise to be almost maximal at TF. M-waves were not altered (P = .88). These findings confirm that TF is due to the subjects reaching their maximal perceived effort rather than any particular central event or neuromuscular limitations since MVC at TF was far from 10% of its original value. It is suggested that strategies minimizing RPE (eg, motivational self-talk) should be employed to enhance endurance performance.     

Heavy‐resistance exercise‐induced increases in jump performance are not explained by changes in neuromuscular function

Post‐activation potentiation (PAP) is the increased involuntary muscle twitch response to stimulation following strong contraction. The enhancement to whole‐body explosive muscular performance (PE) after heavy‐resistance exercise is often attributed to modulations in neuromuscular function that are proposed to reflect PAP, but the evidence to support this is equivocal. We assessed the neuromuscular basis of PE using transcranial magnetic stimulation (TMS) of the primary motor cortex, and electrical stimulation of the femoral nerve. Eleven male athletes performed heavy‐resistance exercise with measures of countermovement jump (CMJ) pre‐ and 8 min post‐exercise. Pre‐exercise and after the final CMJ, single‐ and paired‐pulse TMS were delivered during submaximal isometric knee‐extensor contractions to measure corticospinal excitability, short‐interval intracortical inhibition (SICI), and intracortical facilitation (ICF), with motor evoked potentials recorded from rectus femoris. Twitch responses to motor nerve stimulation during and post maximum‐knee‐extensor contractions were studied to quantify voluntary activation (VA) and potentiated twitch (Q_tw,pot). The experimental protocol successfully induced PE (+4 ± 1% change in CMJ, P = 0.01), but no changes were observed for maximum voluntary force, VA, corticospinal excitability, SICI or ICF (all P > 0.05), and Q_tw,pot declined (P < 0.001). An enhancement of muscular performance after heavy‐resistance exercise was not accompanied by PAP, or changes in measures of neuromuscular function.     

Concentric versus eccentric cycling at equal power output or effort perception: Neuromuscular alterations and muscle pain

This study aimed to compare neuromuscular alterations and perceptions of effort and muscle pain induced by concentric and eccentric cycling performed at the same power output or effort perception. Fifteen participants completed three 30-min sessions: one in concentric at 60% peak power output (CON) and two in eccentric, at the same power output (ECC_POWER) or same perceived effort (ECC_EFFORT). Muscle pain, perception of effort, oxygen uptake as well as rectus femoris and vastus lateralis electromyographic activities were collected when pedaling. The knee extensors maximal voluntary contraction (MVC) torque, the torque evoked by double stimulations at 100 Hz and 10 Hz (Dt100; Dt10), and the voluntary activation level (VAL) were evaluated before and after exercise. Power output was higher in ECC_EFFORT than CON (89.1 ± 23.3% peak power). Muscle pain and effort perception were greater in CON than ECC_POWER (p < 0.03) while muscle pain was similar in CON and ECC_EFFORT (p > 0.43). MVC torque, Dt100, and VAL dropped in all conditions (p < 0.04). MVC torque (p < 0.001) and the Dt10/ Dt100 ratio declined further in ECC_EFFORT (p < 0.001). Eccentric cycling perceived as difficult as concentric cycling caused similar muscle pain but more MVC torque decrease. A given power output induced lower perceptions of pain and effort in eccentric than in concentric yet similar MVC torque decline. While neural impairments were similar in all conditions, eccentric cycling seemed to alter excitation-contraction coupling. Clinicians should thus be cautious when setting eccentric cycling intensity based on effort perception.     

Facilitation of quadriceps activation is impaired following eccentric exercise

Contracting the knee flexor muscles immediately before a maximum voluntary contraction (MVC) of knee extension increases the maximal force that the extensor muscles can exert. It is hypothesized that this phenomenon can be impaired by muscle fiber damage following eccentric exercise [delayed onset muscle soreness (DOMS)]. This study investigates the effect of eccentric exercise and DOMS on knee extension MVC immediately following a reciprocal‐resisted knee flexion contraction. Electromyography (EMG) was recorded from the knee extensors and flexors of 12 healthy men during knee extension MVCs performed in a reciprocal (maximal knee extension preceded by resisted knee flexion), and nonreciprocal condition (preceded by relaxation of the knee flexors). At baseline, knee extension MVC force was greater during the reciprocal condition (P < 0.001), whereas immediately after, 24 and 48 h after eccentric exercise, the MVC force was not different between conditions. Similarly, at baseline, the EMG amplitude of the quadriceps during the MVC was larger for the reciprocal condition (P < 0.001). However, immediately after, 24 and 48 h postexercise the EMG amplitude was similar between conditions. In conclusion, eccentric exercise abolished the facilitation of force production for the knee extensors, which normally occurs when maximum knee extension is preceded by activation of the knee flexors.     

Damage and the repeated bout effect of arm,leg, and trunk muscles induced by eccentric resistance exercises

This study compared nine resistance eccentric exercises targeting arm, leg, and trunk muscles in one session for changes in maximal voluntary isometric contraction strength (MVC), delayed onset muscle soreness (DOMS), plasma creatine kinase (CK) activity, and myoglobin (Mb) concentration after the first and second bouts. Fifteen sedentary men (20‐25 years) performed 5 sets of 10 eccentric contractions with 80% of MVC load for the elbow flexors (EF), elbow extensors (EE), pectoralis, knee extensors (KE), knee flexors (KF), plantar flexors (PF), latissimus, abdominis, and erector spinae (ES) in a randomized order and repeated the same exercises 2 weeks later. MVC decreased at 1 (16%‐57%) to 4 (13%‐49%) days, DOMS developed (peak: 43‐70 mm), and CK activity (peak: 23 238‐207 304 IU/L) and Mb concentration showed large increases after the first bout. The magnitude of decrease in MVC was greater (P < 0.05) for EF, EE, and PEC than others and for KF than KE, PF, and ES. DOMS was greater (P < 0.05) for EF, EE, and ES than others. Changes in all measures were smaller (P < 0.05) after the second than the first bout, and the magnitude of the repeated bout effect was similar among the muscles. Plasma CK activity and Mb concentration did not increase significantly after the second exercise bout. It was concluded that muscle damage was greater for arm than leg muscles, and muscle proteins in the blood increased to a critical level after unaccustomed whole‐body resistance exercises, but the magnitude of damage was largely attenuated for all muscles similarly after the second bout.     

Muscle fiber conduction velocity in the vastus lateralis and medialis muscles of soccer players after ACL reconstruction

The neural factors underlying the persistency of quadriceps weakness after anterior cruciate ligament reconstruction (ACLR) have been only partially explained. This study examined muscle fiber conduction velocity (MFCV) as an indirect parameter of motor unit recruitment strategies in the vastus lateralis (VL) and medialis (VM) muscles of soccer players with ACLR. High-density surface electromyography (HDsEMG) was acquired from VL and VM in nine soccer players (22.7 ± 2.9 years; BMI: 22.08 ± 1.72 kg·m⁻²; 7.7 ± 2.2 months post-surgery). Voluntary muscle force and the relative myoelectrical activity from the reconstructed and contralateral sides were recorded during linearly increasing isometric knee extension contractions up to 70% of maximal voluntary isometric force (MVIF). The relation of MFCV and force was examined by linear regression analysis at the individual subject level. The initial (intercept), peak (MFCV₇₀), and rate of change (slope) of MFCV related to force were compared between limbs and muscles. The MVIF was lower in the reconstructed side than in the contralateral side (−%20.5; P < .05). MFCV intercept was similar among limbs and muscles (P > .05). MFCV₇₀ and MFCV slope were lower in the reconstructed side compared to the contralateral for both VL (−28.5% and −10.1%, respectively; P < .001) and VM (−22.6% and −8.1%, respectively; P < .001). The slope of MFCV was lower in the VL than VM, but only in the reconstructed side (−12.4%; P < .001). These results suggest possible impairments in recruitment strategies of high-threshold motor units (HTMUs) as well as deficits in sarcolemmal excitability, fiber diameter, and discharge rate of knee extensor muscles following ACLR.     

The effects of compression garments on recovery of muscle performance following high-intensity sprint and plyometric exercise

This study compared the effects of compression garments on recovery of evoked and voluntary performance following fatiguing exercise. Eleven participants performed 2 sessions separated by 7 days, with and without lower-body compression garments during and 24 h post-exercise. Participants performed a 10-min exercise protocol of a 20-m sprint and 10 plyometric bounds every minute. Before, following, 2 h and 24 h post-exercise, evoked twitch properties of the knee extensors, peak concentric knee extension and flexion force were assessed, with blood samples drawn to measure lactate [La^?], pH, creatine kinase (CK), aspartate transaminase (AST) and c-reactive protein (C-RP). Heart rate, exertion (RPE) and muscle soreness (MS) measures were obtained pre- and post-exercise. No differences (P = 0.50–0.80) and small effect sizes (d < 0.3) were present for 20-m sprint (3.59 ± 0.22 vs. 3.59 ± 0.18 s) or bounding performance (17.13 ± 1.4 vs. 17.21 ± 1.7 m) in garment and control conditions. The decline and recovery in concentric force were not different (P = 0.40) between conditions. Full recovery of voluntary performance was observed 2 h post-exercise, however, evoked twitch properties remained suppressed 2 h post-exercise in both conditions. No differences (P = 0.40–0.80, d < 0.3) were present between conditions for heart rate, RPE, [La^?], pH, CK or C-RP. However, 24 h post-exercise a smaller change (P = 0.08; d = 2.5) in AST (23.1 ± 3.1 vs. 26.0 ± 4.0) and reduced (P = 0.01; d = 1.1) MS (2.8 ± 1.2 vs. 4.5 ± 1.4) were present in the garments. In conclusion the effects of compression garments on voluntary performance and recovery were minimal; however, reduced levels of perceived MS were reported following recovery in the garments.     

Mechanized pivot shift test achieves greater accuracy than manual pivot shift test

The objective of this study was to design a navigated mechanized pivot shift test setup and evaluate its repeatability in the ACL-deficient knee. It was hypothesized that translations and rotations measured with the mechanized pivot shift would be more repeatable when compared to those obtained with a manual pivot shift. Twelve fresh frozen cadaveric hip-to-toe whole lower extremities were used for this study. A manual pivot shift test was performed in the intact knee and in the ACL-deficient knee and was repeated three times. A navigation system simultaneously recorded tibial translation and rotation. The mechanized pivot shift test consists of a modified continuous passive motion (CPM) machine and a custom-made foot holder to allow for the application of internal rotation moments at the knee. Valgus moments were achieved by a 45° tilt of the CPM machine with respect to the supine position and a Velcro strap secured across the proximal tibia. The mechanized pivot shift was repeated three times. Repeated measures ANOVA was used to compare manual and mechanized pivot shift testing. An intra-class correlation coefficient (ICC) was used to determine variability within each knee at each testing condition. In the ACL-deficient knee, translation with manual pivot shift testing (11.7 ± 2.6 mm) was significantly higher than with mechanized pivot shift testing (7.4 ± 2.5 mm; p < 0.05). Rotation with the manual pivot shift testing (18.6 ± 5.4°) was also significantly higher than with mechanized pivot shift testing (11.0 ± 2.3°; p < 0.05). The intra-class ICC for translations was 0.76 for manual pivot shift and 0.92 for the mechanized pivot shift test. The intra-class ICC for rotations was 0.89 for manual pivot shift and 0.82 for the mechanized pivot shift test. This study introduced a modified CPM for mechanized pivot shift testing. Although recorded translations and rotations with the mechanized pivot shift test were lower than with manual testing, the clinical advantage of mechanized pivot shift testing is a more repeatable measurement of ATT when compared to manual pivot shift testing. This setup may increase consistency of clinical grading of the pivot shift test.     

Cold‐water immersion decreases cerebral oxygenation but improves recovery after intermittent‐sprint exercise in the heat

This study examined the effects of post‐exercise cooling on recovery of neuromuscular, physiological, and cerebral hemodynamic responses after intermittent‐sprint exercise in the heat. Nine participants underwent three post‐exercise recovery trials, including a control (CONT), mixed‐method cooling (MIX), and cold‐water immersion (10 °C; CWI). Voluntary force and activation were assessed simultaneously with cerebral oxygenation (near‐infrared spectroscopy) pre‐ and post‐exercise, post‐intervention, and 1‐h and 24‐h post‐exercise. Measures of heart rate, core temperature, skin temperature, muscle damage, and inflammation were also collected. Both cooling interventions reduced heart rate, core, and skin temperature post‐intervention (P < 0.05). CWI hastened the recovery of voluntary force by 12.7 ± 11.7% (mean ± SD) and 16.3 ± 10.5% 1‐h post‐exercise compared to MIX and CONT, respectively (P < 0.01). Voluntary force remained elevated by 16.1 ± 20.5% 24‐h post‐exercise after CWI compared to CONT (P < 0.05). Central activation was increased post‐intervention and 1‐h post‐exercise with CWI compared to CONT (P < 0.05), without differences between conditions 24‐h post‐exercise (P > 0.05). CWI reduced cerebral oxygenation compared to MIX and CONT post‐intervention (P < 0.01). Furthermore, cooling interventions reduced cortisol 1‐h post‐exercise (P < 0.01), although only CWI blunted creatine kinase 24‐h post‐exercise compared to CONT (P < 0.05). Accordingly, improvements in neuromuscular recovery after post‐exercise cooling appear to be disassociated with cerebral oxygenation, rather reflecting reductions in thermoregulatory demands to sustain force production.     

Muscular and neuromuscular control following soccer‐specific exercise in male youth: Changes in injury risk mechanisms

Poor neuromuscular control has been proposed as a risk factor for non‐contact injuries, thus this study aimed to explore the effects of soccer‐specific fatigue on leg muscle activation, reactive strength, leg stiffness, and functional hamstring/quadriceps ratio (H/Q_FUNC) in elite male youth soccer players. Outcome measures were determined in 18 youth players (age 14.4 ± 0.5 years; stature 169.4 ± 9.9 cm; mass 59.3 ± 8.9 kg; maturity offset 0.86 ± 0.88 years) pre and post simulated soccer match play (SAFT⁹⁰). There was no fatigue‐related change in the H/Q_FUNC; however, reactive strength and leg stiffness were both compromised (P < 0.001) after soccer‐specific fatigue. Muscle activation was also locally compromised (P < 0.001) in the medial hamstring and quadriceps but not in the lateral muscles. Where statistically significant changes were observed, the effect sizes ranged from small to large (0.33–0.97). Compromised stiffness when fatigue is present suggests an increased yielding action, greater ground contact times, greater center of mass displacement, and less efficient movement when the limb comes into contact with the ground. This combined with a reduction in medial quadriceps muscle activation may reflect poor kinetic chain control at the hip and an increase in knee injury risk.     

Alteration of neuromuscular function in squash

The alteration in neuromuscular function of knee extensor muscles was characterised after a squash match in 10 trained players. Maximal voluntary contraction (MVC) and surface EMG activity of vastus lateralis (VL) and vastus medialis (VM) muscles were measured before and immediately after a 1-h squash match. M-wave and twitch contractile properties were analysed following single stimuli. MVC declined (280.5 ± 46.8 vs. 233.6 ± 35.4 N m, ?16%; P < 0.001) after the exercise and this was accompanied by an impairment of central activation, as attested by decline in voluntary activation (76.7 ± 10.4 vs. 71.3 ± 9.6%, ?7%; P < 0.05) and raw EMG activity of the two vastii (?17%; P < 0.05), whereas RMS/M decrease was lesser (VL: ?5%; NS and VM: ?12%; P = 0.10). In the fatigued state, no significant changes in M-wave amplitude (VL: ?9%; VM: ?5%) or duration were observed. Following exercise, the single twitch was characterised by lower peak torque (?20%; P < 0.001) as well as shorter half-relaxation time (?13%; P < 0.001) and reduced maximal rate of twitch tension development (?23%; P < 0.001) and relaxation (?17%; P < 0.05). A 1-h squash match play caused peripheral fatigue by impairing excitation–contraction coupling, whereas sarcolemmal excitability seems well preserved. Our results also emphasise the role of central activation failure as a possible mechanism contributing to the torque loss observed in knee extensors. Physical conditioners should consider these effects when defining their training programs for squash players.     

Neuromuscular adaptations induced by adjacent joint training

Effects of resistance training are well known to be specific to tasks that are involved during training. However, it remains unclear whether neuromuscular adaptations are induced after adjacent joint training. This study examined the effects of hip flexion training on maximal and explosive knee extension strength and neuromuscular performance of the rectus femoris (RF , hip flexor, and knee extensor) compared with the effects of knee extension training. Thirty‐seven untrained young men were randomly assigned to hip flexion training, knee extension training, or a control group. Participants in the training groups completed 4 weeks of isometric hip flexion or knee extension training. Standardized differences in the mean change between the training groups and control group were interpreted as an effect size, and the substantial effect was assumed to be ≥0.20 of the between‐participant standard deviation at baseline. Both types of training resulted in substantial increases in maximal (hip flexion training group: 6.2% ± 10.1%, effect size = 0.25; knee extension training group: 20.8% ± 9.9%, effect size = 1.11) and explosive isometric knee extension torques and muscle thickness of the RF in the proximal and distal regions. Improvements in strength were accompanied by substantial enhancements in voluntary activation, which was determined using the twitch interpolation technique and RF activation. Differences in training effects on explosive torques and neural variables between the two training groups were trivial. Our findings indicate that hip flexion training results in substantial neuromuscular adaptations during knee extensions similar to those induced by knee extension training.     

Neuromuscular and functional responses in professional soccer players during pre-season: implications for injury prevention

Objective

The purpose of this investigation was to assess the muscular strength of the knee extensors and flexors muscles, agility and functionality between dominant leg (DL) vs. non-dominant leg (NDL) of professional soccer players during pre-season.

Methods

Twenty-two healthy male professional soccer players (25.1?±?3.8 years; 182.1?±?5.9 cm; 79.3?±?5.4 kg; body fat 14.6?±?3.8%) from a club of the Brazilian second-division soccer league participated in this study. All soccer players underwent three tests to assess strength, power, and physical performance with an interval of 48 h inter-sessions. During the first assessment session, isokinetic tests were performed in which athletes performed maximal isokinetic concentric and eccentric contractions, as well as maximal isometric contractions. In the second and third assessment sessions, single leg step down and agility tests were performed, respectively.

Results

Maximum voluntary isometric contraction (MVIC) results showed a significant difference (p?<?0.04) between DL compared to the NDL for absolute and relative values. But, the isokinetic variables of knee extensors at 60°/s (total work and power) were significantly higher (p?<?0.03) in NDL. Pearson’s correlation was found to be significant between MVIC and single leg step down (SLSD) test for DL (r?=?0.70, p?=?0.0001) and NDL (r?=?0.58; p?=?0.002).

Conclusion

The results confirm that DL shows greater isometric strength than NDL. However, NDL revealed an increase in the total work and power output in professional soccer players. Furthermore, the SLSD test is recommended because it proved to be a good method for evaluating muscle strength by a positive correlation with MVIC.

     

Fatigue and recovery after high-intensity exercise part I: neuromuscular fatigue

The contribution of central and peripheral factors to muscle fatigue were quantified following a high-intensity uphill running exercise. Eight male volunteers performed an intermittent exercise at 120 % of maximal aerobic speed on a treadmill with an 18 % grade. Electrically evoked and voluntary contractions of the knee extensors and EMG of the two vastii were analyzed before and immediately after the high-intensity exercise. Isometric maximal voluntary contraction decreased slightly (-7+/-8 %; p < 0.05) after exercise but no changes were found in the level of maximal activation or in the torque produced by a 80 Hz maximal stimulation applied to the femoral nerve. Following exercise, the single twitch was characterized by lower peak torque, maximal rate of force development, and relaxation (-28+/-11%, -25+/-12%, -31+/-15% respectively, p < 0.001), and higher surface of the M-wave for both vastii. The ratio between the torques evoked by 20 Hz and 80 Hz stimulation declined significantly (-22+/-10%, p < 0.01) after exercise. These findings indicate that muscle fatigue after high-intensity running exercise is due to significant alteration in excitation-contraction coupling and that this type of exercise does not induce significant central fatigue or changes at the crossbridge level.     

Velocity‐specific and time‐dependent adaptations following a standardized Nordic Hamstring Exercise training

The Nordic Hamstring Exercise (NHE ) is effective for selective hamstring strengthening to improve muscle balance between knee flexors and extensors. The purpose of this study (within subject design of repeated measures) was to determine the effects of a standardized 4‐week NHE training on thigh strength and muscle balance with concomitant kinetic and kinematic monitoring. Sixteen male sprinters (22 years, 181 cm, 76 kg) performed a standardized 4‐week NHE training consisting of three sessions per week (each 3×3 repetitions). Six rope‐assisted and six unassisted sessions were performed targeting at a constant knee extension angular velocity of ~15°/s across a ~90‐100° knee joint range of motion. Kinetic (peak and mean moment, impulse) and kinematic parameters (eg, ROM to downward acceleration, ROM_DWA ) were recorded during selected sessions. Unilateral isokinetic tests of concentric and eccentric knee flexors and extensors quantified muscle group‐, contraction mode‐, and velocity‐specific training adaptations. Peak moments and contractional work demonstrated strong interactions of time with muscle group, contraction modes, and angular velocities (η²>.150). NHE training increased eccentric hamstring strength by 6%‐14% as well as thigh muscle balance with biggest adaptations at 150°/s 2 weeks after NHE training. Throughout the training period significant increases (P <.001) of peak (η²=.828) and mean moments (η²=.611) became apparent, whereas the impulse and the ROM_DWA of unassisted NHE repetitions remained unchanged (P >.05). A 4‐week NHE training significantly strengthened the hamstrings and improved muscle balance between knee flexors and extensors. Despite the slow training velocity, biggest adaptations emerged at the highest velocity 2 weeks after training ended.     

Central and peripheral fatigue in knee and elbow extensor muscles after a long‐distance cross‐country ski race

Although elbow extensors (EE) have a great role in cross‐country skiing (XC) propulsion, previous studies on neuromuscular fatigue in long‐distance XC have investigated only knee extensor (KE) muscles. In order to investigate the origin and effects of fatigue induced by long‐distance XC race, 16 well‐trained XC skiers were tested before and after a 56‐km classical technique race. Maximal voluntary isometric contraction (MVC) and rate of force development (RFD) were measured for both KE and EE. Furthermore, electrically evoked double twitch during MVC and at rest were measured. MVC decreased more in KE (?13%) than in EE (?6%, P = 0.016), whereas the peak RFD decreased only in EE (?26%, P = 0.02) but not in KE. The two muscles showed similar decrease in voluntary activation (KE ?5.0%, EE ?4.8%, P = 0.61) and of double twitch amplitude (KE ?5%, EE ?6%, P = 0.44). A long‐distance XC race differently affected the neuromuscular function of lower and upper limbs muscles. Specifically, although the strength loss was greater for lower limbs, the capacity to produce force in short time was more affected in the upper limbs. Nevertheless, both KE and EE showed central and peripheral fatigue, suggesting that the origins of the strength impairments were multifactorial for the two muscles.     

Protective effect by maximal isometric contractions against maximal eccentric exercise-induced muscle damage of the knee extensors

This study investigated whether maximal voluntary isometric contractions (MVIC) performed before maximal eccentric contractions (MaxEC) would attenuate muscle damage of the knee extensors. Untrained men were placed to an experimental group that performed 6 sets of 10 MVIC at 90° knee flexion 2 weeks before 6 sets of 10 MaxEC or a control group that performed MaxEC only (n = 13/group). Changes in muscle damage markers were assessed before to 5 days after each exercise. Small but significant changes in maximal voluntary concentric contraction torque, range of motion (ROM) and plasma creatine kinase (CK) activity were evident at immediately to 2 days post-MVIC (p < 0.05), but other variables (e.g. thigh girth, myoglobin concentration, B-mode echo intensity) did not change significantly. Changes in all variables after MaxEC were smaller (p < 0.05) by 45% (soreness)–67% (CK) for the experimental than the control group. These results suggest that MVIC conferred potent protective effect against MaxEC-induced muscle damage.     

Effects of muscle activation on shear between human soleus and gastrocnemius muscles

Lateral connections between muscles provide pathways for myofascial force transmission. To elucidate whether these pathways have functional roles in vivo, we examined whether activation could alter the shear between the soleus (SOL) and lateral gastrocnemius (LG) muscles. We hypothesized that selective activation of LG would decrease the stretch‐induced shear between LG and SOL. Eleven volunteers underwent a series of knee joint manipulations where plantar flexion force, LG, and SOL muscle fascicle lengths and relative displacement of aponeuroses between the muscles were obtained. Data during a passive full range of motion were recorded, followed by 20° knee extension stretches in both passive conditions and with selective electrical stimulation of LG. During active stretch, plantar flexion force was 22% greater (P < 0.05) and relative displacement of aponeuroses was smaller than during passive stretch (P < 0.05). Soleus fascicle length changes did not differ between passive and active stretches but LG fascicles stretched less in the active than passive condition when the stretch began at angles of 70° and 90° of knee flexion (P < 0.05). The activity‐induced decrease in the relative displacement of SOL and LG suggests stronger (stiffer) connectivity between the two muscles, at least at flexed knee joint angles, which may serve to facilitate myofascial force transmission.     

Effect of tunnel position for anatomic single-bundle ACL reconstruction on knee biomechanics in a porcine model

Attention has been focused on the importance of anatomical tunnel placement in anterior cruciate ligament (ACL) reconstruction. The purpose of this study was to compare the effect of different tunnel positions for single-bundle (SB) ACL reconstruction on knee kinematics. Ten porcine knees were used for the following reconstruction techniques: three different anatomic SB [AM–AM (antero-medial), PL–PL (postero-lateral), and MID–MID] (n = 5 for each group), conventional SB (PL–high AM) (n = 5), and anatomic double-bundle (DB) (n = 5). Using a robotic/universal force–moment sensor testing system, an 89 N anterior load (simulated KT1000 test) at 30, 60, and 90° of knee flexion and a combined internal rotation (4 N m) and valgus (7 N m) moment (simulated pivot-shift test) at 30 and 60° were applied. Anterior tibial translation (ATT) (mm) and in situ forces (N) of reconstructed grafts were calculated. During simulated KT1000 test at 60° of knee flexion, the PL–PL had significantly lower in situ force than the intact ACL (P < 0.01). In situ force of the MID–MID was higher than other SB reconstructions (at 30°: 94.8 ± 2.5 N; at 60°: 85.2 ± 5.3 N; and 90°: 66.0 ± 8.7 N). At 30° of knee flexion, the PL–high AM had the lowest in situ values (67.1 ± 19.3 N). At 60 and 90° of knee flexion the PL–PL had the lowest in situ values (at 60°: 60.8 ± 19.9 N; 90°: 38.4 ± 19.2 N). The MID–MID and DB had no significant in situ force differences at 30 and 60° of knee flexion. During simulated pivot-shift test at 60° of knee flexion, the PL–PL and PL–high AM reconstructions had a significant lower in situ force than the intact ACL (P < 0.01). During simulated KT1000 test at 30, 60, and 90° of knee flexion, the PL–PL and PL–high AM had significantly lower ATT than the intact ACL (P < 0.01). During simulated KT1000 test at 60 and 90°, the MID–MID, AM–AM, and DB had significantly lower ATT than the ACL deficient knee (P < 0.01). During simulated KT1000 test at 90°, every reconstructed knee had significantly higher ATT than the intact knee (P < 0.01). In conclusion, the MID–MID position provided the best stability among all anatomic SB reconstructions and more closely restored normal knee kinematics.