Background:
The acute effects of exercise on anterior knee laxity (AKL) and anterior knee stiffness (AKS) have been documented in healthy participants, but only limited evidence has been provided for ...athletes cleared to return to sports after anterior cruciate ligament (ACL) reconstruction (ACLR).
Purpose/Hypothesis:
The purpose was to determine if 45 minutes of a soccer match simulation lead to acute changes in AKL and AKS in soccer players returning to sport within 12 months after ACLR. We hypothesized that the reconstructed knee of the ACLR group would exhibit an altered response to sport-specific exercise.
Study Design:
Controlled laboratory study.
Methods:
A total of 13 soccer players cleared to return to sport after ACLR and 13 healthy control soccer players matched for age, physical activity level, limb dominance, and anthropometric characteristics were recruited. To assess the effects of a standardized soccer match simulation (Soccer Aerobic Field Test SAFT45) on AKL and AKS, an arthrometric evaluation was carried out bilaterally before and immediately after SAFT45. To conduct a comprehensive examination of the force-displacement curve, the absolute and side-to-side difference (SSD) values of both AKL and AKS were extracted at 67, 134, and 200 N.
Results:
The ACLR and control groups showed similar AKL and AKS at baseline (P > .05). In response to SAFT45, laxity increased bilaterally at all force levels by 14% to 17% only in the control group (P < .025). Similarly, AKS at 134 and 200 N decreased in response to SAFT45 only in the control group (10.5% and 20.5%, respectively; P < .025). After SAFT45, the ACLR group had 1.9 and 2.5 times higher SSDs of AKS at 67 and 134 N compared with the control group, respectively (P < .025), as well as a 1.9 times higher SSD of AKS at 134 N compared with baseline (P = .014).
Conclusion:
Soccer players at the time of return to sport after ACLR showed an altered mechanical response to a sport-specific match simulation consisting of bilaterally unchanged AKL and AKS.
Clinical Relevance:
Soccer players showing altered AKL and AKS in response to exercise after ACLR may not be ready to sustain their preinjury levels of sport, thus potentially increasing the risk of second ACL injuries.
The persistence of quadriceps weakness represents a major concern following anterior cruciate ligament reconstruction (ACLR). The underlying adaptations occurring in the activity of spinal ...motoneurons are still unexplored. This study examined the discharge patterns of large populations of motor units (MUs) in the vastus lateralis (VL) and vastus medialis muscles following ACLR. Nine ACLR individuals and 10 controls performed unilateral trapezoidal contractions of the knee extensor muscles at 35%, 50% and 70% of the maximal voluntary isometric force (MVIF). High‐density surface electromyography (HDsEMG) was used to record the myoelectrical activity of the vasti muscles in both limbs. HDsEMG signals were decomposed with a convolutive blind source separation method and MU properties were extracted and compared between sides and groups. The ACLR group showed a lower MVIF on the reconstructed side compared to the contralateral side (28.1%; P < 0.001). This force deficit was accompanied by reduced MU discharge rates (∼21%; P < 0.05), lower absolute MU recruitment and derecruitment thresholds (∼22% and ∼22.5%, respectively; P < 0.05) and lower input–output gain of motoneurons (27.3%; P = 0.009). Deficits in MU discharge rates of the VL and in absolute recruitment and derecruitment thresholds of both vasti MUs were associated with deficits in MVIF (P < 0.05). A strong between‐side correlation was found for MU discharge rates of the VL of ACLR individuals (P < 0.01). There were no significant between‐group differences (P > 0.05). These results indicate that mid‐ to long‐term strength deficits following ACLR may be attributable to a reduced neural drive to vasti muscles, with potential changes in excitatory and inhibitory synaptic inputs.
Key points
Impaired expression and control of knee extension forces is common after anterior cruciate ligament reconstruction and is related to high risk of a second injury.
To provide novel insights into the neural basis of this impairment, the discharge patterns of motor units in the vastus lateralis and vastus medialis were investigated during voluntary force contractions.
There was lower knee extensor strength on the reconstructed side with respect to the contralateral side, which was explained by deficits in motor unit discharge rate and an altered motoneuronal input–output gain. Insufficient excitatory inputs to motoneurons and increased inhibitory afferent signals potentially contributed to these alterations.
These results further our understanding of the neural underpinnings of quadriceps weakness following anterior cruciate ligament reconstruction and can help to develop effective rehabilitation protocols to regain muscle strength and reduce the risk of a second injury.
figure legend Changes in activity of motor units underlying deficits in knee extension strength following anterior cruciate ligament reconstruction (ACLR). The behaviour of motor units was assessed by decomposing high‐density surface electromyographic signals that were recorded from the vasti muscles of ACLR and control soccer players during submaximal trapezoidal contractions. The lower knee extension strength of the reconstructed side with respect to the contralateral side was explained by deficits in motor unit recruitment and derecruitment thresholds, by deficits in motor unit discharge rate of the vastus lateralis and by an altered input–output gain of motoneurons. Synaptic alterations and changes at muscle unit level potentially contributed to changes in motor unit activity. Correction made on 15 November 2021, after first online publication: The two preceding sentences have been updated to provide greater clarity.
There is a growing interest in decomposing high-density surface electromyography (HDsEMG) into motor unit spike trains to improve knowledge on the neural control of muscle contraction. However, the ...reliability of decomposition approaches is sometimes questioned, especially because they require manual editing of the outputs. We aimed to assess the inter-operator reliability of the identification of motor unit spike trains. Eight operators with varying experience in HDsEMG decomposition were provided with the same data extracted using the convolutive kernel compensation method. They were asked to manually edit them following established procedures. Data included signals from three lower leg muscles and different submaximal intensities. After manual analysis, 126 ± 5 motor units were retained (range across operators: 119–134). A total of 3380 rate of agreement values were calculated (28 pairwise comparisons × 11 contractions/muscles × 4–28 motor units). The median rate of agreement value was 99.6%. Inter-operator reliability was excellent for both mean discharge rate and time at recruitment (intraclass correlation coefficient > 0.99). These results show that when provided with the same decomposed data and the same basic instructions, operators converge toward almost identical results. Our data have been made available so that they can be used for training new operators.
The application of neuromuscular electrical stimulation superimposed on voluntary muscle contractions (NMES+) has demonstrated a considerable potential to enhance or restore muscle function in both ...healthy and individuals with neurological or orthopedic disorders. Improvements in muscle strength and power have been commonly associated with specific neural adaptations. In this study, we investigated changes in the discharge characteristics of the tibialis anterior motor units, following three acute exercises consisting of NMES+, passive NMES and voluntary isometric contractions alone.
Seventeen young participants participated in the study. High-density surface electromyography was used to record myoelectric activity in the tibialis anterior muscle during trapezoidal force trajectories involving isometric contractions of ankle dorsi flexors with target forces set at 35, 50% and 70% of maximal voluntary isometric contraction (MVIC). From decomposition of the electromyographic signal, motor unit discharge rate, recruitment and derecruitment thresholds were extracted and the input-output gain of the motoneuron pool was estimated.
Global discharge rate increased following the isometric condition compared to baseline at 35% MVIC while it increased after all experimental conditions at 50% MVIC target force. Interestingly, at 70% MVIC target force, only NMES + led to greater discharge rate compared to baseline. Recruitment threshold decreased after the isometric condition, although only at 50% MVIC. Input-output gain of the motoneurons of the tibialis anterior muscle was unaltered after the experimental conditions.
These results indicated that acute exercise involving NMES + induces an increase in motor unit discharge rate, particularly when higher forces are required. This reflects an enhanced neural drive to the muscle and might be strongly related to the distinctive motor fiber recruitment characterizing NMES+.
This study aimed at: (1) Reporting COVID‐19 symptoms and duration in professional football players; (2) comparing players’ pulmonary function before and after COVID‐19; (3) comparing players’ ...metabolic power (Pmet) before and after COVID‐19. Thirteen male players (Age: 23.9 ± 4.0 years, V̇O2peak: 49.7 ± 4.0 mL/kg/min) underwent a medical screening and performed a running incremental step test and a spirometry test after COVID‐19. Spirometric data were compared with the ones collected at the beginning of the same season. Players’ mean Pmet of the 10 matches played before COVID‐19 was compared with mean Pmet of the 10 matches played after COVID‐19. Players completed a questionnaire on COVID‐19 symptoms and duration 6 months following the disease. COVID‐19 positivity lasted on average 15 ± 5 days. “General fatigue” and “muscle fatigue” symptoms were reported by all players during COVID‐19 and persisted for 77% (general fatigue) and 54% (muscle fatigue) of the players for 37 ± 28 and 38 ± 29 days after the disease, respectively. No significant changes in spirometric measurements were found after COVID‐19, even though some impairments at the individual level were observed. Conversely, a linear mixed‐effects model analysis showed a significant reduction of Pmet (−4.1 ± 3.5%) following COVID‐19 (t = −2.686, p < 0.05). “General fatigue” and “muscle fatigue” symptoms may persist for several weeks following COVID‐19 in professional football players and should be considered for a safer return to sport. Players’ capacity to compete at high intensities might be compromised after COVID‐19.
“General fatigue” and “muscle fatigue” symptoms were the most common long‐term COVID‐19 symptoms reported. Players’ capability to exercise at high intensity was compromised following COVID‐19. Players’ pulmonary function was not affected by COVID‐19, even though some impairments were observed at the individual level.
Abstract Emerging questions in neuromuscular physiology revolve around whether males and females share similar neural control in diverse tasks across a broad range of intensities. In order to explore ...these features, high‐density electromyography was used to record the myoelectrical activity of biceps brachii during trapezoidal isometric contractions at 35% and 70% of maximal voluntary force (MVF) on 11 male and 13 female participants. Identified motor units were then classified as lower‐threshold (recruited at ≤30%MVF) and higher‐threshold (recruited at >30%MVF). The discharge rate, interspike interval variability, recruitment and derecruitment thresholds, and estimates of neural drive to motor neurons were assessed. Female lower‐threshold motor units showed higher neural drive ( P < 0.001), accompanied by higher discharge rate at recruitment ( P = 0.006), plateau ( P = 0.001) and derecruitment ( P = 0.001). On the other hand, male higher‐threshold motor units showed greater neural drive ( P = 0.04), accompanied by higher discharge rate at recruitment ( P = 0.005), plateau ( P = 0.04) and derecruitment ( P = 0.01). Motor unit discharge rate normalised by the recruitment threshold was significantly higher in female lower‐threshold motor units ( P < 0.001), while no differences were observed in higher‐threshold motor units. Recruitment and derecruitment thresholds are higher in males across all intensities ( P < 0.01). However, males and females have similar activation and deactivation strategies, as evidenced by similar recruitment‐to‐derecruitment ratios ( P > 0.05). This study encompasses a broad intensity range to analyse motor unit sex‐related differences, highlighting higher neural drive and discharge rates in female lower‐threshold motor units, elevated recruitment and derecruitment thresholds in males, and convergences in activation and deactivation strategies. Highlights What is the central question of the study? Do male and female motor units behave similarly in low‐ and high‐intensity contractions? What is the main finding and its importance? Female motor units show higher discharge rates in low‐intensity tasks and lower discharge rates in high‐intensity tasks, with no differences in recruitment behaviour. A broader inter‐spike interval variability was also observed in females. These findings underline that there are sex‐specific differences concern the firing strategies based on task intensity.
We proposed a new method for the identification and quantification of cross talk at the motor unit level. We show that surface EMG cross talk can lead to physiological misinterpretations of EMG ...signals such as overestimations in the muscle activity and intermuscular correlation. Cross talk had little influence on the EMG power spectrum, which indicates that conventional temporal filtering cannot minimize cross talk. Spatial filter (single and double differential) effectively reduces but not abolish cross talk.
Cross talk is an important source of error in interpreting surface electromyography (EMG) signals. Here, we aimed at characterizing cross talk for three groups of synergistic muscles by the identification of individual motor unit action potentials. Moreover, we explored whether spatial filtering (single and double differential) of the EMG signals influences the level of cross talk. Three experiments were conducted. Participants (total 25) performed isometric contractions at 10% of the maximal voluntary contraction (MVC) with digit muscles and knee extensors and at 30% MVC with plantar flexors. High-density surface EMG signals were recorded and decomposed into motor unit spike trains. For each muscle, we quantified the cross talk induced to neighboring muscles and the level of contamination by the nearby muscle activity. We also estimated the influence of cross talk on the EMG power spectrum and intermuscular correlation. Most motor units (80%) generated significant cross-talk signals to neighboring muscle EMG in monopolar recording mode, but this proportion decreased with spatial filtering (50% and 42% for single and double differential, respectively). Cross talk induced overestimations of intermuscular correlation and has a small effect on the EMG power spectrum, which indicates that cross talk is not reduced with high-pass temporal filtering. Conversely, spatial filtering reduced the cross-talk magnitude and the overestimations of intermuscular correlation, confirming to be an effective and simple technique to reduce cross talk. This paper presents a new method for the identification and quantification of cross talk at the motor unit level and clarifies the influence of cross talk on EMG interpretation for muscles with different anatomy.
NEW & NOTEWORTHY We proposed a new method for the identification and quantification of cross talk at the motor unit level. We show that surface EMG cross talk can lead to physiological misinterpretations of EMG signals such as overestimations in the muscle activity and intermuscular correlation. Cross talk had little influence on the EMG power spectrum, which indicates that conventional temporal filtering cannot minimize cross talk. Spatial filter (single and double differential) effectively reduces but not abolish cross talk.
Crosstalk is an important source of error in interpreting surface electromyography (EMG) signals. Here, we aimed at characterizing crosstalk for three groups of synergistic muscles by the ...identification of individual motor unit action potentials. Moreover, we explored whether spatial filtering (single and double differential) of the EMG signals influences the level of crosstalk. Three experiments were conducted. Participants (total twenty-five) performed isometric contractions at 10% of the maximal voluntary contraction (MVC) with digit muscles and knee extensors, and at 30% MVC with plantar flexors. High-density surface EMG signals were recorded and decomposed into motor unit spike trains. For each muscle, we quantified the crosstalk induced to neighboring muscles and the level of contamination by the nearby muscle activity. We also estimated the influence of crosstalk on the EMG power spectrum and intermuscular correlation. Most motor units (80%) generated significant crosstalk signals to neighboring muscle EMG in monopolar recording mode, but this proportion decreased with spatial filtering (50% and 42% for single and double differential, respectively). Crosstalk induced overestimations of intermuscular correlation and has a small effect on the EMG power spectrum, which indicates that crosstalk is not reduced with high-pass temporal filtering. Conversely, spatial filtering diminished the crosstalk magnitude and the overestimations of intermuscular correlation, confirming to be an effective and simple technique to reduce crosstalk. This paper presents a new method for the identification and quantification of crosstalk at the motor unit level and clarifies the influence of crosstalk on EMG interpretation for muscles with different anatomy.
The purpose of our study was to identify the low-dimensional latent components, defined hereafter as motor unit modes, underlying the discharge rates of the motor units in two knee extensors (vastus ...medialis and lateralis, eight men) and two hand muscles (first dorsal interossei and thenars, seven men and one woman) during submaximal isometric contractions. Factor analysis identified two independent motor unit modes that captured most of the covariance of the motor unit discharge rates. We found divergent distributions of the motor unit modes for the hand and vastii muscles. On average, 75% of the motor units for the thenar muscles and first dorsal interosseus were strongly correlated with the module for the muscle in which they resided. In contrast, we found a continuous distribution of motor unit modes spanning the two vastii muscle modules. The proportion of the muscle-specific motor unit modes was 60% for vastus medialis and 45% for vastus lateralis. The other motor units were either correlated with both muscle modules (shared inputs) or belonged to the module for the other muscle (15% for vastus lateralis). Moreover, coherence of the discharge rates between motor unit pools was explained by the presence of shared synaptic inputs. In simulations with 480 integrate-and-fire neurons, we demonstrate that factor analysis identifies the motor unit modes with high levels of accuracy. Our results indicate that correlated discharge rates of motor units that comprise motor unit modes arise from at least two independent sources of common input among the motor neurons innervating synergistic muscles.
It has been suggested that the nervous system controls synergistic muscles by projecting common synaptic inputs to the engaged motor neurons. In our study, we reduced the dimensionality of the output produced by pools of synergistic motor neurons innervating the hand and thigh muscles during isometric contractions. We found two neural modules, each representing a different common input, that were each specific for one of the muscles. In the vastii muscles, we found a continuous distribution of motor unit modes spanning the two synergistic muscles. Some of the motor units from the homonymous vastii muscle were controlled by the dominant neural module of the other synergistic muscle. In contrast, we found two distinct neural modules for the hand muscles.
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