An anterior cruciate ligament (ACL) injury is one of the most common severe knee injuries in sports. The purpose of this review was to summarize the studies that directly quantified in vivo ACL loading as a function of knee motion in healthy individuals during static, slow-speed and athletic tasks.
A systematic review of the literature in multiple databases was conducted using different combinations of the terms “anterior cruciate ligament” or “ACL” and “in vivo” combined with “tensile” “strain” “stress” “force” and “loading”.
A total of 27 studies were identified. Eleven studies utilized strain sensors, while 16 studies applied imaging techniques. The numbers of studies for static or semi-static postures, slow-speed tasks, and athletic tasks were 12, 11 and 4, respectively.
There were strong and negative correlations between ACL elongation and knee flexion angles across different tasks. Peak ACL elongation mostly occurred when the knee flexion angle was minimal. Increased tibial anterior shear forces and patellar tendon forces would increase ACL loading when the knee is kept at a constantly small angle. In addition, a high patellar tendon force could be generated by preparatory quadriceps activation to load the ACL even when the lower extremity was not in contact with the ground during athletic tasks. Furthermore, while exercise modalities might affect peak ACL loading, the relationship between exercise intensities and ACL loading was complex and should not be assumed to be linear.
The purpose of this investigation was to examine the acute effects of low-load unilateral submaximal leg extension muscle actions with and without blood flow restriction (BFR) on maximal voluntary isometric contraction (MVIC) torque, electromyographic (EMG) amplitude (AMP) and EMG mean power frequency (MPF).
Twelve (mean ± SD; 23 ± 4 years) men performed 75 submaximal (1 × 30, 3 × 15) unilateral leg extension muscle actions with or without BFR. Before and immediately after the 75 reps, ultrasound measures and MVIC muscle actions were performed, and surface EMG was simultaneously assessed from the vastus lateralis. BFR was applied at 60% of total arterial occlusion. Separate repeated measures ANOVA’s, and Bonferroni corrected t-tests were performed to examine MVIC, EMG AMP, and EMG MPF. An alpha of P < 0.05 was considered statistically significant for all comparisons.
There was no significant (P = 0.077) interaction or main effect for Condition (P = 0.442) for EMG AMP. There was, however, an interaction (P = 0.014) for EMG MPF (posttest BFR decrease > posttest non-BFR decrease). There was a main effect for Time, collapsed across Condition, for MVIC torque (P < 0.001; (mean ± SD; 294.9 ± 20.1 N·m to 138.6 ± 12.2 N·m), but no main effect for Time for EMG AMP.
The findings of the present study indicated there were similar fatigue-induced decreases in MVIC torque and mode-specific decreases in EMG mean power frequency between the BFR and non-BFR conditions, but no changes in EMG amplitude. The decrease in MVIC torque may be due, in part, to the fatigue-induced buildup of metabolic byproducts that adversely affects excitation–contraction coupling and force output.
The purpose of this study was to examine the responses of electromyographic (EMG) and mechanomyographic (MMG) amplitude across the torque spectrum in pre- and post-pubescent males and females.
Forty pre-pubescent (mean ± 95% confidence interval, age = 9.79 ± 0.35 years, n = 10 males, n = 10 females) and post-pubescent (age = 17.23 ± 0.58 years, n = 10 males, n = 10 females) participants completed this study. Participants completed maximal voluntary isometric contractions (MVICs) of the forearm flexors and extensors, as well as isometric ramp muscle actions. EMG and MMG amplitude were quantified from the biceps brachii, brachialis, and brachioradialis during all muscle actions. EMG and MMG amplitude during the isometric ramp muscle actions were normalized to EMG and MMG amplitude from the MVICs.
The pre-pubertal group tended to have greater relative EMG amplitude across intensity (P < 0.050), while the post-pubertal group had a more pronounced increase in EMG amplitude at higher intensities. Similarly, the pre-pubertal group tended to have greater relative MMG amplitude across intensity (P ≤ 0.004) that plateaued earlier than the post-pubertal group (55% vs. 65%–75% of MVIC). Additionally, the pre-pubertal group had greater coactivation across intensity (P ≤ 0.001).
The greater relative EMG and MMG amplitude in the pre-pubertal group, in conjunction with the earlier plateau in MMG amplitude for the pre-pubertal group and greater coactivation, suggests less efficient muscle activation and motor unit recruitment strategies during pre-pubescence. Taken together, the findings of the present study suggest that growth-mediated changes in neuromuscular function lead to improvements in the efficiency of muscular activation and augmentations in motor unit recruitment strategies.
Special tactical units differ from other police departments, for having more physically demanding tasks and occupations. Therefore, the aim was to analyze: (i) the differences in anthropometrics, body composition, and physical performance variables between those officers with the highest and lowest lean mass (LM) and fat mass (FM); and (ii) the associations between body composition (i.e., FM and LM) and some selected performance variables.
Thirty-six special operations officers (n = 36, age: 35.97 ± 5.50 years) volunteered to participate in this study. Participants were assessed for anthropometrics and body composition through skin-fold measures. Additionally, fitness was evaluated using appropriate physical tests (i.e. 30-m sprints, vertical jump, strength and endurance). Afterwards, participants were divided according to their level of LM and FM into: high (LMhigh and FMhigh) and low (LMlow and FMlow).
Regarding strength and jump performance, LMhigh and FMlow obtained better estimated values in Squat (1 repetition maximum [SQ1RM]), and jump height (P < 0.05; ES = 0.62–1.29), although non-significant but small differences were observed for relative strength (P = 0.107; ES = 0.54). In terms of sprint and endurance, the results indicated that LMhigh and FMlow obtained significantly better performances across all measures (P < 0.05; ES > 0.89), except for endurance between FMhigh and FMlow (ES = 0.25–0.65). In addition, FM and LM were significantly associated with physical performance (P < 0.05; r > 0.383) in most of the variables of this study.
Higher LM and lower FM are determinant factors of physical performance in this population. Moreover, FM and LM seemed to be detrimental for physical performance as shown by the moderate to large correlations observed.