The difference between squat jump and countermovement jump in 770 male and female participants from different sports.

Traditionally, a larger difference between countermovement (CMJ) and squat jump (SJ) was seen as beneficial, as it reflects the ability to utilize the stretch-shortening cycle. However, strong arguments have been made that this might not always be the case, as larger differences between the jumps could also suggest higher muscle-tendon slack, or poor capability to take this slack up quickly. The purpose of this study was to explore SJ and CMJ, as well as the CMJ to SJ difference (CMJSJDiff) in 9 groups of young athletes. In total, 712 athletes from various disciplines (mean age range: 15.7-36.3 years) and 58 physical education students (mean age: 19.6 years) participated in the study. The major finding of this study was that the groups that showed better SJ and CMJ performance did not show the larger CMJSJDiff. For instance, SJ and CMJ heights were highest in short-distance runners and lowest in long-distance runners, while the largest and smallest CMJSJDiff was shown in physical education students and speed skaters, respectively. Male athletes had a higher CMJSJDiff than female athletes, but the difference was very small. While a larger CMJSJDiff has been traditionally associated viewed as positive, our results could indicate both superior ability to utilize the stretch-shortening cycle, as well as poor ability of rapid force development and excessive muscle slack. Further studies are needed to directly investigate the associations between CMJSJDiff and indicators of athletic performance.

Investigation of Inter-Limb Symmetry in Knee Extensors Using Different Strength Outcome Measures.

Muscle performance between contra-lateral knee extensors is most often assessed using maximal test for isometric/isokinetic torque evaluation. Recently, the rate of force development scaling factor (RFD-SF) has been used to evaluate neuromuscular capacity with a range of submaximal target peak torques, which could highlight other aspects of inter-limb (a)symmetry. The aim of our study was to investigate the differences, associations, and agreement between inter-limb symmetries of knee extensors using maximal torque (Tmax) rate of torque development (RTD), slope of the RFD-SF regression line (k), and theoretical peak of RTD (TPRTD). A total of 236 young, healthy athletes participated in the cross-sectional study. All participants performed unilateral knee extension (maximal voluntary contraction protocol and RFD-SF protocol) with both legs in the isometric knee dynamometer. Inter-limb symmetries were calculated for each outcome measure. Our results showed significant differences between all symmetry values (Tmax (91.7%), RTD (85.2%), k (94.2%), TPRTD (95.9%)). Significant strong correlations were found between symmetry values calculated from k and TPRTD (r = 0.88, p < 0.001), while weak correlation was found between Tmax and RTD (r = 0.17, p < 0.01. Fair agreement regarding leg dominance was found between Tmax and RTD values. Our results suggest that inter-limb (a)symmetries are metric- and task-specific.

Quantification of Inter-Limb Symmetries With Rate of Force Development and Relaxation Scaling Factor.

The inter-limb (a)symmetries have been most often assessed with the tests that quantify the maximal muscle capacity. However, the rapid force production and relaxation during submaximal tasks is equally important for successful sports performance. This can be evaluated with an established rate of force development and relaxation scaling factor (RFD-SF/RFR-SF). The aims of our study were (1) to assess the intra-session reliability of shortened RFD-SF/RFR-SF protocol and its absolute and symmetry outcome measures, (2) to compare the main absolute RFD-SF/RFR-SF outcome measures (slopes of RFD-SF and RFR-SF: kRTD-SF and kRFR-SF, theoretical peak RFD/RFR: TPRFD and TPRFR) across gender and sports groups, and (3) to compare inter-limb symmetries across gender and sports groups for main outcome measures (kRFD-SF, kRFR-SF, TPRFD, and TPRFR). A cross-sectional study was conducted on a group of young health participants (basketball and tennis players, and students): 30 in the reliability study and 248 in the comparison study. Our results showed good to excellent relative and excellent absolute reliability for the selected absolute and symmetry outcome measures (kRFD-SF, kRFR-SF, TPRFD, and TPRFR). We found significantly higher absolute values for kRFD-SF and TPRFD in males compared to females for the preferred (kRFD-SF: 9.1 ± 0.9 vs. 8.6 ± 0.9/s) and the non-preferred leg (kRFD-SF: 9.1 ± 0.9 vs. 8.5 ± 0.8/s), while there was no effect of sport. Significantly lower symmetry values for kRFR-SF (88.4 ± 8.6 vs. 90.4 ± 8.0%) and TPRFR (90.9 ± 6.8 vs. 92.5 ± 6.0%) were found in males compared to females. Moreover, tennis players had significantly higher symmetry values for kRFR-SF (91.1 ± 7.7%) and TPRFR (93.1 ± 6.0%) compared to basketball players (kRFR-SF: 88.4 ± 8.7% and TPRFR: 90.9 ± 6.7%) and students (kRFR-SF: 87.6 ± 8.7% and TPRFR: 90.5 ± 6.7%). Our results suggest that the reduced RFD-SF/RFR-SF protocol is a valuable and useful tool for inter-limb (a)symmetry evaluation. Differences in symmetry values in kRFR-SF and TPRFR (relaxation phase) were found between different sports groups. These may be explained by different mechanisms underlying the muscle contraction and relaxation. We suggest that muscle contraction and relaxation should be assessed for in-depth inter-limb symmetry investigation.

Advancements in the Protocol for Rate of Force Development/Relaxation Scaling Factor Evaluation.

Brief submaximal actions are important for wide range of functional movements. Until now, rate of force development and relaxation scaling factor (RFD-SF and RFR-SF) have been used for neuromuscular assessment using 100-120 isometric pulses which requires a high level of attention from the participant and may be influenced by physiological and/or psychological fatigue. All previous studies have been conducted on a smaller number of participants which calls into question the eligibility of some of the outcome measures reported to date. Our aims were: (1) to find the smallest number of rapid isometric force pulses at different force amplitudes is still valid and reliable for RFD-SF slope (k R F D -SF) and RFR-SF slope (k RFR-SF ) calculation, (2) to introduce a new outcome measure - theoretical peak of rate of force development/relaxation (TP RFD and TP RFR ) and (3) to investigate differences and associations between k RFD-SF and k RFR-SF . A cross-sectional study was conducted on a group of young healthy participants; 40 in the reliability study and 336 in the comparison/association study. We investigated the smallest number of rapid isometric pulses for knee extensors that still provides excellent reliability of the calculated k RFD-SF and k RFR-SF (ICC2,1 ≥ 0.95, CV < 5%). Our results showed excellent reliability of the reduced protocol when 36 pulses (nine for each of the four intensity ranges) were used for the calculations of k RFD-SF and k RFR-SF . We confirmed the negligibility of the y-intercepts and confirmed the reliability of the newly introduced TP RFD and TP RFR . Large negative associations were found between k RFD-SF and k RFR-SF (r = 0.502, p < 0.001), while comparison of the absolute values showed a significantly higher k RFD-SF (8.86 ± 1.0/s) compared to k RFR-SF (8.03 ± 1.3/s) (p < 0.001). The advantage of the reduced protocol (4 intensities × 9 pulses = 36 pulses) is the shorter assessment time and the reduction of possible influence of fatigue. In addition, the introduction of TP RFD and TP RFR as an outcome measure provides valuable information about the participant's maximal theoretical RFD/RFR capacity. This can be useful for the assessment of maximal capacity in people with various impairments or pain problems.