Determinant factors of efficiency when the Counter Movement Jump is performed in acute fatigue

This work analysed the changes that occur in kinetic-temporal parameters and the electromyographic activity of lower limb muscles during the performance of maximum Counter Movement Jumps (CMJ) done with and without muscular fatigue, to explain the changes in performance in both situations. The vertical strength of fifty jumps performed by ten male sportsmen before and after a fatigue protocol was registered. Records of surface electromyography were obtained for six lower limb muscles; in addition, activation level and intermuscular coordination were analysed. For analysis purposes, negative and positive mechanic working times were considered separately. In both conditions, the temporal course of strength distribution is more important for the performance than any other instantaneous parameter. The negative external working time was significantly lower in fatigue conditions. The electromyographic analysis showed an increase in the activation level of all muscles studied and significant changes in the activation sequence. The specific kinetic-temporal variables were not good predictors of jump height. When CMJ is performed without muscle fatigue, a high value of active state favours the positive work. During fatigue, there could be a partial compensation of the performance due to the increasing activity of the contractile elements, although the activation sequence undergoes significant changes. Thus, the changes in performance would be mainly associated with the decrease in the capacity to transmit power in the proximal-distal direction.


INTRODUCTION
Counter Movement Jump (CMJ) is among vertical jumps commonly used in evaluations 1 .The presence of eccentric phase for leg extenders considering the jump allows achieving greater height, which is associated with different aspects.Among them, stored elastic energy 2,3 , increased prior activation 4 and the contribution of the stretching reflection 5 stand out.
Several biomechanical approaches such as simulations 6 , electromyographic tests 7 and force platforms 8,9 have been used to identify factors associated with changes in performance during CMJ.All of these approaches resulted in a greater or less useful measure.However, the same vertical force curve results in the same performance, which does not occur with other variables 8 .
A recent study confirmed that the height in CMJ, considered the primary indicator of performance, is related to the maximum normalized force, confirming previous studies on Squat Jump and CMJ [10][11][12][13] .
It must be considered that records from force platforms have fundamental information for the study of performance on jumps and do not add data regarding the main causes that determine performance changes.This is the way that combined measures of variables arising from force platforms and electromyography activity (EMG) of muscles involved during the different phases of CMJ could help to better understand the factors that determine performance (height reached in a jump).
Although in real conditions jumps are performed with a degree of muscle fatigue, CMJ studies usually do not include this condition.
It is well established that acute fatigue determines changes in metabolic level 14,15 and that these changes may contribute to decrease capacity by changing the success-contraction coupling process 15 or the reduction of sensitivity in the stretching reflex 16 .Thus, it is possible that fatigue affects the whole jump dynamics during the boost phase.This difference may be associated with the fatigue condition, the degree of activity and the force capacity of contractile elements that may change significantly 15,16 .
The aim of this study was to identify the kinetic-temporal and electromyographic parameters that allow discussing the changes in CMJ performance when these are performed with fatigue.For this, the vertical force curve is analyzed during periods of electromechanical events corresponding to different time during the jump and electromyographic signals from six muscles of the lower limb of 10 athletes.It was observed that the time course of force is a parameter more important than any other in both conditions, the time of negative work decreases significantly in fatigue and changes occur both in the level and sequence of activation of muscle associated with fatigue.

Characteristics of the sample subjects
A population of ten male athletes (age 23.5 ± 3.0 years, body mass 72.3 ± 4.1 kg, height 1.75 ± 0.09 m) was studied.Inclusion criteria were individuals who train more than three times per week with a maximum of 30 km of jogging per week and rely on previous experience in vertical jumping test.In addition, exclusion criterion was not presenting lesions in the last year.Of selected individuals, six practiced amateur soccer and four middle-distance running (two professionals).An intentional non-probabilistic selection was used given the similarity of training among athletes in order to maintain sample homogeneity and to fulfill the proposed overall objective of this study.
All subjects signed the informed consent form containing clarifications approved by the Ethics Research Committee of the Faculty of Medicine, University of Uruguay (Process No. 071140-001764-09).

Experimental Protocol
Each subject was instructed to perform five maximum CMJ in each of two conditions fatigue (F) and no fatigue (NF), performing a total of 50 jumps at each condition.During the execution of jumps, the subjects were in the upright position and performed a counter-movement to reach an angle of 90° of knee flexion.The angular variation during jumps was controlled by kinematics, discarding those whose knee flexion varied over 5º in relation to the reference angle.
Fatigue condition was obtained by performing 1 minute of continuous jumping quantified by the drop in average power analyzed in periods of 15 seconds 17 .
Jumps were performed on a force platform (AMTI OR6-5) (Advanced Mechanical Technology Inc., Watertown, Massachusetts) while the electrical activity of the rectus femoris (RF), vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA), medial gastrocnemius (GM) and Soleus (SOL) muscles was synchronously recorded by surface electromyography EMG recordings were made with two Miotec® electromyography devices (Miotool 400) and Miograph® 2.0 software with acquisition rate of 1000 Hz and yield of 1600 per channel.Disposable surface electrodes (Kendall MEDI-TRACE-100 Ag / AgCl) are used in a bipolar configuration with a separation of 2 m from center to center.The electrodes were located in the muscles of the right leg according to criteria established in 18 .For the purposes of standardizing electromyographic recordings, recordings were performed during maximal voluntary contractions (MVC) for 5 s.

Data processing
Only the vertical component (Fy) of force curves obtained from the force platform was analyzed.The records were exported by MATLAB® and filtered through a third-order Butterworth filter with maximum frequency determined by the residues method of Winter 19 .
Jump height (Hmax) was calculated as: where g is the gravity acceleration (9.81 m / s 2 ) and V v is the vertical velocity of the subject's center of mass (CM), which was determined considering the time of flight (Tair) directly obtained from the F e curve 17 : The support time of the Fy curve was divided into two parts, the descending phase (T1) and the ascending phase (T2) of the CM at each jump.
Besides the value of the maximum vertical force (Fmax), impulses (integral of the closed area under the Fe-time curve) for T1 (Imp 1 ) and T2 (Imp 2 ) in each condition were calculated.
The EMG signals captured were filtered with a 3 rd order Butterworth filter between 10 and 500 Hz and were synchronized with the corresponding records of vertical force (Fy) considering an electromechanical delay of 0.012 s 5 .The records were analyzed in the time domain by calculating by the root mean square (RMS) values using a temporal window of 0.25s 7 .
To evaluate the coordination and the activation level of the RMS values, they were divided into time periods T1 and T2.
The level of activity of each muscle was quantified by calculating the integral of RMS.The values obtained during T1 and T2 for the different muscles were normalized by dividing them by the length of the interval and the maximum RMS value obtained during MVC, thus obtaining values of integral relative to T1 and T2.The effects that results in activation amplitude obtained in this study were comparable to those in literature 7,20 , and the values of each muscle varied for each subject and condition.
To study the relative differences in the activation sequence, we considered the moment in which the maximum RMS value normalized with respect to T1 and T2.This procedure was performed for each muscles analyzed in each condition.

Data Analysis
For each series of jumps, mean and standard deviation were calculated for all variables, evaluating the data fitting through the normal distribution by Shapiro-Wilk test and homogeneity was tested by the Levene test.

Pearson correlation test was performed between each kinetic-temporal variable and jump height in each condition.
The difference in T1, T2, Fmax, IMP1, Imp2 was quantified and the level of activation by "t" test for averages compared between values obtained in F and NF condition for each time interval.(p < 0.05).
For the analysis of activation sequence, the time of maximum activation was measured for each subject and the change between conditions by "t" test for comparison measures was verified (p < 0.05).
All statistical analyses were performed with the SPSS 13.0 software (Statistical Package for the Social Sciences, Sun Microsystems, USA).

RESULTS
The heights reached in NF (0.31 ± 0.06 m) and F (0.29 ± 0.06 m) were significantly different (p <0.05).Table 1 shows the results obtained in the force platform in each condition, while Tables 2 and 3 show the results of the electromyographic analysis.(*) indicate those cases in which significant correlation of maximum force (Fmax), time of descending phase (T1), time of ascending phase (T2), impulse during T1 (IMP1) and during T2 (Imp 2), jump height (* p < 0.05) was found.( †) indicates the presence of significant differences for the same variable between conditions.Biceps femoris (BF), rectus femoris (RF), vastus lateralis (VL), tibialis anterior (TA), medial gastrocnemius (GM) and soleus (SOL).Results of the "t" test for compared measures before and after fatigue in two time periods analyzed are included.(*) indicate significant difference (p <0.05).

DISCUSSION
The height achieved in CMJ without fatigue (0.31 ± 0.06 m) and with fatigue (0.29 ± 0.06 m) was comparable to that reported in other studies, in which subjects jumped using an equivalent technique 7 .
The record of maximum vertical force (Fmax) is the first result to be observed, as it was solely associated with the height achieved when the jumps are performed in the absence of fatigue.Whereas in real conditions vertical jumps are performed with a degree of muscle fatigue, this result reports that Fmax cannot be considered a good predictor of performance on CMJ.
Previous studies have associated the decreased performance in the condition with fatigue to changes in the capacity of different muscle groups 7 .These changes can assign the combined effect of the capacity of utilization of elastic energy and change in contractile activity 4,21 , and this information may vary for each muscle group considered 7 .
In general, our results showed that the kinetic-temporal variable correlated with jump height change in each condition (Table 1).The significant correlations of T2 with height in condition NF corroborate reports of a similar experimental study where eighteen variables defined from the record of vertical force were analyzed 8 .On the other hand, it was observed that in the fatigue condition, there are no correlations of time of each phase with jump height.The correlations of times found would indicate that without fatigue determines that the performance on CMJ occurs is a positive work phase and that with fatigue, this situation changes.
The results of t-test showed that there is a marked decrease in T1 (negative work time) when CMJ is carried out with fatigue.This is consistent with the fact that F max is not a determining factor in fatigue, in the sense that the maximum vertical force that the subject performs against the floor is observed just before the moment that this condition changes.
The values found correlating impulse with height in different periods in fatigue support the idea that in ballistic movements, performance is based on optimizing the impulse and not necessarily in any instantaneous value of force or power 8, i.e., in the form of the vertical force curve.This form of force curve during early negative work would be determined by the participation of contractile elements, which would be determined by both pre-activation as the participation of the stretching reflex, since both aspects influence the force produced during T1 16 .
An increase in the level of activation of all muscles studied was observed (Table 2).Considering these results, it is suggested that during CMJ on condition of fatigue, optimization of force-time product during positive work would be favored by the participation of contractile elements.
Studies conducted to verify the sequence of activation have shown significant changes fundamentally during T2.During this time, a significant delay in the time of maximum peak in RF, BF and GM SOL and a significant advance in the VL could be observed.Changes in bi-articular muscles (RF, BF and GM) may be fundamental as these are determinants of the transmission of power at the distal proximally direction 6 .Advances in peak VI added to changes in RF and BF could reflect an important imbalance in the level of control of the knee joint in relation to fatigue.However, these changes should be considered carefully, since the evaluation of coordination through a value such as activity peak can lead to erroneous interpretations.
The fact that the sample does not allow interference on the general population is among the main limitations of this study, although all statistical tests obtained low error probability.Moreover, this study performed an analysis of the electromyographic signals based on the two most commonly used parameters.These do not yet have a general consensus, by which a more complex analysis of signals may in future add new elements to the discussion.

CONCLUSION
The results of this study support the idea that the pattern of force distribution in time determines a good performance at CMJ in any condition.By increasing fatigue the contribution given by contractile elements during positive work also increases.On the other hand, the activation sequence is changed so that it is possible that changes in performance are mainly associated with a decrease in the ability to effectively transmit the power in the proximal-distal direction.

Table 1 .
Values (mean and standard deviation) of variables analyzed from records of vertical force in fatigue (F) and no fatigue (NF) conditions

Table 2 .
Results of the integral of RMS of muscles studied in fatigue (F) and no fatigue (NF) conditions over two periods of analysis.

Table 3 .
Results for the relative time of the peak activity of muscles analyzed before and after the fatigue protocol in two time periods analyzed.