200-m Front Crawl Research Articles

Biomechanics, energetics and coordination during extreme swimming intensity: effect of performance level

ABSTRACTThe present study aimed to examine how high- and low-speed swimmers organise biomechanical, energetic and coordinative factors throughout extreme intensity swim. Sixteen swimmers (eight high- and eight low-speed) performed, in free condition, 100-m front crawl at maximal intensity and 25, 50 and 75-m bouts (at same pace as the previous 100-m), and 100-m maximal front crawl on the measuring active drag system (MAD-system). A 3D dual-media optoelectronic system was used to assess speed, stroke frequency, stroke length, propelling efficiency and index of coordination (IdC), with power assessed by MAD-system and energy cost by quantifying oxygen consumption plus blood lactate. Both groups presented a similar profile in speed, power output, stroke frequency, stroke length, propelling efficiency and energy cost along the effort, while a distinct coordination profile was observed (F(3, 42) = 3.59, P = 0.04). Speed, power, stroke frequency and propelling efficiency (not significant, only a tendency) were higher in high-speed swimmers, while stroke length and energy cost were similar between groups. Performing at extreme intensity led better level swimmers to achieve superior speed due to higher power and propelling efficiency, with consequent ability to swim at higher stroke frequencies. This imposes specific constraints, resulting in a distinct IdC magnitude and profile between groups.

The Relationship Between Propulsive Force in Tethered Swimming and 200-m Front Crawl Performance

Santos, KB, Bento, PCB, Pereira, G, and Rodacki, ALF. The relationship between propulsive force in tethered swimming and 200-m front crawl performance. J Strength Cond Res 30(9): 2500-2507, 2016-The aims of this study were to determine whether propulsive force (peak force, mean force, impulse, and rate of force development) and stroke rate change during 2 minutes of front crawl tethered swimming and to correlate them with the stroke rate and swimming velocity in 200-m front crawl swimming. Twenty-one swimmers (21.6 ± 4.8 years, 1.78 ± 0.06 m, 71.7 ± 8.1 kg), with 200-m front crawl swimming performance equivalent to 78% of the world record (140.4 ± 10.1 seconds), were assessed during 2 minutes of maximal front crawl tethered swimming (propulsive forces and stroke rate) and 200-m front crawl swimming (stroke rate and clean velocity). Propulsive forces decreased between the beginning and the middle instants (∼20%; p ≤ 0.05) but remained stable between the middle and the end instants (∼6%; p > 0.05). The peak force was positively correlated with the clean velocity in the 200-m front crawl swimming (mean r = 0.61; p < 0.02). The stroke rates of the tethered swimming and 200-m front crawl swimming were positively correlated (r = 45; p≤ 0.01) at the middle instant. Therefore, the propulsive force and stroke rate changed throughout the 2 minutes of tethered swimming, and the peak force is the best propulsive force variable tested that correlated with 200-m front crawl swimming performance.

The Effect of Intensity on 3-Dimensional Kinematics and Coordination in Front-Crawl Swimming.

Coaches are often challenged to optimize swimmers' technique at different training and competition intensities, but 3-dimensional (3D) analysis has not been conducted for a wide range of training zones. To analyze front-crawl 3D kinematics and interlimb coordination from low to severe swimming intensities. Ten male swimmers performed a 200-m front crawl at 7 incrementally increasing paces until exhaustion (0.05-m/s increments and 30-s intervals), with images from 2 cycles in each step (at the 25- and 175-m laps) being recorded by 2 surface and 4 underwater video cameras. Metabolic anaerobic threshold (AnT) was also assessed using the lactate-concentration-velocity curve-modeling method. Stroke frequency increased, stroke length decreased, hand and foot speed increased, and the index of interlimb coordination increased (within a catch-up mode) from low to severe intensities (P ≤ .05) and within the 200-m steps performed above the AnT (at or closer to the 4th step; P ≤ .05). Concurrently, intracyclic velocity variations and propelling efficiency remained similar between and within swimming intensities (P > .05). Swimming intensity has a significant impact on swimmers' segmental kinematics and interlimb coordination, with modifications being more evident after the point when AnT is reached. As competitive swimming events are conducted at high intensities (in which anaerobic metabolism becomes more prevalent), coaches should implement specific training series that lead swimmers to adapt their technique to the task constraints that exist in nonhomeostatic race conditions.

Hypoventilation Training at Supramaximal Intensity Improves Swimming Performance.

This study aimed to determine whether hypoventilation training at supramaximal intensity could improve swimming performance more than the same training conducted under normal breathing conditions. For a 5-wk period, 16 triathletes (12 men and 4 women) were asked to include one supramaximal set of 12 to 20 × 25-m front crawl swimming twice a week into their usual swimming session, performed either with hypoventilation at low lung volume (VHL group) or with normal breathing (CONT group). Before (Pre) and after (Post) training, all triathletes performed all-out front crawl trials for 100, 200, and 400 m. Time performance was significantly improved in VHL in all trials (100 m: -3.7 ± 3.7 s [-4.4% ± 4.0%]; 200 m: -6.9 ± 5.0 s [-3.6% ± 2.3%]; 400 m: -13.6 ± 6.1 s [-3.5% ± 1.5%]) but did not change in CONT. In VHL, the maximal lactate concentration (+2.35 ± 1.3 mmol·L on average) and the rate of lactate accumulation in blood (+41.7% ± 39.4%) were higher at Post than at Pre in the three trials, whereas they remained unchanged in CONT. Arterial oxygen saturation, heart rate, breathing frequency, and stroke length were not altered in both groups at the end of the training period. On the other hand, stroke rate was higher at Post compared with Pre in VHL but not different in CONT. The measurements of gas exchange for the 400-m trial revealed no change in peak oxygen consumption as well as in any pulmonary variable in both groups. This study demonstrated that VHL training, when performed at supramaximal intensity, represents an effective method for improving swimming performance, partly through an increase in the anaerobic glycolysis activity.

Swimming Training Assessment: The Critical Velocity and the 400-m Test for Age-Group Swimmers.

To verify the metabolic responses of oxygen consumption (V[Combining Dot Above]O2), heart rate (HR), blood lactate concentrations [La], and rate of perceived exertion (RPE) when swimming at an intensity corresponding to the critical velocity (CV) assessed by a 4-parameter model (CV4par), and to check the reliability when using only a single 400-m maximal front crawl bout (T400) for CV4par assessment in age-group swimmers. Ten age-group swimmers (14-16 years old) performed 50-, 100-, 200-, 400- (T400), 800-, and 1,500-m maximal front crawl bouts to calculate CV4par. V[Combining Dot Above]O2, HR, [La], and RPE were measured immediately after bouts. Swimmers then performed 3 × 10-minute front crawl (45 seconds rest) at CV4par. V[Combining Dot Above]O2, HR, [La], and RPE were measured after 10 minutes of rest (Rest), warm-up (Pre), each 10-minute repetition, and at the end of the test (Post). CV4par was 1.33 ± 0.08 m·s. V[Combining Dot Above]O2, HR, [La], and RPE were similar between first 10-minute and Post time points in the 3 × 10-minute protocol. CV4par was equivalent to 92 ± 2% of the mean swimming speed of T400 (v400) for these swimmers. CV4par calculated through a single T400 (92%v400) showed excellent agreement (r = 0.30; 95% CI: -0.04 to 0.05 m·s, p = 0.39), low coefficient of variation (2%), and root mean square error of 0.02 ± 0.01 m·s when plotted against CV4par assessed through a 4-parameter model. These results generated the equation CV4par = 0.92 × v400. A single T400 can be used reliably to estimate the CV4par typically derived with 6 efforts in age-group swimmers.

Effects of hypercapnic-hypoxic training on respiratory muscle strength and front crawl stroke performance among elite swimmers

The aim of this resent study was to determine the effects of an 8-week hypercapnic-hypoxic (H-H or apnea) training program on respiratory muscles strength and 100 meter crawl swimming performance. The study was conducted on a sample of 26 Croatian elite swimmers (experimental group [EG] n=12, control group [CG] n=14). Both groups were subjected to the same swimming training programs and training sessions on a treadmill. The experimental group was additionally subjected to hypercapnic-hypoxic training program with increased muscular activity. Date on the following outcome variables were collected: the strength of respiratory muscles (maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP)), 100m front crawl swimming time (R100m) and breathing frequency during the same test (BF 100m ). A series of two way repeated measures ANOVAs has shown significant interactions between group (EG and CG) and the repeated-measure factor (pre- and post-test) (MIP: p = 0.006, MEP: p < 0.001, R100m, p < 0.001, FB100m, p < 0.001), all showing greater efficacy of the experimental program. It seems that the hypercapnic-hypoxic training program may provide substantial benefits for elite swimmers, in addition to their standard training sessions.

VO2 Kinetics in All-out Arm Stroke, Leg Kick and Whole Stroke Front Crawl 100-m Swimming.

The VO2 response to extreme-intensity exercise and its relationship with sports performance are largely unexplored. This study investigated the pulmonary VO2 kinetics during all-out 100-m front crawl whole stroke swimming (S), arm stroke (A) and leg kick (L). 26 male and 10 female competitive swimmers performed an all-out S trial followed by A and L of equal duration in random order. Breath-by-breath VO2 was measured using a snorkel attached to a portable gas analyzer. Mean (±SD) primary component parameters and peak blood lactate (Lapeak) during S, A, and L were, respectively: time delay (s), 14.2 ± 4.7, 14.3 ± 4.5, 15.6 ± 5.1; amplitude (ml·kg(-1)·min(-1)), 46.8 ± 6.1, 37.3 ± 6.9, 41.0 ± 4.7; time constant (τ, s): 9.2 ± 3.2, 12.4 ± 4.7, 10.1 ± 3.2; Lapeak (mmol·l(-1)), 6.8 ± 3.1, 6.3 ± 2.5, 7.9 ± 2.8. During A and L respectively, 80% and 87% of amplitude in S was reached, whereas A+L were 68% greater than in S. 100-m performance was associated to shorter cardiodynamic phase and greater VO2 amplitude and Lapeak (accounting up to 61% of performance variance), but not to τ. We conclude that (i) VO2 gain was proportional to exercise intensity and muscle mass involved, (ii) kicking is metabolically less efficient, and (iii) the main limiting factor of peak VO2 appears to be O2 delivery and not muscle extraction.

Front Crawl Sprint Performance: A Cluster Analysis of Biomechanics, Energetics, Coordinative, and Anthropometric Determinants in Young Swimmers.

The aim of this study was to evaluate the determinants of front crawl sprint performance of young swimmers using a cluster analysis. 103 swimmers, aged 11- to 13-years old, performed 25-m front crawl swimming at 50-m pace, recorded by two underwater cameras. Swimmers analysis included biomechanics, energetics, coordinative, and anthropometric characteristics. The organization of subjects in meaningful clusters, originated three groups (1.52 ± 0.16, 1.47 ± 0.17 and 1.40 ± 0.15 m/s, for Clusters 1, 2 and 3, respectively) with differences in velocity between Cluster 1 and 2 compared with Cluster 3 (p = .003). Anthropometric variables were the most determinants for clusters solution. Stroke length and stroke index were also considered relevant. In addition, differences between Cluster 1 and the others were also found for critical velocity, stroke rate and intracycle velocity variation (p < .05). It can be concluded that anthropometrics, technique and energetics (swimming efficiency) are determinant domains to young swimmers sprint performance.

Effect of swim speed on leg-to-arm coordination in unilateral arm amputee front crawl swimmers

The aim of this study was to examine the effect of swimming speed on leg-to-arm coordination in competitive unilateral arm amputee front crawl swimmers. Thirteen well-trained swimmers were videotaped underwater during three 25-m front crawl trials (400 m, 100 m and 50 m pace). The number, duration and timing of leg kicks in relation to arm stroke phases were identified by video analysis. Within the group, a six-beat kick was predominantly used (n = 10) although some swimmers used a four-beat (n = 2) or eight-beat kick (n = 1). Swimming speed had no significant effect on the relative duration of arm stroke and leg kick phases. At all speeds, arm stroke phases were significantly different (P < 0.05) between the affected and unaffected sides. In contrast, the kicking phases of both legs were not different. Consequently, leg-to-arm coordination was asymmetrical. The instant when the leg kicks ended on the affected side corresponded with particular positions of the unaffected arm, but not with the same positions of the affected arm. In conclusion, the ability to dissociate the movements of the arms from the legs demonstrates that, because of their physical impairment, unilateral arm amputee swimmers functionally adapt their motor organisation to swim front crawl.

Changes in mean swimming speed and efficiency in the front crawl at 2x25m track

Between the years 2009 and 2014 our team have collected data from more than 2000 measurements of four different swimming techniques. The swimmers that have participated in the research study include members of the Czech national swimming team and, competitive swimmers at national level. This article focuses on the measurements conducted during 50m front crawl swimming at a 25m pool, using data of 96 swimmers (52 men, 44 women). The aim of the study was to compare swimming efficiency and mean speed during two consecutive 25m segments. The average swimming efficiency and standard deviation during two successive 25m segments of the 50m distance were: 1) all measurements 0,979; s =0,013396; 0,972 s = 0,017616. 2) males 0,974; s = 0,018071; 0,962 s = 0,024814. 3) females 0,983; s = 0,008431; 0,979 s = 0,004562. The mean speed and standard deviation during two successive 25m segments of the 50m distance were: 1) all measurements 1,639; s = 0,129161; 1,556; s = 0,121743. 2) males 1,504; s = 0,042109; 1,443; s = 0,056049. 3) females 1,734; s = 0,060302; 1,637; s = 0,082359. Results show that swimming efficiency and mean speed decrease during the second 25m segment of the 50 m distance in both groups - men and women. The decrease in medium speed and efficiency between the first and the second section was higher in the male category.

An electromyographic evaluation of dual role breathing and upper body muscles in response to front crawl swimming.

The upper body trunk musculature is key in supporting breathing, propulsion, and stabilization during front crawl swimming. The aim of this study was to determine if the latissimus dorsi, pectoralis major, and serratus anterior contributed to the development of inspiratory muscle fatigue observed following front crawl swimming. Fourteen trained swimmers completed a 200-m front crawl swim at 90% of race pace. Maximal inspiratory and expiratory mouth pressures (PImax and PEmax) were assessed before (baseline) and after each swim, and electromyography was recorded from the three muscles. Post-swim PImax fell by 11% (P < 0.001, d = 0.57) and the median frequency (MDF: a measure of fatigue) of the latissimus dorsi, pectoralis major, and serratus anterior fell to 90% (P = 0.001, d = 1.57), 87% (P = 0.001, r = -0.60) and 89% (P = 0.018, d = 1.04) of baseline, respectively. The fall in serratus anterior MDF was correlated with breathing frequency (r = 0.675, P = 0.008) and stroke rate (r = 0.639, P = 0.014). The results suggest that the occurrence of inspiratory muscle fatigue was partly caused by fatigue of these muscles, and that breathing frequency and stroke rate particularly affect the serratus anterior.

Influência de diferentes padrões respiratórios na cinemática do nado crawl

The aim of this study was to investigate the effects of different breathing patterns on the kinematics of front crawl. Eleven college swimmers (10 men and one woman) performed seven 25-m front crawl trials at maximum intensity, with 2-min intervals between trials. One trial was performed in the breath-holding condition and six trials were performed using variable breathing frequencies (every two, four and six strokes) and sides (preferred and opposite side). Twenty-five meter time (T25) and average stroke rate (SR), stroke length (SL) and swimming velocity (SV) were obtained by video analysis and compared between conditions. The results showed that breathing side had no effect on T25, SR, SL, or SV. The breathing frequency was unable to change SV, but T25 and SR were higher for the two-stroke breathing pattern. Stroke length was greater when the swimmer breathed every two strokes compared to six strokes. The breath-holding condition produced lower T25 values and higher SV and SR compared to the other breathing patterns. In conclusion, the breathing side does not seem to interfere with the kinematic variables and breath-holding resulted in shorter swim times.

The influence of the hand’s acceleration and the relative contribution of drag and lift forces in front crawl swimming

The aim of this study was to assess the effect of the hand’s acceleration on the propulsive forces and the relative contribution of the drag and lift on their resultant force in the separate phases of the front crawl underwater arm stroke. Ten female swimmers swam one trial of all-out 25-m front crawl. The underwater motion of each swimmer’s right hand was recorded using four camcorders and four periscope systems. Anatomical landmarks were digitised, and the propulsive forces generated by the swimmer’s hand were estimated from the kinematic data in conjunction with hydrodynamic coefficients. When the hand’s acceleration was taken into account, the magnitude of the propulsive forces was greater, with the exception of the mean drag force during the final part of the underwater arm stroke. The mean drag force was greater than the mean lift force in the middle part, while the mean lift force was greater than the mean drag force in the final part of the underwater arm stroke. Thus, swimmers should accelerate their hands from the beginning of their backward motion, press the water with large pitch angles during the middle part and sweep with small pitch angles during the final part of their underwater arm stroke.

Selected anthropometrical, muscular architecture, and biomechanical variables as predictors of 50-m performance of front crawl swimming in young male swimmers

Summary Background Swimming performance may be influenced by anthropometric, physiology and biomechanical factors. Purpose The purpose of this study was to examine the influence of different parameter on 50-m front crawl swimming performance in young male swimmers. Methods Selected anthropometrical (body height, body mass, body mass index, arm span, shoulders width, thigh, leg, and upper arm lengths), muscle architectural (muscle thickness (MT), pennation angle (PA), and fascicle length (FL)), and biomechanical (stroke rate (SR), stroke length (SL), and stroke index (SI)) parameters have been measured on 50-m front crawl swimming performance in young male swimmers 13.86 ± 0.86 years of age (n = 23). Results Stepwise regression analysis revealed that biomechanical parameters (84%) characterized best 50-m front crawl swimming performance, followed by muscular architecture (52%) and anthropometrical (22%) parameters. Also, SI, SL, and MT of Vastus Lateralis (VL) explained 89% of 50-m front crawl swimming performance variability in these young male swimmers. Conclusion These results suggest that 50-m front crawl swimming performance can be generally explained by biomechanical and muscular architecture parameters than anthropometrical factors in young male swimmers.

Inter-Limb Coordinative Structure in a 200 m Front Crawl Event

The Index of Coordination has been currently used as a tool that measures the inter-arm lag time between propulsive phases in front crawl swimming. The aim of the present study was to assess the inter-arm coordinative structure during a 200m front crawl maximal effort, as well as to understand its interplay with the stroking parameters. One complete arm stroke cycle, without breathing, was digitized and analysed the relative duration of the different phases and identified the model of arm coordination by using the Index of Coordination in each length of 50m of the 200m front crawl. A one way repeated measures ANOVA, with Tukey post-hoc test was used to compare all the variables, and Pearson correlation coefficients were computed between Index of Coordination and the biomechanical (velocity, stroke rate, stroke length) and coordinative parameters (p<0.05). Results showed an increase in the relative duration of the propulsive phases, which induced an increase in the Index of Coordination as fatigue developed in relation to a decrease in velocity, stroke rate and stroke length. Additionally, Index of Coordination was inversely related with entry/catch (r=-0.78) and the non-propulsive phases (r=-0.97), and positive related with the pull (r=0.54), push (0.78) and propulsive phases (r=0.97). The results highlighted that, in fatigue stages, swimmers tend to increase their propulsive continuity, corresponding to a reduction of the non-propulsive lag time between the two arms' propulsive actions.

Effect of aerobic training on inter-arm coordination in highly trained swimmers

The effect of three months of aerobic training on spatio-temporal and coordination parameters was examined during a swim trial at maximal aerobic speed. Nine male swimmers swam a 400-m front crawl at maximal speed twice: in trial 1, after summer break, and trial 2, after three months of aerobic training. Video analysis determined the stroke (swimming speed, stroke length, and stroke rate) and coordination (Index of Coordination and propulsive phase duration) parameters for every 50-m segment. All swimmers significantly increased their swimming speed after training. For all swimmers except one, stroke length increased and stroke rate remained constant, whereas the Index of Coordination and the propulsive phase duration decreased (p<.05). This study suggests that aerobic training developed a greater force impulse in the swimmers during the propulsive phases, which allowed them to take advantage of longer non-propulsive phases. In this case, catch-up coordination, if associated with greater stroke length, can be an efficient coordination mode that reflects optimal drag/propulsion adaptation. This finding thus provides new insight into swimmers’ adaptations to the middle-distance event.

Characterization of speed fluctuation and drag force in young swimmers: A gender comparison

The aim of this study was to compare the speed fluctuation and the drag force in young swimmers between genders. Twenty-three young pubertal swimmers (12 boys and 11 girls) volunteered as subjects. Speed fluctuation was measured using a kinematical mechanical method (i.e., speedo-meter) during a maximal 25-m front crawl bout. Active drag, active drag coefficient and power needed to overcome drag were measured with the velocity perturbation method for another two maximal 25m front crawl bouts with and without the perturbation device. Passive drag and the passive drag coefficient were estimated using the gliding decay velocity method after a maximal push-off from the wall while being fully immersed. The technique drag index was also assessed as a ratio between active and passive drag. Boys presented meaningfully higher speed fluctuation, active drag, power needed to overcome drag and technique drag index than the girls. There were no significant gender differences for active drag coefficient, passive drag and passive drag coefficient. There were positive and moderate-strong associations between active drag and speed fluctuation when controlling the effects of swim velocity. So, increasing speed fluctuation leads to higher drag force values and those are even higher for boys than for girls.

Comparison Between Aerobic Power Parameters at Different Time- Averaging Intervals in Swimming: An Update

Sousa et al. (Open Sports Sci J, 3: 22 – 24, 2010) showed that different time averaging intervals lead to distinct VO2 values in a maximal 200m front crawl effort, evidencing higher VO2 values for breath-by-breath sampling, and differences between this latter data acquisition and all the other less frequent time intervals studied (5, 10, 15 and 20 s). These are interesting outputs in the field of exercise physiology applied to swimming once: (1) VO2 assessment is conducted in a swimming pool with a portable gas analyser which allowed breath-by-breath measurements, and not in a swimming flume with a Douglas bag technique or mixing chamber analyser, as traditionally occurs, and (2) the comparison between different time-averaging intervals used to remove breath-by-breath fluctuations during exercise periods has remained neglected, in sport in general and swimming in particular. Therefore, in the present study, we investigate the influence that different time averaging intervals have in aerobic power related parameters (VO2peak and VO2max). Ten subjects performed 200m front crawl effort at supra-maximal intensities (all-out test) and other ten subjects performed 200m front crawl effort at maximal aerobic intensities (100% of VO2max).The intensity at which the 200m front crawl was performed (supra-maximal and maximal intensities) had a significant effect on VO2peak and VO2max values obtained for each averaging intervals studied.

Phase-dependence of elbow muscle coactivation in front crawl swimming

Propulsion in swimming is achieved by complex sculling movements with elbow quasi-fixed on the antero-posterior axis to transmit forces from the hand and the forearm to the body. The purpose of this study was to investigate how elbow muscle coactivation was influenced by the front crawl stroke phases. Ten international level male swimmers performed a 200-m front crawl race-pace bout. Sagittal views were digitized frame by frame to determine the stroke phases (aquatic elbow flexion and extension, aerial elbow flexion and extension). Surface electromyograms (EMG) of the right biceps brachii and triceps brachii were recorded and processed using the integrated EMG to calculate a coactivation index (CI) for each phase. A significant effect of the phases on the CI was revealed with highest levels of coactivation during the aquatic elbow flexion and the aerial elbow extension. Swimmers stabilize the elbow joint to overcome drag during the aquatic phase, and act as a brake at the end of the recovery to replace the arm for the next stroke. The CI can provide insight into the magnitude of mechanical constraints supported by a given joint, in particular during a complex movement.

Upper- and lower-limb muscular fatigue during the 200-m front crawl

The aim of this study was to investigate how upper- and lower-limb muscle fatigue evolves in a 200-m front crawl swimming race. Surface electromyography signals were collected from the flexor carpi radialis, biceps brachii, triceps brachii, pectoralis major, upper trapezius, tibialis anterior, biceps femoris, and rectus femoris muscles of 10 international-level swimmers; 4 underwater cameras were used for kinematic analysis. In addition, blood lactate was measured before and after the test using capillary blood samples. Swimming speed and stroke length decreased from the beginning to the end of the effort, whereas stroke frequency increased after an initial decrease to maintain speed. Concomitant with the decrease in speed, blood lactate increased to 11.12 (1.65) mmol·L(-1). The changes in stroke parameters were associated with an increase in integrated electromyography (20%-25%) and a decrease in spectral parameters (40%-60%) for all of the upper-limb muscles, indicating the reaching of submaximal fatigue. The fatigue process did not occur regularly during the 8 laps of the 200 m but was specific for each muscle and each subject. Lower-limb muscles did not present signals of fatigue, confirming their lower contribution to swimming propulsion. The test was conducted to individualize the training process to each muscle and each subject.