Isotonic resistance training has been widely applied in sport and exercise. Design of resistance training programs depends on numerous training variables, such as intensity, volume, time under tension (TUT), repetition duration (tempo), etc. It has been shown that manipulation of repetition duration affects TUT and training volume, which in turn could results in distinct morphological and contractile adaptative response of skeletal muscle. The aim of the research was three-fold: i) to compare two different tempos of resistance exercise, which differ in the duration of eccentric phase on hypertrophy, strength and contractile properties of the elbow flexors and knee extensors muscles, ii) to compare sex-specific adaptations in hypertrophy, strength and contractile properties of the elbow flexors and knee extensors muscles produced by training intervention, iii) to investigate potential cause-and-effect association between changes in size, strength and contractile properties of the tested muscles.

The subjects were assigned to two experimental groups, based on the duration of the eccentric muscle contraction in resistance training. All participants were fully informed about experimental procedures and potential risks and they signed written informed consent prior to participation in the study. The training intervention was performed twice per week for a 7-week period. Before training intervention, a 2-week familiarization period was conducted. Elbow flexor and knee extensors muscle dimensions and strength were assessed 2 days before and after training intervention, while contractile properties were evaluated 5 days before and after experiment. The number of repetitions, training volume (number of repetitions x number of series), and TUT were recorded after first session of training intervention for both groups.

Muscle strength was assessed by one-repetition maximum test (1RM) for two exercises: elbow flexion on Scott's bench and parallel barbell squat. Each participant had 5 attempts to lift the maximum weight with pauses between trials of 3 minutes.

Muscles cross-section area (CSA) were evaluated by ultrasonic device (Siemens Antares, Erlangen, Germany), using the 2D ellipse diagnostic method with variable high frequency transducer (from 7 MHz to 13 MHz), for 5 muscles: elbow flexor (biceps brachii) and 4 knee extensors (rectus femoris , vastus intermedius, vastus medialis, vastus lateralis). The measurements were performed while the subjects seated with their elbows and knees extended and relaxed. The biceps brachii CSA was measured at two-thirds of the distance from the acromion to the antecubital crease. CSA of rectus femoris was measured at the height of proximal section of its distal third, above musculoskeletal joint. CSA of vastus intermedius and vastus medialis were measured at the height of distal part right above the patella. The visible part of vastus lateralis was measured at the distal third directly above the patella, under rectus femoris level.

The contractile properties of biceps brachii, rectus femoris and vastus lateralis muscles were evaluated by tensiomyography according to the manufacturer's instructions (TMG-Science for Body Evaluation, Ljubljana, Slovenia). The values of contraction time (Tc) and radial deformation (Dm) were taken for analysis. The biceps brachii testing was performed while subjects were in a sitting position with the dominant arm bent at 90°. During the assessment of the knee extensors subjects were lying on the back, where dominant leg was placed on the support, forming a knee angle of 120°. Subjects were asked to perform a voluntary contraction, in order to mark the point of placement of the TMG sensor by the palpation method. Two self-adhesive electrodes (Pals Platinum, model 895220 with multi-stick gel, AxelGaard Manufacturing Co. Ltd) were placed proximal and distal at 3 cm from the marked point, emitting an electrical impulse. A sensor (GK40, Panoptik d.o.o., Ljubljana, Slovenia) was placed between the electrodes to detect muscle changes initiated by electrical stimulation. The initial impulse was 25 milliamperes (mA) and it increased proportionally by 10 mA, until the maximum (muscle no longer responds to electrical stimulus).

Both experimental groups performed two exercises to target elbow flexor and knee extensor muscles: biceps curl on a Scott bench and parallel barbell squat, respectively. All sessions were performed at the same time (13-15 h), with a minimum 48 h rest between sessions on the same days during the week. For both groups, the intensity of load (1RM%) and number of sets were as follows: in the first three weeks the subjects exercises with ̴ 60% of 1RM in 3 sets, while during the next four weeks the load was set at ̴ 70% of 1RM in 4 sets. The difference in training protocols was in the duration of the eccentric phase, where one group performed training with a tempo of 1 seconds for eccentric and concentric phase, while for other group tempo was 4 seconds for eccentric and 1 seconds for concentric phase. All repetitions were performed until muscular failure. The pause between sets was 2 minutes.

Prior to analysis data were checked for normality and that the relevant assumptions for each test were met. Differences in the training volume and the TUT between the groups were determined by an independent t-test. One-way ANCOVA (using baseline values as covariates) was used to examine differences in changes of CSA, 1RM and TMG parameters between the groups. If ANCOVA showed statistical significance, the bonferroni post-hoc test was used for further estimation of differences between groups. Additionally, using sex as between-group factor and baseline values as covariates, ANCOVA was used to determine possible differences between males and females in changes of morphological, mechanical and contractile variables. Relative changes for each variable was derived from Pre to Post change in percentage, and for each participant. Pearson's correlation coefficient was used to examine the association between relative pre-to-post changes of CSA, 1RM and TMG for tested muscles. Data were reported as mean ± standard deviation. The level of significance was set at p \< 0.05. The program used for statistical analysis was Statistical Package for Social Sciences (IBM SPSS Statistics), version 20.0.