Abstract:

Introduction: Manipulating resistance training (RT) variables has been suggested as a means to optimize muscular adaptations. For instance, training volume and muscle activation are two variables that quantify the demands placed on the muscle within a session. However, there is a paucity of data on how these elements are influenced by varying joint angles within a training session.
Purpose: Therefore, the aim of this study was to examine how altering the shoulder angle (glenohumeral joint [GH]) affects volume load and muscle activation of the biceps brachii within a training session.
Methods: Eleven resistance trained individuals (5 males, 6 females, age: 21 years ± 1.47, height: 166.8cm ± 7.1, body mass: 66.6kg ± 10.4, RT experience: 4.7years ± 1.91) volunteered for this study. Subjects performed three familiarization sessions prior to the experimental sessions. Baseline 10 repetition-maximum (RM) elbow flexion values were recorded during the familiarization sessions in three different GH joint angles (-30°, 0°, 90°). This 10RM load was used for the first working set of each experimental session and the load was adjusted during subsequent sets. Forty-eight hours after the familiarization sessions, subjects underwent two experimental conditions with a one-week washout period between conditions, control (CON) and varying GH joint angle (VAR), in a randomized, cross-over design. Before any experimental session, surface electromyography (EMG) was applied to the mid-belly of the biceps brachii. In order to normalize the EMG data, a maximum voluntary isometric contraction (MVIC) was performed at each shoulder position before starting the experimental conditions working sets. During the MVIC, the root mean square (RMS) of the EMG signal was calculated in 500ms windows, and the highest value was used for normalization of the dynamic contractions. During the working sets, repetitions of each set were divided into quartiles and raw EMG signal was converted into RMS values for each quartile. The CON condition performed all 9 sets of elbow flexion with their GH joint at 0°(i.e., neutral position). The VAR condition performed 3 sets of elbow flexion in each position (-30°, 0°, 90°), in a randomized fashion. A paired t-test was used to compare volume load and muscle activation between conditions. The significance level was set at p< 0.05. RESULTS: For muscle activation, the overall session EMG amplitude was significantly higher (p=0.0001) in the VAR condition compared to CON (CI: 8.4% to 23.3%). Interestingly , regarding volume load, there were no differences between conditions (VAR 596 kg ± 170, CON 606 kg ± 175).
Conclusion: Our results suggest that despite similar volume load between conditions, varying joint angles increases muscle activation within a training session. Practical Application: Our data suggest that altering joint angles during exercise increases muscle activation in an acute fashion. However, these findings should be examined with caution since these acute responses may not be associated with long term adaptations. Therefore, future investigations are warranted to determine any chronic effects.