Exercise Type, Elbow Angle Effects During the Push-up Plus
The present study examined the effect of PUP variant and elbow position on EMG activity of four scapular stabilizer muscles and vertical ground reaction force during concentric contraction. To our knowledge, this is the first study that looked at EMG activity of shoulder muscle stabilizer and vGRF at various elbow position during push-up plus. The current data demonstrated that traditional PUP was characterized with greater EMG muscle activity in all the muscles tested during the entire elbow range of motion compared to modified PUP. In addition, the traditional PUP resulted in larger vGRF compared to modified PUP.
The push-up plus exercise is considered to be the best exercise to activate the Serratus Anterior. It has been shown that SA EMG activity ranged from 69% to 120% maximum voluntary isometric contraction (MVIC) during some variants of push-up exercise. In the current study, the SA EMG activity ranged from 45% to 57% MVC, which is below the value reported in the literature. The difference between the EMG values may be due to the fact that the current study performed the MVIC in a different method. However, in a recent study by Sciascia et al., they reported similar SA EMG activity to the present study during the push-up plus exercise. Although, their results were from subjects with multi-directional instability and isolated anterior instability of the shoulder, they reported no significant differences between healthy and shoulder instability patients in SA EMG activity during the PUP exercise.
The present data also showed increased SA activity during elbow extension. During the modified PUP, there was only a 3% change in the entire concentric phase of the PUP. This shows that when performing the modified version of the PUP, the added motion of protracting the shoulder blade upon completion of the push-up repetition is not necessitated. The highest SA EMG activity during the concentric phase of the modified PUP happened at 55° of elbow extension, while in the traditional PUP, the highest SA EMG activity occurred at 20° of concentric elbow extension, which is during the plus phase of the exercise.
The UT and LT are considered to be two of the main scapular stabilizers. Both the UT and LT contribute to scapular upward rotation during humeral elevation. However, the UT also contributes to anterior tilting by drawing the clavicle medially and upwards, which could decrease the subacromial space and can increase the risk of subacromial impingement. The LT also assists in posterior tilting during humeral elevation. Thus, the goal in most shoulder impingement rehabilitation protocol is to decrease the activity of the UT compared to the LT during arm elevation. Consequently, the scapula will be in a more posteriorly tilted position, which increases the subacromial space. In the current study, both variants of PUP started with the UT (24% MVC) having an increased muscle activity compared to the LT (19% MVC). However, during the mid-range of the concentric phase of the PUP, there was a shift in activation pattern. The UT (20% MVC) decreased and the LT (24.8% MVC) increased its EMG activation starting at 70° of elbow extension up to the plus phase (i.e. 20° of elbow extension) of the traditional PUP exercise. These results are in accordance with previous studies that examined different variance of push-up plus exercise with the UT ranging from 20.5% - 25.2% MVC. For the LT, Park et al. examined the LT during wall push-up plus and wall slide device and reported a range of 11.6% - 16.1% MVC. The current study had a higher LT EMG activity. This difference can be attributed to the position that the push-up plus was implemented. The PUP exercise performed in Park et al. study was accomplished standing up against the wall while the current study had the subjects complete the PUP on the ground. The wall PUP is considered to be less demanding than the traditional on the ground PUP. The modified PUP did not change muscle activation patterns until the elbow was at 55° of elbow extension. The shift of muscle activity into a more LT than UT during the PUP is important to avoid impingement of soft tissues under the subacromial space.
The infraspinatus is one of the rotator cuff muscles. Its main function is to externally rotate the humerus. It also serves to assist in centralizing the humeral head into the glenoid during shoulder motion. In the current study, the infraspinatus was activated during both PUP variants. The traditional PUP had the greatest EMG activity (30% MVC) when the elbow was less than 50° of elbow flexion. This result is in accordance with Sciascia et al. while subjects performed the PUP exercise. During humeral elevation in the scapular plane, infraspinatus activity has been reported to be between 10% - 25% MVC. Conversely, it has been shown that sidelying external rotation exercise and prone external rotation can elicit a great amount of EMG activity equal to 62% and 63% MVIC, respectively. However, these exercises were designed to isolate the INF. The result of the present study demonstrates that PUP exercise is not an ideal exercise to strengthen the INF. Even with the increased demand or complexity to maintain stability in the shoulder musculature during the PUP exercise, the INF had the same amount of activation with scapular plane elevation.
Our results indicated greater vGRF across the ROM in the traditional versus the modified PUP variant. This finding confirms those of previous studies examining ground reaction forces in these variants in both static positions and dynamic movement. In the traditional variant, vGRF ranged from 70.18% - 75.99% body weight, while in the modified variant, vGRF ranged from 52.95% - 57.95%. These ranges compare well to the percentage of body weight supported in the "up" and "down" positions of the traditional and modified variants reported by Suprak, Dawes, and Stephenson. Ebben et al. reported similar findings with respect to the peak GRF in various push-up exercises, which included traditional and modified variants, as well as those with feet elevated, and those with hands elevated on boxes of increasing height. These investigators reported increasing peak vGRFs as the push-up variant was altered from hands elevated 60.96 cm to modified to hands elevated 30.48 cm to traditional to feet elevated 30.48 cm and 60.98 cm. These results are in support of the present data in that they confirm the pattern of greater vGRF in the traditional versus the modified push-up. Gouvali and Boudolos also reported greater vGRF in the traditional variant, as compared to the modified. However, they reported peak vGRFs of 66% and 53% in the traditional and modified push-up ROM, respectively. This difference between their findings and those in the present study may be related to the different subjects included in the two studies. In their study, Gouvali and Boudolos included only male subjects, while the present study included both males and females. This difference may have impacted the distribution of body mass in subjects in the two studies, leading to a greater percentage observed in the present data.
The second important finding with regards to the vGRF in this study was the significant linear decrease in vGRF with elbow extension in the concentric portion of both variants. We hypothesize that the increase in vGRF with elbow extension is the result of the whole-body center of mass location moving further from the point of contact (feet or knees) with the support surface (floor) in the horizontal direction, resulting in greater gravitational torque that must be overcome by the vGRF in order to perform the exercise. This finding, again, supports those of Suprak, Dawes, and Stephenson, who reported greater vGRF in the "down" versus the "up" position of both variants.
Lastly, one of the more surprising finding in the current study was that the subjects did not lock their elbows at the end of the concentric phase of the push-up plus. The minimum elbow extension angle was 20° during the concentric phase of the PUP exercise. The investigators made sure that all the subjects were performing the exercises correctly, and they were protracting their shoulders at the end of the concentric phase of the PUP. To our knowledge, this is the first study that examined the entire elbow angle range of motion during the concentric phase of the PUP. The usual direction to subject performing the PUP is to extend their elbow to a standard push-up position and continued rise up by protracting the scapula. There are no studies that reported the specific elbow angle position throughout the entire concentric phase of the push-up plus exercise.
There were limitations that needed to be acknowledged and addressed regarding the present study. The first limitation concerns the elbow kinematics in 2D during the PUP. In order to avoid error in projection angle of the elbow, the investigator made sure that the camera was directly perpendicular to the elbow motion. Additionally, the hand placement of each subject was clearly marked on the force plate to maintain consistent subject location between trials. The second limitation is EMG cross talk between adjacent muscles. This limitation is inherent in every surface EMG study. In order to address this limitation, the investigator did a specific manual muscle test for every muscle tested before the normalization of the signal and during each trial.
Discussion
The present study examined the effect of PUP variant and elbow position on EMG activity of four scapular stabilizer muscles and vertical ground reaction force during concentric contraction. To our knowledge, this is the first study that looked at EMG activity of shoulder muscle stabilizer and vGRF at various elbow position during push-up plus. The current data demonstrated that traditional PUP was characterized with greater EMG muscle activity in all the muscles tested during the entire elbow range of motion compared to modified PUP. In addition, the traditional PUP resulted in larger vGRF compared to modified PUP.
The push-up plus exercise is considered to be the best exercise to activate the Serratus Anterior. It has been shown that SA EMG activity ranged from 69% to 120% maximum voluntary isometric contraction (MVIC) during some variants of push-up exercise. In the current study, the SA EMG activity ranged from 45% to 57% MVC, which is below the value reported in the literature. The difference between the EMG values may be due to the fact that the current study performed the MVIC in a different method. However, in a recent study by Sciascia et al., they reported similar SA EMG activity to the present study during the push-up plus exercise. Although, their results were from subjects with multi-directional instability and isolated anterior instability of the shoulder, they reported no significant differences between healthy and shoulder instability patients in SA EMG activity during the PUP exercise.
The present data also showed increased SA activity during elbow extension. During the modified PUP, there was only a 3% change in the entire concentric phase of the PUP. This shows that when performing the modified version of the PUP, the added motion of protracting the shoulder blade upon completion of the push-up repetition is not necessitated. The highest SA EMG activity during the concentric phase of the modified PUP happened at 55° of elbow extension, while in the traditional PUP, the highest SA EMG activity occurred at 20° of concentric elbow extension, which is during the plus phase of the exercise.
The UT and LT are considered to be two of the main scapular stabilizers. Both the UT and LT contribute to scapular upward rotation during humeral elevation. However, the UT also contributes to anterior tilting by drawing the clavicle medially and upwards, which could decrease the subacromial space and can increase the risk of subacromial impingement. The LT also assists in posterior tilting during humeral elevation. Thus, the goal in most shoulder impingement rehabilitation protocol is to decrease the activity of the UT compared to the LT during arm elevation. Consequently, the scapula will be in a more posteriorly tilted position, which increases the subacromial space. In the current study, both variants of PUP started with the UT (24% MVC) having an increased muscle activity compared to the LT (19% MVC). However, during the mid-range of the concentric phase of the PUP, there was a shift in activation pattern. The UT (20% MVC) decreased and the LT (24.8% MVC) increased its EMG activation starting at 70° of elbow extension up to the plus phase (i.e. 20° of elbow extension) of the traditional PUP exercise. These results are in accordance with previous studies that examined different variance of push-up plus exercise with the UT ranging from 20.5% - 25.2% MVC. For the LT, Park et al. examined the LT during wall push-up plus and wall slide device and reported a range of 11.6% - 16.1% MVC. The current study had a higher LT EMG activity. This difference can be attributed to the position that the push-up plus was implemented. The PUP exercise performed in Park et al. study was accomplished standing up against the wall while the current study had the subjects complete the PUP on the ground. The wall PUP is considered to be less demanding than the traditional on the ground PUP. The modified PUP did not change muscle activation patterns until the elbow was at 55° of elbow extension. The shift of muscle activity into a more LT than UT during the PUP is important to avoid impingement of soft tissues under the subacromial space.
The infraspinatus is one of the rotator cuff muscles. Its main function is to externally rotate the humerus. It also serves to assist in centralizing the humeral head into the glenoid during shoulder motion. In the current study, the infraspinatus was activated during both PUP variants. The traditional PUP had the greatest EMG activity (30% MVC) when the elbow was less than 50° of elbow flexion. This result is in accordance with Sciascia et al. while subjects performed the PUP exercise. During humeral elevation in the scapular plane, infraspinatus activity has been reported to be between 10% - 25% MVC. Conversely, it has been shown that sidelying external rotation exercise and prone external rotation can elicit a great amount of EMG activity equal to 62% and 63% MVIC, respectively. However, these exercises were designed to isolate the INF. The result of the present study demonstrates that PUP exercise is not an ideal exercise to strengthen the INF. Even with the increased demand or complexity to maintain stability in the shoulder musculature during the PUP exercise, the INF had the same amount of activation with scapular plane elevation.
Our results indicated greater vGRF across the ROM in the traditional versus the modified PUP variant. This finding confirms those of previous studies examining ground reaction forces in these variants in both static positions and dynamic movement. In the traditional variant, vGRF ranged from 70.18% - 75.99% body weight, while in the modified variant, vGRF ranged from 52.95% - 57.95%. These ranges compare well to the percentage of body weight supported in the "up" and "down" positions of the traditional and modified variants reported by Suprak, Dawes, and Stephenson. Ebben et al. reported similar findings with respect to the peak GRF in various push-up exercises, which included traditional and modified variants, as well as those with feet elevated, and those with hands elevated on boxes of increasing height. These investigators reported increasing peak vGRFs as the push-up variant was altered from hands elevated 60.96 cm to modified to hands elevated 30.48 cm to traditional to feet elevated 30.48 cm and 60.98 cm. These results are in support of the present data in that they confirm the pattern of greater vGRF in the traditional versus the modified push-up. Gouvali and Boudolos also reported greater vGRF in the traditional variant, as compared to the modified. However, they reported peak vGRFs of 66% and 53% in the traditional and modified push-up ROM, respectively. This difference between their findings and those in the present study may be related to the different subjects included in the two studies. In their study, Gouvali and Boudolos included only male subjects, while the present study included both males and females. This difference may have impacted the distribution of body mass in subjects in the two studies, leading to a greater percentage observed in the present data.
The second important finding with regards to the vGRF in this study was the significant linear decrease in vGRF with elbow extension in the concentric portion of both variants. We hypothesize that the increase in vGRF with elbow extension is the result of the whole-body center of mass location moving further from the point of contact (feet or knees) with the support surface (floor) in the horizontal direction, resulting in greater gravitational torque that must be overcome by the vGRF in order to perform the exercise. This finding, again, supports those of Suprak, Dawes, and Stephenson, who reported greater vGRF in the "down" versus the "up" position of both variants.
Lastly, one of the more surprising finding in the current study was that the subjects did not lock their elbows at the end of the concentric phase of the push-up plus. The minimum elbow extension angle was 20° during the concentric phase of the PUP exercise. The investigators made sure that all the subjects were performing the exercises correctly, and they were protracting their shoulders at the end of the concentric phase of the PUP. To our knowledge, this is the first study that examined the entire elbow angle range of motion during the concentric phase of the PUP. The usual direction to subject performing the PUP is to extend their elbow to a standard push-up position and continued rise up by protracting the scapula. There are no studies that reported the specific elbow angle position throughout the entire concentric phase of the push-up plus exercise.
There were limitations that needed to be acknowledged and addressed regarding the present study. The first limitation concerns the elbow kinematics in 2D during the PUP. In order to avoid error in projection angle of the elbow, the investigator made sure that the camera was directly perpendicular to the elbow motion. Additionally, the hand placement of each subject was clearly marked on the force plate to maintain consistent subject location between trials. The second limitation is EMG cross talk between adjacent muscles. This limitation is inherent in every surface EMG study. In order to address this limitation, the investigator did a specific manual muscle test for every muscle tested before the normalization of the signal and during each trial.
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