Subject Characteristics and Inclusion Criteria
Twelve healthy males (average age (standard deviation) 27.09 years(1.23), average height (SD) 179.08 cm(3.75), average weight (SD)78.25 kg (5.23)), with greater than 6 months of weight training experience, with out back pain or upper limb injuries were recruited from a convenience sample of college students. Subjects signed an informed consent form approved by the Internal Review Board of the Canadian Memorial Chiropractic College (CMCC).
The muscle activation level, expressed as a percentage of a maximum voluntary contraction (MVC), of the Latissimus dorsi (LD), Biceps Brachii (BB) and middle trapezius/rhomboid muscle (MTR) groups during a series of different exercise tasks was quantified. Four different exercise tasks and three normalization procedures occurred during one test session.
Data Collection Hardware Characteristics
Disposable bipolar Ag-AgCl disc surface electrodes with a diameter of one cm were adhered bilaterally over the muscle groups with a centre to centre spacing of 2.5 cm. For the right biceps brachii electrodes were placed on the middle of the muscle belly when the elbow was flexed at 90 degrees. For the latissimus dorsi, electrodes were placed one cm lateral to the inferior border of the right scapula. A pair of electrodes was adhered superiorly to the skin above the middle trapezius and rhomboid minor between the spine of the scapula and the 2nd thoracic spinous process. Raw EMG was amplified between 1000 and 20,000 times depending on the subject. The amplifier had a CMRR of 10,000:1 (Bortec EMG, Calgary AB, Canada). Raw EMG was band pass filtered (10 and 1000 Hz) and A/D converted at 2000 Hz using a National Instruments data acquisition system and collected using EMG acquisition software (Delsys, Boston MA).
Normalization task procedure
Three different maximal voluntary contractions for the three muscle groups studied were collected for each subject. Subjects performed 1–2 practice MVCs before the collection of EMG. For the latissimus dorsi, subjects were required to perform a 3 second maximal isometric Lat pull down against an immovable resistance. For the biceps brachii subjects were required to perform a maximum isometric bicep curl (i.e. attempted elbow flexion) against an immoveable object with the arm at 90 degrees of flexion. The maximum voluntary contraction to recruit the MiddleTrapezius/Rhomboid muscle required the participants to perform a maximum isometric scapular retraction against experimenter provided manual resistance. The muscle activity during the exercise tasks was then subsequently expressed as a percentage of the peak activity found during the previously described normalization tasks.
During all exercises subjects used the same weight on a standard lat pulldown and seated row pulley machine. This weight was chosen by the subject based on their perceived ability to perform between 10 and 12 reps until failure for the pronated grip lat pulldown. For each exercise, two repetitions of a ten second isometric contraction were performed. Following each repetition, a three minute rest occurred. The two repetition protocol was then repeated for each exercise. The exercises performed were:
1. Wide grip pull down (WGP): From a seated position with the thighs restricted, subjects used an overhand grip on a straight pull down bar at 150% of the bi acromial distance (BAD). The weight was pulled into an isometric position with the arms at 90 degrees of shoulder flexion and elbow flexion. (Essentially a bar position which finds the bar 1–2 inches above eye level). Subjects held this position for 10 seconds.
2. Reverse grip pull down (RGP): From a seated position with thighs fixed subjects used an underhand grip on a straight bar at 100% BAD. The isometric contraction was held at a position with 90 degrees of shoulder forward flexion and 90 degrees of elbow flexion (Essentially a bar position which finds the bar 1–2 inches above eye level)
3. Seated row, shoulders retracted (SRR): Subjects started from a seated position, arms extended with forearms at a mid pronated position 6 inches apart. The participants pulled the weight to a position where the shoulder was at 0 degrees of flexion and 90 degrees of elbow flexion with maximal scapular retraction. During the isometric portion of the exercise, the subject was asked to approximate the shoulder blades (Retraction).
4. Seated Row, shoulders slack (SRR): Subjects performed the same movement as exercise #3 however the subject was instructed to allow the scapula to roll forward during the isometric hold portion of the exercise.
During all of these exercises the isometric portion (the portion that was analysed) was preceded by a concentric contraction that positioned the subjects arm and then followed by an eccentric contraction where the participant lowered the weight to the stack.
EMG Processing and data analysis
The root mean square (sliding window of 128 ms with an overlap of 64 ms) of the raw EMG during each exercise task and the normalization tasks was calculated using an EMG analysis software package (Delsys, Boston USA). The average activity was then calculated for the middle two seconds of the isometric portion of each exercise and repetition. The average of the two repetitions for each exercise was then calculated for each subject and presented as a percentage of the maximum activity found during the normalization tasks.
Separate repeated-measures ANOVA with post-hoc Tukey tests were then used to determine the influence of exercise type on muscle activity within the latissimus dorsi, biceps brachii and middle trapezius/rhomboids.