The lower lumbar segment reaches peak extension angular velocity early in the period of peak force production, at approximately 14% postcatch (Table 2). This demonstrates that the activity of the extensors of the spine quickly extend this segment so it is not maintained at its position of maximal flexion during peak force production. As displayed in Figure 4, the lower lumbar segment extends quickly as force production would be developing, along with a steady increase in extension angular velocity of the pelvis. During the period of peak force production, the pelvis is the segment exhibiting an increasing angular velocity.
Sequencing of muscle activity, as determined by the onset of the EMG bursts, demonstrated that the extensors of the pelvis (GM and BF) and the spine (ES lumbar, ES thoracic, and LD) were initiated within the first 6% of the stroke postcatch (Table 4). Onsets of EMG bursts of all pelvic and spinal extensors were not significantly different from each other.
Period of peak force production.
The clear grouping of activation among the extensor muscles is maintained as indicated by timing of peak EMG amplitude (Table 4). The BF, GM, ES lumbar, and ES thoracic peak first postcatch (BF at 18.3% ± 2.4%, GM at 17.1% ± 4.7%, ES lumbar at 18.5% ± 3.5%, and ES thoracic at 18.5% ± 4.8%), with no significant difference among the timing of peak amplitudes. Collectively, the prime extensor muscles of the pelvis, lumbar, and mid-lower thoracic spine demonstrate peak activity during this phase, occurring within approximately a 1.5% range of the stroke cycle.
The LD reaches its peak amplitude (23.1% ± 6.4%) significantly later in the stroke compared with the preceding extensors. The peak amplitude of LD falls near the end of the period of peak force production of the drive; perhaps as it is differentiated from the remaining extensors by contributing also to glenohumeral joint extension during force production of the stroke. This is reflective of the overall sequence of the stroke (legs followed by trunk, followed by arms in completing the stroke).
During the period of peak force production, there were minimal amounts of coactivation among the flexors and the extensors of the trunk (Table 5). The amount of activity of each flexor and extensor muscle, as demonstrated by percentage of total burst area, during this period was significantly different (P = 0.0001). Extensor activity predominates during the period of peak force production with over 60% of the total burst area occurring during this period (Table 5). The flexor group demonstrates less than 6% of total burst area during the period of peak force production. It is unlikely that EMG activity displayed within the flexor group represents a spinal stabilizing coactivation strategy in response to the extensor activity generated during the period of peak force production within the stroke.
Among the extensor group, the hamstrings (BF) and the gluteal (GM) muscles demonstrate similar percentage of burst area during the period of peak force production consistent with being the main muscles of hip extension during the leg drive (Table 5). It is interesting to note that the percentage of burst area of the ES lumbar (62.9%) is similar to that of the hip extensors (60.7% and 66.5%) during the period of peak force production (Table 5). These muscles have also similar burst durations. In contrast, ES thoracic and LD demonstrate more prolonged bursts and therefore less percentage of total activity within this period specifically (Figs. 2 and 5). This suggests a differentiation of the erector spinae roles during the stroke.
During the period of peak force production, the pelvis and all segments of the spine are extending. Table 3 displays the range of motion of each segment during this percentage of the stroke. The pelvis is the main segment extending the trunk, by 9.5° ± 3.4°, with very little extension of the spinal segments. The coordination of activity of the extensor muscles appears to achieve effective stabilization of the spine with the pelvis, facilitating a somewhat rigid lever on which to transfer forces. The lower lumbar segment (L3-S1) demonstrates the most extension of the spinal segments (2.9° ± 1.0°, P = 0.0001). Although this is a small excursion, it demonstrates an increase in mobility of the lower lumbar segment during the period of peak force production.
Period of coactivation.
The period of coactivation is defined as the EMG activity between the onset of the first flexor muscle and the end of the last extensor muscle burst for each subject. This period of coactivation did not include the period of peak force production in any subject. The mean period of coactivation was from 28.2% ± 2.2% postcatch to 36.8% ± 3.2%. The mean duration of the period of coactivation was 7.9% ± 2.9% of the stroke. This represents mid to late drive as the end of the drive occurs at 44.3% ± 1.2% of the stroke.
During the period of coactivation within the stroke, flexor muscles demonstrate increasing EMG activity; however, the peak of flexor activity occurs outside of this period, approximately 40% postcatch (Figs. 2 and 5). The extensor group demonstrates decreasing activity, without any extensor muscle demonstrating peak activity in this period. This period represents the transition into the finish of the stroke and the slowing of trunk extension and the influence of the gravitational force. The flexor group, TrA/IO, RA, and EO, shows consistency with no significant difference found between timing of onset (Table 4). This consistency is also seen in the timing of peak amplitude of the flexor group (40.4-41.3%; Table 4). The flexor group demonstrates similar levels of activity as ES thoracic and LD (approximately 20% of total burst area; Table 5).
Pelvic peak extension angular velocity occurred at the initiation of the period of coactivation (27.9% ± 1.4% postcatch; Table 2). This coincides with onset of the flexor muscle group (at approximately 29% postcatch) directly following the peak extension angular velocity of the pelvis. Peak extension angular velocities of T10-L1 and L1-L3 segments occurred at 29.2% ± 9.3% and 32.7% ± 2.3% of stroke, respectively (Table 2). Although both lumbar segments and lower thoracic segment continue to extend during this period, the thoracic segments of T4-T7 and T7-T10 began to flex. This resulted in flexion being experienced superiorly through the trunk as the pelvis is displaying a slowing of extension angular velocity. The flexion of the upper thoracic segments may be influenced by multiple factors such as the arm pull of the handle toward the body, the goal of the subject to maintain a somewhat level horizontal linear trajectory of the head and handle throughout the drive.
The main findings of the current study include the following: 1) the pelvic and the spinal extensor muscles demonstrated similar timing of activation, as did the flexor muscles; 2) during the period of peak force production, there was minimal coactivation of the flexors and the extensors; 3) the motion of the spine was minimal during peak force production, although there was significantly more movement at L3-S1 than at any other segment; and 4) low levels of coactivation of the trunk flexors and extensors existed in late drive during the transition between predominately extension to predominately flexion activity.
Extensor muscle recruitment in early drive.
The group of elite female rowers investigated in the current study closely resembles the group of rowers investigated by McGregor et al. (21). Within that study, it was reported that peak force occurred at 18.8% ± 1.8% postcatch, in mid-drive of the stroke. However, McGregor et al. (21) defined catch as the onset of force production at the handle, whereas catch was defined as the most forward position of the wrist in the current study. This will introduce discrepancies in the timing between the two studies because McGregor et al. (21) noted that the point at which the handle ceased moving toward the ergometer (analogous to the furthest point forward of the wrist and catch as defined by our study) occurred 2.7-3.0% of the stroke cycle before the onset of force production at the handle. The current study is not trying to match precisely one point in our data set with that of McGregor et al. (21); instead, we created a window of 10% of the rowing stroke centered around McGregor et al.'s (21) point of peak force production (18.8%) to characterize the period of peak force production. Although force was not measured in the current study, the timing of the peak of EMG activity of BF, GM, ES lumbar, and ES thoracic was 17-20.6% postcatch, suggesting that peak force production of the stroke occurred in this period of the drive.
Our findings suggest that coordination of the extensors of the spine and the pelvis, after the catch, may be an effective strategy to support the spine as forces increase with the initiation of the drive. There was remarkable consistency of the onset of EMG activity among extensors of the spine and the pelvis. The peak extension angular velocity of the lower lumbar (L3-S1) segment and the upper two thoracic segments (T4-T7, T7-T10) occurs relatively early in the drive before peak loading. The early extension of these segments may serve to prevent further flexion of the spine before the occurrence of peak forces.
Kinematics associated with peak extensor EMG activity highlight the effectiveness of the motor control strategies of the subjects in maintaining the spine position relatively fixed. The trunk experiences forces inferiorly and superiorly created by the leg drive and the resistance of the handle through the pull of the arms. If the spine were to flex in response to the transfer of forces, the result would be decreased force transmission to the handle (2). Greater extension range of motion of a spinal segment could create a greater risk of shear forces introduced to the segment while experiencing high compression forces. The lower lumbar spine appears to be the segment at risk as compared with the upper lumbar spine. The larger movement at the lower lumbar spine may be a consequence of its proximity to the pelvis, which demonstrates high extension angular velocity.
Recruitment of trunk flexors in late drive and recovery.
The pattern of trunk flexor EMG activity demonstrates the main role of the abdominals in slowing extension of the trunk as the center of mass is brought further into extension, the layback. During the late drive, gravitational forces increase and resistance from the handle decreases once the pull of the arms is completed. The trunk EMG activity will shift increasingly toward flexion as it must stop extension and reverse direction, flexing toward the catch.
The onset of the flexors of the trunk is associated with coactivation of flexor and extensor activity. Of the erector spinae group, ES thoracic remains active into the coactivation period, more so than ES lumbar. ES lumbar activity is similar to BF and GM, which also demonstrate less EMG burst area during this period than ES thoracic. This may be reflective of differentiation of the two erector spinae muscles based on mechanical advantage of the longer, increasingly multisegmental thoracic portions of this group to demonstrate improved effectiveness as global stabilizers as well as extensors of the spine (7). Latissimus dorsi (LD) also demonstrates a longer duration of burst. The peak activity of LD occurs during the period of peak force and remains active as flexor activity is initiated. The activity of LD may be differentiated from the remainder of the extensors due to its involvement with the extension of the humerus during the pull of the handle to the trunk, signifying the end of the stroke.
Peak burst amplitude of TrA/IO, RA, and EO occurs at approximately 40% postcatch, with the end of the drive occurring at approximately 44% when force on the handle is minimal. The peak activity of TrA/IO, RA, and EO falls outside the range of which the trunk muscles exhibit coactivation demonstrating that the peak force production of the flexors occurs at the end of the drive phase when there is limited activity of the extensors. The flexor group is able to flex the trunk forward over the extended legs without having to overcome forces of extension that would be created if the extensor group was strongly active during this action.
Coactivation of trunk musculature.
During the period of peak force production, there were minimal amounts of coactivation between the trunk flexor and the extensor muscle groups. During this phase of the stroke, the extensors of the pelvis and the spine predominate, with peak activity of extensor bursts occurring with very low levels of flexor EMG. Currently there are limited data to develop empirical measures of spinal stability and trunk muscle coactivation within the literature. Therefore, it remains unclear whether low levels of abdominal muscle activity, as seen in this study, contribute to stabilization of the spine.
Lavender et al. (14) found similar levels of low trunk flexor and extensor coactivation during a symmetrical pull activity performed leaning back in a semisquat position. Of studies investigating coactivation of trunk flexors and extensors during trunk extension tasks, this investigation represents the task most closely resembling the activity of rowing. However, levels of coactivation displayed by subjects within our study are low. This may be reflective of the subjects and the activity investigated. The current investigation involved elite athletes performing a skilled, trained activity. A higher level of coactivation may be seen in novice rowers as compared with elite rowers as they may demonstrate less refined ability to maintain balance within these conditions (5,20). Higher levels of coactivation of the flexor muscles during peak force production would negatively impact the peak extension force created by the extensor muscle group and hence the power of the stroke. If the antagonist activity were heightened, the trunk extensor muscle activity would need to increase to maintain the overall power of the stroke. Increased coactivation of the flexors and the extensors would also result in increased compressive spinal forces and may therefore be injurious (9).
Coactivation has been shown to increase in tasks introducing asymmetrical or unstable loads (14,28,30). There may be more coactivation in sweep rowing than that seen on the ergometer because sweep rowing incorporates an element of spinal rotation and asymmetrical loading. It is also possible that the unstable base of the rowing shell in water versus the stable base of the ergometer may increase the level of trunk coactivation to maintain balance, a condition further challenged by rough water. Reeves et al. (24) demonstrated that increased coactivation of the trunk muscles while seated on an unstable surface degraded postural control, as seen by an increase in center of pressure velocity and excursion. Therefore, higher coactivation may be seen on the water or with novice rowers than with the ergometer in elite athletes in the current study.
Implications for rowing.
This investigation into the kinematics and the EMG of the trunk musculature revealed that muscle recruitment is quite precise in elite female rowers. This is associated with limited motion of the trunk in peak force production, thereby creating a stable lever to transfer the forces generated by the legs and a stable base on which the upper extremities are able to generate and transfer these forces to the handle. Timing of muscle activation has been demonstrated to be impacted by skill level or stage of motor learning (5), presence of pain within the segment of interest (30), kinematic position of the segment (18), direction of load (14), unanticipated external perturbations (28), and fatigue (3). Poor timing of muscle recruitment may result in technique that may compromise how the spine is loaded, as suggested by McGregor et al. (21) concerning novice rowers who demonstrated altered lumbopelvic rhythm. Further research is needed to explore the kinematics and the muscle recruitment strategies of the trunk under conditions of injury, fatigue, and different levels of rowing experience (novice rowers). Investigation with bilateral EMG recordings would also enable the identification of any motor control strategies unique to sweep or sculling rowing technique.
The authors wish to thank Dr. Trevor Birmingham, codirector of the Wolf Orthopaedic Biomechanics Laboratory, for his assistance with data collection and Angela Kedgley for her assistance with data analysis. Dr. Volker Nolte, head coach of the Western Men's Rowing team, provided helpful commentary on a draft of this article. This work was funded, in part, through a grant from the Natural Sciences and Engineering Research Council of Canada awarded to SJ Garland. The authors report no conflict of interest or endorsement by ACSM.
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Keywords:©2009The American College of Sports Medicine
SPINE; COACTIVATION; MUSCLE ACTIVATION PATTERNS; PEAK FORCE