John W. Chow

A Biomechanical Comparison of Back and Front Squats in Healthy Trained Individuals

Gullett, JC, Tillman, MD, Gutierrez, GM, and Chow, JW. A biomechanical comparison of back and front squats in healthy trained individuals. J Strength Cond Res 23(1): 284-292, 2008-The strength and stability of the knee plays an integral role in athletics and activities of daily living. A better understanding of knee joint biomechanics while performing variations of the squat would be useful in rehabilitation and exercise prescription. We quantified and compared tibiofemoral joint kinetics as well as muscle activity while executing front and back squats. Because of the inherent change in the position of the center of mass of the bar between the front and back squat lifts, we hypothesized that the back squat would result in increased loads on the knee joint and that the front squat would result in increased knee extensor and decreased back extensor muscle activity. A crossover study design was used. To assess the net force and torque placed on the knee and muscle activation levels, a combination of video and force data, as well as surface electromyographic data, were collected from 15 healthy trained individuals. The back squat resulted in significantly higher compressive forces and knee extensor moments than the front squat. Shear forces at the knee were small in magnitude, posteriorly directed, and did not vary between the squat variations. Although bar position did not influence muscle activity, muscle activation during the ascending phase was significantly greater than during the descending phase. The front squat was as effective as the back squat in terms of overall muscle recruitment, with significantly less compressive forces and extensor moments. The results suggest that front squats may be advantageous compared with back squats for individuals with knee problems such as meniscus tears, and for long-term joint health.

Decreased static and dynamic postural control in children with autism spectrum disorders

The purpose of this study was to investigate postural control in children with Autism Spectrum Disorders (ASD) during static and dynamic postural challenges. We evaluated postural sway during quiet stance and the center of pressure (COP) shift mechanism during gait initiation for 13 children with ASD and 12 age-matched typically developing (TD) children. Children with ASD produced 438% greater normalized mediolateral sway (p<0.05) and 104% greater normalized anteroposterior sway (p<0.05) than TD children. Consequently, normalized sway area was also significantly greater (p<0.05) in the group with ASD. Similarly, the maximum separation between the COP and center of mass (COM) during quiet stance was 100% greater in the anteroposterior direction (p<0.05) and 146% greater in the resultant direction (p<0.05) for children with ASD. No significant difference was observed in the mediolateral direction, in spite of the 123% greater separation detected in children with ASD. During gait initiation, no group differences were detected in the posterior COP shift mechanism, suggesting the mechanism for generating forward momentum is intact. However, significantly smaller lateral COP shifts (p<0.05) were observed in children with ASD, suggesting instability or an alternative strategy for generating momentum in the mediolateral direction. These results help to clarify some discrepancies in the literature, suggesting an impaired or immature control of posture, even under the most basic conditions when no afferent or sensory information have been removed or modified. Additionally, these findings provide new insight into dynamic balance in children with ASD.

Agreement between temporospatial gait parameters of an electronic walkway and a motion capture system in healthy and chronic stroke populations.

Stokic DS, Horn TS, Ramshur JM, Chow JW: Agreement between temporospatial gait parameters of an electronic walkway and a motion capture system in healthy and chronic stroke populations. Objectives:To determine agreement between temporospatial gait parameters derived from an electronic walkway and a video-based motion capture system in healthy and chronic stroke subjects. Design:Data were acquired simultaneously by a walkway (GAITRite) and an eight-camera motion capture system (Motion Analysis Corporation) in 52 healthy subjects (age 47 ± 15 yrs) and 20 people with stroke (age, 58 ± 20 yrs; 4 ± 7 yrs poststroke). Gait velocity, stride time, stride length, step length, percent single support, and percent total support were compared by the limits-of-agreement method. Results:The mean differences between the two methods were small and measured 1.5% or less of the parameter mean value. The velocity difference was significantly (P < 0.001) larger in healthy (1.37 ± 0.93 cm/sec) than in stroke subjects (0.50 ± 0.63) and in stroke subjects who did not use an assistive device (0.75 ± 0.67 cm/sec) compared with those who did (0.13 ± 0.32). Conclusions:Electronic walkway and video-based gait analysis provide comparable temporospatial gait information in healthy and stroke subjects. The differences between the two methods were greater with increasing speed but overall still negligible considering expected changes resulting from stroke recovery, experimental manipulations, or therapeutic interventions. Therefore, electronic walkway and video-based gait analysis may be used interchangeably for evaluating temporospatial gait parameters after stroke for clinical and research purposes.

Lower trunk kinematics and muscle activity during different types of tennis serves

BackgroundTo better understand the underlying mechanisms involved in trunk motion during a tennis serve, this study aimed to examine the (1) relative motion of the middle and lower trunk and (2) lower trunk muscle activity during three different types of tennis serves - flat, topspin, and slice.MethodsTennis serves performed by 11 advanced (AV) and 8 advanced intermediate (AI) male tennis players were videorecorded with markers placed on the back of the subject used to estimate the anatomical joint (AJ) angles between the middle and lower trunk for four trunk motions (extension, left lateral flexion, and left and right twisting). Surface electromyographic (EMG) techniques were used to monitor the left and right rectus abdominis (LRA and RRA), external oblique (LEO and REO), internal oblique (LIO and RIO), and erector spinae (LES and RES). The maximal AJ angles for different trunk motions during a serve and the average EMG levels for different muscles during different phases (ascending and descending windup, acceleration, and follow-through) of a tennis serve were evaluated.ResultsThe repeated measures Skill × Serve Type × Trunk Motion ANOVA for maximal AJ angle indicated no significant main effects for serve type or skill level. However, the AV group had significantly smaller extension (p = 0.018) and greater left lateral flexion (p = 0.038) angles than the AI group. The repeated measures Skill × Serve Type × Phase MANOVA revealed significant phase main effects in all muscles (p < 0.001) and the average EMG of the AV group for LRA was significantly higher than that of the AI group (p = 0.008). All muscles showed their highest EMG values during the acceleration phase. LRA and LEO muscles also exhibited high activations during the descending windup phase, and RES muscle was very active during the follow-through phase.ConclusionSubjects in the AI group may be more susceptible to back injury than the AV group because of the significantly greater trunk hyperextension, and relatively large lumbar spinal loads are expected during the acceleration phase because of the hyperextension posture and profound front-back and bilateral co-activations in lower trunk muscles.

Kinematic and Electromyographic Analysis of Wheelchair Propulsion on Ramps of Different Slopes for Young Men With Paraplegia

OBJECTIVE To gain insight into the biomechanics of upslope wheelchair stroking by examining the changes in kinematic and electromyographic characteristics of wheelchair propulsion over ramps of different slopes. DESIGN Repeated-measures design. Each subject pushed up a wooden ramp (7.3m long) 3 times at self-selected normal and fast speeds for each of these slopes: 0 degrees , 2 degrees , 4 degrees , 6 degrees , 8 degrees , 10 degrees , and 12 degrees . SETTING A biomechanics laboratory. PARTICIPANTS Young men (N=10) with paraplegia. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Electromyographic activity of extensor carpi radialis, triceps brachii, antero-middle and postero-middle deltoids, pectoralis major, and latissimus dorsi, and stroking kinematics. RESULTS Forward lean of the trunk increased as the slope increased. The triceps brachii, antero-middle deltoid, and pectoralis major were more active during the push phase, while the postero-middle deltoid was more active during the recovery phase. Both extensor carpi radialis and latissimus dorsi were active throughout a stroke. Major adjustments in stroking kinematics and significant increases in muscle activity occurred at slopes between 4 degrees and 10 degrees . CONCLUSION In addition to a decrease in stroking speed, the stroking pattern becomes more compact (decreased push angle and relative recovery time, increased stroke frequency) and the trunk becomes more active with increasing slope.

Effects Of Hip Flexor Training On Sprint, Shuttle Run, And Vertical Jump Performance

Although hip flexion is integral in sports, hip flexion exercises are seldom emphasized in strength and conditioning for sports performance. This study aimed to determine whether a hip flexor resistance-training program could improve performance on a variety of tasks. Thirteen men and 11 women completed an 8-week hip flexion resistance-training program. Eleven men and 13 women served as controls. Isometric hip flexion strength, 40-yd dash time and the time for the first 10-yds, 4 × 5.8-m shuttle run time, and vertical jump height were evaluated at the beginning and end of the training and control period. Improvements were observed in the training group but not in the control group. Individuals in the training group improved hip flexion strength by 12.2% and decreased their 40-yd and shuttle run times by 3.8% and 9.0%, respectively. An increase in hip flexion strength can help to improve sprint and agility performance for physically active, untrained individuals.

Kinematic analysis of shot-putting performed by wheelchair athletes of different medical classes

The aim of this study was to identify those kinematic characteristics that are most closely related to an athletes medical classification and measured distance of a put. Two S-VHS camcorders (60 fields per second) were used to record the performance of 17 males of different classes. Each participant performed six trials and the best trial for each was selected for analysis. Three-dimensional kinematics of the shot and upper body segments at the instant of release and during the forward thrust (delivery) were determined. The average speeds and angles of the shot at release for different classes (5.3-7.8 ms -1 and 21.2 to 34.4°, respectively) were smaller than those exhibited by elite male able-bodied throwers. The height of the shot at release, the angular speed of the upper arm at release, the range of motion of the shoulder girdle during the delivery, and the average angular speeds of the trunk, shoulder girdle and upper arm during the delivery, were all significantly correlated with both the classification and measured distance ( P ≪ 0.05). The results indicate the importance of achieving a high average angular speed for each upper body segment during the delivery.

Discus throwing performances and medical classification of wheelchair athletes

PURPOSE The purpose of this study was to identify those kinematic characteristics that are most closely related to the medical classification and measured distance of a throw. METHODS Two S-VHS camcorders (60 fields x s(-1) were used to record the performance of 14 males of different classes. Each subject performed 10 trials and the best two trials from each subject were selected for analysis. Three-dimensional kinematics of the discus and upper body segments at the instant of release and the range of motion and average angular speed of different segments during the final forward swing were determined. RESULTS The speeds of the discus at release, ranging from 9.9 to 17.2 m x s(-1), were smaller than those exhibited by elite male able-bodied throwers. However, the angles of release, ranging from 24.6 to 41.4 degrees, were comparable with those observed in able-bodied throwers. Of the segmental kinematic parameters, (a) the inclination and angular speed of the upper arm at release; (b) the ranges of motion of the shoulder girdle, upper arm, and forearm during the forward swing; and (c) the average angular speed of the shoulder girdle during the forward swing were significantly correlated to both the classification and measured distance. The inclinations of different segments at the instant of release suggested that athletes with high level of spinal cord injury emphasized the elbow flexion to compensate for the deficiency in shoulder girdle movement. CONCLUSIONS In addition to the speed of the discus at release, the shoulder girdle movement during the forward swing is an important determinant of classification and measured distance.

Biomechanical comparison of two racing wheelchair propulsion techniques.

PURPOSE The purpose was to compare the conventional (CVT) and para-backhand (PBT) techniques used for racing wheelchair propulsion. Selected 3-D kinematic characteristics of the upper body and the electromyographic (EMG) signals of selected muscles during racing wheelchair propulsion over a roller system were examined. METHODS Eight CVT and seven PBT elite performers served as the subjects. Each subject performed maximum effort pushing for 30 s at a load that simulated overground pushing. Two S-VHS camcorders (60 Hz) were used to obtain 3-D kinematic parameters and muscle activity was monitored using surface electrodes. RESULTS The CVT was found to have significant shorter push time, smaller relative push time, and greater relative recovery time than the PBT. The CVT is a more compact stroke (smaller joint range of motion) and the PBT has a faster overall movement speed. Significant differences in arm positions were found between the two techniques at the instants of hand contact and hand release, and the upper arm was more internally rotated at these two instants in the CVT when compared with the PBT. The EMG data showed that large variations in muscle activation patterns existed in each technique group. In general, the flexor carpi radialis and triceps brachii were most active in the push phase. The upper trapezius and postero-middle deltoids were most active in the ascending recovery phase, whereas the extensor carpi radialis, biceps brachii, antero-middle deltoids, and pectoralis major were most active during the descending recovery phase. CONCLUSIONS The greater push time and push angle associated with the PBT suggest that the PBT may be more suitable for endurance athletes who are less explosive in their pushing strokes. The greater time and angle allow PBT users the opportunity to transmit more force to the wheel.

Coactivation of ankle muscles during stance phase of gait in patients with lower limb hypertonia after acquired brain injury.

OBJECTIVE Examine (1) coactivation between tibialis anterior (TA) and medial gastrocnemius (MG) muscles during stance phase of gait in patients with moderate-to-severe resting hypertonia after stroke or traumatic brain injury (TBI) and (2) the relationship between coactivation and stretch velocity-dependent increase in MG activity. METHODS Gait and surface EMG were recorded from patients with stroke or TBI (11 each) and corresponding healthy controls (n=11) to determine the magnitude and duration of TA-MG coactivation. The frequency and gain of positive (>0) and significant positive (p<0.05) EMG-lengthening velocity (EMG-LV) slope in MG were related to coactivation parameters. RESULTS The magnitude of coactivation was increased on the more-affected (MA) side, whereas the duration was prolonged on the less-affected (LA) side of both stroke and TBI patients. The difference reached significance during the initial and late double support. The magnitude of coactivation positively correlated with the gain of significant positive EMG-LV slope in TBI patients. CONCLUSIONS Increased coactivation between TA and MG during initial and late double support is a unique feature of gait in stroke and TBI patients with muscle hypertonia. SIGNIFICANCE Increased coactivation may represent an adaptation to compensate for impaired stability during step transition after stroke and TBI.