Effects of technique variations on knee biomechanics during the squat and leg press.

PURPOSE: The specific aim of this project was to quantify knee forces and muscle activity while performing squat and leg press exercises with technique variations. METHODS: Ten experienced male lifters performed the squat, a high foot placement leg press (LPH), and a low foot placement leg press (LPL) employing a wide stance (WS), narrow stance (NS), and two foot angle positions (feet straight and feet turned out 30 degrees ). RESULTS: No differences were found in muscle activity or knee forces between foot angle variations. The squat generated greater quadriceps and hamstrings activity than the LPH and LPL, the WS-LPH generated greater hamstrings activity than the NS-LPH, whereas the NS squat produced greater gastrocnemius activity than the WS squat. No ACL forces were produced for any exercise variation. Tibiofemoral (TF) compressive forces, PCL tensile forces, and patellofemoral (PF) compressive forces were generally greater in the squat than the LPH and LPL, and there were no differences in knee forces between the LPH and LPL. For all exercises, the WS generated greater PCL tensile forces than the NS, the NS produced greater TF and PF compressive forces than the WS during the LPH and LPL, whereas the WS generated greater TF and PF compressive forces than the NS during the squat. For all exercises, muscle activity and knee forces were generally greater in the knee extending phase than the knee flexing phase. CONCLUSIONS: The greater muscle activity and knee forces in the squat compared with the LPL and LPH implies the squat may be more effective in muscle development but should be used cautiously in those with PCL and PF disorders, especially at greater knee flexion angles. Because all forces increased with knee flexion, training within the functional 0-50 degrees range may be efficacious for those whose goal is to minimize knee forces. The lack of ACL forces implies that all exercises may be effective during ACL rehabilitation.

Ground reaction force profiles from force platform gait analyses of clinically normal mesomorphic dogs at the trot.

Force platform analysis of gait provides ground reaction force information that can be used to study limbs with normal or abnormal function. When combined, the interrelated variables of ground reaction forces give a more thorough description of gait than when used individually. To describe the pattern of ground reaction forces in clinically normal, conditioned, mesomorphic dogs, we studied the data from platform gait analyses of 43 dogs. Mediolateral (Fx), craniocaudal (Fy), and vertical (Fz) forces were measured and recorded. Torque (Tz) around the vertical axis also was calculated. Mean stance times for forelimbs and hind limbs were 0.278 and 0.261 second, respectively. Among dogs, ground reaction forces were normalized and expressed as percentage of body weight (%bw). The vertical (Fz) peak, average force during stance phase, and force vs time impulses were 106.68, 60.82, and 17.2 %bw in forelimbs, and were 65.11, 35.3, and 9.33 %bw in hind limbs. The forelimb braking/propulsive (Fy) peaks were -16.74 and +6.73 %bw. In hind limbs, these peaks were -3.76 and +7.69 %bw. The usual mediolateral force (Fx) pattern found in forelimbs was laterally directed, with average peak magnitude of 6.69 %bw, whereas the hind limb patterns were variable.

The effectiveness of weight-belts during multiple repetitions of the squat exercise.

The purpose of this study was to examine the effectiveness of weight-belts during multiple repetitions of the parallel back squat exercise. Five subjects were filmed (50 fps) as they performed eight consecutive trials at each of two weight-belt conditions [with belt = WB, without belt = WOB] in random order at their eight-repetition maximum effort. Other parameters examined were ground reaction forces, intra-abdominal pressure (IAP), and mean electromyography (mEMG) for the external oblique (EO), erector spinae (ES), vastus lateralis (VL), and bicep femoris (BF) muscles. All parameters were collected and interfaced to a computer via an A/D converter. WB repetitions were generally performed faster than WOB repetitions, especially by the later repetitions (3.34 vs 3.56 s). WB IAP values were consistently greater (P less than 0.05) than WOB values by 25-40%. IAP increased by approximately 11.5% from the first to the last repetitions. No differences were observed for ES and EO mEMG for belt usage, but values increased by up to 20% across repetitions. Several differences were observed between WB and WOB for the VL and BF mEMG, with WB values being significantly greater. These data suggest that a weight-belt aids in supporting the trunk by increasing IAP, and that any differential effect due to wearing a weight-belt did not occur over eight repetitions.

The effectiveness of weight-belts during the squat exercise.

The purpose of this study was to examine the effectiveness of weight-belts during the performance of the parallel squat exercise. Six subjects were filmed (40 fps) as they performed three trials at each of three belt conditions (NB, none; LB, light; HB, heavy) in random order and three load conditions (70, 80, 90% 1RM (one repetition maximum] in increasing order. The parameters examined were collected and interfaced to a computer via an analog-to-digital (A/D) converter: ground reaction forces, intra-abdominal pressure (IAP), and EMG for the rectus abdominus (RA), external oblique (EO), and erector spinae (ES) muscles. Most differences were observed during the 90% 1RM condition, and only they are presented in this paper. Maximum IAP values were always greater (P less than 0.05) for the weight-belt conditions (LB, 29.2; HB, 29.1 greater th an NB, 26,8 kPa). Similar results were observed for the mean IAP. The integrated EMG (iEMG) activity of the muscles and adjusted mean values for back compressive force and back muscle force followed a similar but opposite pattern, with NB being the greatest. ES mEMG/(L5/S1) values for HB (18.1%) were the least, followed by LB (20.01%) and NB (22.3%). Few differences were observed between belt types. These data suggest that a weight-belt can aid in supporting the trunk by increasing IAP.

Co-activation of sprinter and distance runner muscles in isokinetic exercise.

The purpose of this study was to investigate the extent of co-activation of quadriceps and hamstring musculature in sprinters and distance runners. Nine female intercollegiate track athletes performed maximal knee extensions and flexions on a modified orthotron isokinetic dynamometer at two speeds (100 degrees and 400 degrees X s-1). Simultaneous recordings of torque, joint position, and agonist/antagonist electromyographic activity from the quadriceps and hamstrings were computer-processed. The results revealed the hamstrings to be considerably more active during knee extension than the quadriceps during flexion. The integrated electromyographic activity of co-contracting hamstrings and quadriceps, throughout the joint range, averaged 33 and 6%, respectively, of the same muscle group during its agonist phase. Hamstring co-activation increased sharply during the last 25% of knee extension, generating 58% of the integrated electromyographic agonist activity. Co-activation of the sprinters' hamstrings was four times that of distance runners (57/14%), however, the faster speed of movement (400 degrees X s-1) increased hamstring coactivation of distance runners more acutely than sprinters in the final phase of extension. The data suggest that the hamstrings are used to a much greater extent than quadriceps for limb deceleration and that the distraction of antagonist muscle tension should be considered when analyzing agonist isokinetic torques. Furthermore, the relatively high co-activation of the hamstrings, particularly during the last 25% of extension, may induce hamstring soreness or strain in vulnerable subjects.

Biomechanics of the squat exercise using a modified center of mass bar.

The purpose of the study was to investigate the effects of load height on selected performance characteristics of a squat exercise. A lower center of mass bar was designed that allowed the integrity of the squat exercise to be maintained while possibly reducing the chances of injury. Five trials were performed with the center of mass of the bar was set at shoulder height (C1) and lowered 18% (C2) and 36% (C3) of the subject's height below the normal bar position using the inverted "U" bar. All trials were filmed as the subjects lifted on a force platform. A balloon catheter was inserted into the subject's recta to monitor intra-abdominal pressure (IAP). High correlations were found between IAP, joint moment, and force data. Many of the critical parameters occurred just after the lowest squat position. Significant differences (P less than 0.05) in trunk angle excursion and trunk angular velocity indicated a greater ease of hip extension for the center of mass bar conditions. No differences were observed between conditions for thigh and knee angles and joint moments indicating kinematic similarity for the lower extremity. IAP was always least for C2 and C3, while compression, shear, and back muscle forces did not differ. It was estimated that the greater IAP was responsible for relieving back muscle forces and compression by up to 15 and 21%, respectively, and increased stress with the weight at shoulder height stimulated a response for greater IAP to help alleviate the stresses on the spine.

A comparison between free-weight and isokinetic bench pressing.

The purpose of the study was to evaluate selected parameters describing performance characteristics of a free-weight and isokinetic bench press. A secondary purpose was an attempt to clarify the technique requirements essential for a successful lift. Parameters describing the free-weight condition were generated from cinematographic data (150 fps) for five trials each at 90 and 75% of the subject's maximal performance (1RM). Isokinetic data were obtained from an instrumented Cybex Power Bench Press at two speeds corresponding to the average speeds for the free-weight conditions. Despite differences, accommodation appeared to occur for both methods when the lifts were performed maximally. A "sticking region" was defined as the portion of the free-weight activity when the subjects' force application was less than the weight of the bar. No significant difference (P less than 0.05) was observed between the 90% 1RM (26.02%) and 75% 1RM (26.94%) mean relative time values for these regions. For the Cybex device, the percentage of the activity which was isokinetic was longer for the slower speeds of rotation (0.47 rad X s-1 = 70%) and steadily decreased until the movement was only 50% isokinetic at 1.74 rad X s-1. The observed relationships between applied force-time data along with anatomical considerations suggest an ideal technique for the lift.