PURPOSEAccording to the force-velocity relationship of human skeletal muscle, the maximal load one can lift is limited by the concentric movement phase, and the eccentric phase is always underloaded. In the present study, we hypothesized that acute exercise training using an eccentric overload compared with standard loading would lead to greater neuromuscular and strength adaptations.METHODSSedentary women (age 20.9 yr) were tested for concentric and eccentric three-repetition maximum (3RM), maximal isokinetic eccentric and concentric and isometric force and associated EMG activity of selected thigh muscles before and after 7 consecutive days of exercise training of the left quadriceps. The exercise program was designed so that the total weight lifted was similar between the eccentric overload (EO, N = 10) and standard group (ST, N = 10), but EO exercised with about 50% greater eccentric load whereas the controls did not exercise (N = 10).RESULTSThere was a 22% increase in the total weight lifted over 7 d. On the average, EOs compared with STs strength gains were approximately twofold greater. Changes in EMG paralleled the changes in muscle strength without changes in biceps femoris coactivity during knee extension.CONCLUSIONBecause the strength gains were achieved by exercising at low intensities and over a short time period, exercise prescription of eccentric overloading appears especially suitable for elders, individuals deconditioned due to an injury, and the chronically diseased.

PURPOSE:Sprinting, bouncing, and spontaneous landings are associated with a forefoot contact whereas walking, running, and jumping are associated with heel-toe foot placement. Because such foot placement strategies influence landing mechanics or the ensuing performance, the purpose of this work was to compare lower extremity kinematics and kinetics and muscle activation patterns between drop vertical jumps performed with heel-toe (HTL) and forefoot (FFL) landings. METHODS:Ten healthy male university students performed two types of drop jump from a 0.4-m high box placed 1.0-m from the center of the force plate. They were instructed to either land first on the ball of the feet without the heels touching the ground during the subsequent vertical jump, i.e., forefoot landing jump (FFL), or to land on the heels followed by depression of the metatarsals, i.e., heel-toe landing jump (HTL). Three successfully performed trials per jump type were included in the analysis. The criteria for selection of the correct jumps was proper foot position at contact as judged from video records and the shape of force-time curve. RESULTS:The first peak and second peak determined from the vertical force-time curves were 3.4 times greater and 1.4 times lower for HTL compared with those with FFL (P<0.05). In the flexion phase of HTL, the hip and knee joints contributed 40% and 45% to the total torque, whereas during FFL the greatest torque contributions were 37% for both the knee and ankle joints. During the extension phase, the greatest torque contributions to the total torque were 41% and 45% by the knee and ankle joints during HTL and 34% and 55% during FFL. During the flexion phase, power production was 20% greater (P<0.05) in HTL than in FFL, whereas during the extension phase power production was 40% greater in FFL than in HTL. In the flexion phase of HTL the hip and knee joints produced the greatest power, and during the extension phase the knee and ankle joints produced the greatest power. In contrast, during both the flexion and extension phases of FFL, the knee and ankle joints produced the greatest power. The EMG activity of gluteus, vastus lateralis, and plantar flexor muscles was similar between HTL and FFL in most cases except for the greater vastus lateralis EMG activity during precontact phase in HTL than in FFL and the greater gastrocnemius activity in FFL than in HTL. CONCLUSION:Foot placement strategy modifies the individual joint contributions to the total power during drop jumping.

1909 In extreme models of exercise with eccentric contractions, a few days after exercise both young (Gibala et al. JAP 78:702, 1995) and elderly (Clarkson& Dedrick, J Gerontol 43:M91, 1988) humans reveal myofibrillar disruption, disorganization, and z-line streaming. Because aging specifically affects fast twitch fibers and eccentric contractions are associated with unique activation of these fibers (Enoka JAP 81:2339, 1996), two resistive training paradigms were compared with respect to muscle soreness and muscle damage. Six elderly(age 68) and 6 young women (age 21.7) trained with conventional and 6 elderly(age 70) and 6 young women (age 19.8) trained with an eccentric overload for 7 consecutive days using the left knee extensors. The total work load was similar within age groups but the eccentric overload groups performed 50% more eccentric work. Training intensity was 75% of 3 repetition maximum. Thigh muscle soreness was evaluated daily on a scale of 0 to 10. The vastus lateralis was biopsied before training and after bouts 1 and 7 in the young and before training and after bout 7 in the elderly. For soreness, the group by time interaction (F=0.49) and the group main (F=0.18) effects were not significant (p >.05) but there was a significant time main effect (F=17.9, p = 0.0001) with the highest soreness of 4.4 (±2.0) recorded on the day after bout 1 and soreness declining thereafter. Electromicrographs revealed a total absence of muscle damage. These data suggest that training with an eccentric overload is a viable alternative to conventional resistive training because it causes substantially greater strength gains (as reported in an accompanying paper at this meeting) without apparent muscle soreness or damage.

1908 There is strong evidence suggesting that muscle stretch plays a vital role in increasing muscle strength and size. With the unique recruitment pattern of large motor units during eccentric contractions (Enoka JAP 81:2339, 1996) and the aging-related atrophy of the same motor units, one would expect that a training protocol that emphasizes eccentric actions would be more effective to retard strength and muscle loss in the aged. Hence, we compared the strength gains in eccentric and concentric 3-repetition maximum and maximal isometric force (Kin-Com) after two resistive training paradigms. Six elderly (age 68) and 6 young women (age 22) trained with standard (ST) and 6 elderly (age 70) and 6 young women (age 20) trained with an eccentric overload (EO) for 7 consecutive days using the left knee extensors. The total work load was similar in ST and EO within each age group but the EO performed 50% more eccentric work. Training intensity was 75% of 3 repetition maximum. For each measure the significant group by time interaction showed significant pre/post gains (*, see Table), greater changes by EO than ST within (†) and between (‡) age groups.TableThese data suggest that eccentric overload compared with conventional resistive training is more effective to induce strength gains in young and elderly women.

1439 One method of assessing neural activation of muscles in different conditions or subject populations is to compare the EMG to force ratios. A long-debated issue is whether exercise training influences muscle activation with the implied hypothesis of athletes compared with sedentary subjects possessing a greater neuromuscular efficiency (i.e., a lower EMG to force ratio). Thus, we have compared EMG-to-force ratios in sedentary women (n = 21, age 21) and Olympic level male and female sprinters (n = 14, age 20). Subjects performed maximal effort isokinetic 90 °/s eccentric and concentric and isometric knee extensions on a dynamometer (Kin-Com, Chattanooga, TN). Surface EMG activity of the vastus lateralis was monitored and analyzed for the peak EMG activity (Noraxon, Scottsdale, AZ). The Table shows the EMG-to-force ratios (±S.D.) for each group of subjects under the 3 conditions and that the Group by Contraction mode interaction was not significant. The Contraction mode main effect was significant (F = 4.1, p = 0.0001); the eccentric (1.86 ±0.98) and isometric (2.59 ±1.29) ratios were significantly smaller than the concentric ratio (3.67±1.71). These data suggest that, as reported previously, the neural cost of muscle contraction is the least for eccentric and the greatest for concentric muscle actions. However, the current data suggest that the EMG-to-force ratios are independent of training status.Table

1912 During rehabilitation from ACL-reconstruction surgery, patients walk with greater knee flexion and a longer lasting knee extensor torque compared to healthy people (1). A functional knee brace (FKB) reduced both the amount of knee flexion and the extensor torque in these patients (2). The altered mechanics suggest that an FKB may reduce the force placed upon the newly reconstructed ACL during walking. The purpose of this study was to compare the shear force at the knee in recent ACL-reconstructed patients while walking with and without an FKB. Seven subjects (age 20 yr.; mass 75 kg) were tested 5 weeks after ACL-reconstruction. Subjects walked at a self-selected speed (≈1.5 m/s) with and without an FKB. A planar, mathematical model was developed to predict forces in the triceps surae, gastrocnemius, hamstrings and patellar tendon during the stance phase of walking. These forces, excluding triceps surae, and the knee joint reaction force derived through inverse dynamics were applied to the tibia with their directions based on previously reported data (3) to determine total knee shear force. Model results were validated by comparing predicted muscle forces from 5 healthy subjects(age 21 yr; mass 73 kg) with those from other models (4,5) and were within 17% of those predictions. Peak knee shear force directed anteriorly and shear impulse during stance were reduced ≈30% (t-test, p<.05) in the injured subjects while walking with the FKB. Results suggest that an FKB may provide increased protection from re-injury in patients with recent ACL reconstruction. TableTable

PURPOSE:Accelerated rehabilitation for anterior cruciate ligament (ACL) injury and reconstruction surgery is designed to return injured people to athletic activities in approximately 6 months. The small amount of empirical data on this population suggests, however, that the torque at the knee joint may not return until 22 months after surgery during walking and even longer during running. Although the rehabilitation has ended and individuals have returned to preinjury activities, gait mechanics appear to be abnormal at the end of accelerated programs. The purpose of this study was to compare lower extremity joint kinematics, kinetics, and energetics between individuals having undergone ACL reconstruction and accelerated rehabilitation and healthy individuals.METHODS:Eight ACL-injured and 22 healthy subjects were tested. Injured subjects were tested 3 wk and 6 months (the end of rehabilitation) after surgery. Ground reaction force and kinematic data were combined with inverse dynamics to predict sagittal plane joint torques and powers from which angular impulse and work were derived.RESULTS:The difference in all kinematic variables between the two tests for the ACL group averaged 38% (all P < 0.05). The kinematics were not different between the ACL group after rehabilitation and healthy subjects. Angular impulses and work averaged 100% difference for all joints (all P < 0.05) between tests for the ACL group. After rehabilitation, the differences between injured and healthy groups in angular impulse and work at both the hip and knee remained large and averaged 52% (all P < 0.05).CONCLUSIONS:Results indicated that after reconstruction surgery and accelerated rehabilitation for ACL injury, humans walk with normal kinematic patterns but continue to use altered joint torque and power patterns.

Fully rehabilitated, ACL-deficient people walk with more hip extensor torque and hamstring EMG and less knee extensor torque and quadricep EMG compared to healthy people (1,2,3). The developmental pattern of the adaptations and their causes are unknown. New evidence showed the initial response may be to increase knee torque(4) which indicated the adaptations are not quickly learned, automatic responses to ACL injury. The purpose of this pilot study was to compare quadricep (VL) and hamstring (BF) EMG during walking in 2-week post injury, ACL-deficient and healthy subjects. VL and BF EMG from 5 ACL-deficient and 8 healthy subjects were measured during 7 gait cycles walked at 1.5 m/s. Peak EMG and integrated (I) EMG from the main burst (late swing to midstance) were normalized to a maximal voluntary contraction (MVC) and I MVC over a 0.4 s contraction. T-test was used to compare sample means. Peak and I EMG in VL were 202% and 171% greater in ACL-deficient subjects. Peak and I EMG in BF were 30% and 35% lower in ACL-deficient subjects. Results show the initial adaptations were different than the final adaptations and the final adaptations were not an automatic response to ACL injury. Newly ACL-injured people use more knee flexion in stance compared to healthy controls(4). The results show the initial adaptations provide additional support for the overly flexed knee regardless of the increased anterior tibial load the VL might induce. Table

Several studies have examined the effects of landing surfaces or the manipulation of stiffness (MSSE 24:108, 1992) on lower extremity joint kinetics but these studies did not specifically control for foot placement strategy. Such information could be valuable for coaches and clinicians for the selection of appropriate paradigms to load specific lower extremity muscle groups. The purpose of the present study was to compare lower extremity kinetics between toe-heel (TH) versus heel-toe (HT) landing during 0.4-m drop jumps. Ten college age men performed 3 trials each of TH and HT landing on a force platform. All trials were video taped at 200 Hz. TheTable shows the data for extension angular impulse and positive and negative work at each joint. These data suggest that HT landing primarily loads knee and hip extensors while TH landing loads the ankle plantarflexors. Thus if only one type of landing is performed specific muscle groups may be favored while others underloaded.

Gait analyses of rehabilitated individuals with anterior cruciate ligament(ACL) deficiency and reconstruction have identified the final adaptations of increased hip extensor torque and hamstring electromyography (EMG) and decreased knee extensor torque and quadriceps EMG during stance. The initial adaptations to injury and surgery are, however, unknown as are the factors that influence the development of the adaptations. Identification of the initial response to injury would provide a basis for determining whether the final adaptations are learned automatically or if they are the result of a lengthy training period in which various factors may affect their development. The purpose of the study was to evaluate the initial effects of ACL injury and reconstruction surgery on joint kinematics, kinetics, and energetics during walking. Injured limbs from nine subjects with ACL injury were tested 2 wk after injury, and 3 and 5 wk after surgery. Ten healthy subjects were tested. Kinematic and ground reaction data were collected and combined with inverse dynamics to calculate the joint torques and powers. A knee extensor torque throughout most of stance was observed in the injured limbs at all test sessions. This result was in conflict with previous observations of reduced extensor torque or a flexor torque in rehabilitated patients with ACL reconstruction and patients with ACL deficiency. This result also differed from the typical midstance extensor then flexor torque in healthy control subjects. Trend analysis showed a significant (P < 0.001) change in average position at the hip and knee, extensor angular impulse at the hip, and positive work done at the hip 3 wk after surgery followed by a partial rehabilitation at 5 wk after surgery. Power and work produced at the knee were reduced fivefold (P < 0.001) after 5 wk of rehabilitation and did not recover to pre-surgical levels. The existence of a long-lasting knee extensor torque 2 wk after injury indicated that the adaptation process to ACL deficiency is lengthy, requiring many gait cycles, and that numerous factors could be involved in learning the adaptations.

The purpose of this study was to compare the short-term strength and neural adaptations to eccentric and concentric training at equal force levels. Forty-two sedentary women (age = 21.5 yr) were ranked based on the initial quadriceps strength score, and trios of subjects were randomly assigned to either an eccentric (n = 14), a concentric (n = 14), or a nonexercising control group (n = 14). Training involved a total of 824 eccentric or concentric quadriceps actions at 1.05 rad.s-1 administered in four sets of 6-10 repetitions, four times per week for 6 wk. Before and after training, all subjects were tested for unilateral maximal isometric and eccentric and concentric actions at 1.05 rad.s-1 and for a 40-repetition eccentric and concentric fatigue series of the left and right quadriceps. Surface electromyographic activity of the vastus lateralis and medialis was monitored during testing. Concentric training increased concentric (36%, P < 0.05), isometric (18%, P < 0.05), and eccentric strength (13%), and eccentric training increased eccentric (42%, P < 0.05), isometric (30%, P < 0.05), and concentric (13%) strength. Eccentric training improved eccentric and isometric strength more (P < 0.05) than did concentric training. The electromyographic adaptations were greater with eccentric training. Cross-education was 6%, and neither training mode modified fatigability. The data suggest that training of the quadriceps muscle with submaximal eccentric actions brings about greater strength adaptations faster than does training with maximal-level concentric actions in women. This greater adaptation is likely to be mediated by both mechanical and neural factors.

Rehabilitated ACL-injured people use a greater extensor torque at the hip and a reduced extensor torque at the knee during the stance phase of gait. Based on previous data that showed a functional knee brace (FKB) caused healthy subjects to walk and run with larger hip and lower knee extensor torques (DeVita et al., J. Biomechanics, in press), it was hypothesized the adaptations in ACL-injured gait may be mediated by training effects produced by the FKB used during rehabilitation. The purpose of this pilot study was to test the hypothesis that an FKB can cause recent ACL-injured and reconstructed subjects to walk with the torque adaptations seen in fully rehabilitated ACL-injured gait. Four volunteers, mean age=23 yrs, were tested about 1 month after ACL injury and surgery while walking with and without an FKB over a force plate. Sagittal plane video data were collected. 5 to 10 trials were analyzed per subject/condition. Force and video data were combined using inverse dynamics to compute the hip and knee torques in the injured limb. Angular impulse in the extensor direction was derived from each torque curve for comparison purposes. Knee torque was extensor and relatively high for most of stance indicating the subjects had not yet developed the rehabilitated ACL-injured adaptations. Knee extensor angular impulse was reduced in all subjects and by 28%, on average, while wearing the FKB (No FKB: 14.8 Nms{sd=2.2}, FKB: 10.7 Nms {sd=1.9}). Hip extensor angular impulse was increased by 16% with the FKB (No FKB: 22.5 Nms {sd=3.2}, FKB: 26.1 Nms {sd=2.3}) in two subjects and unchanged in the others. These preliminary results support the hypothesis that an FKB can cause ACL-injured individuals to adapt the unique knee and hip torque patterns seen in fully rehabilitated ACL-injured gait.

The purpose of this study was to compare the strength and neural adaptations following eccentric and concentric training at equal force levels. Forty-two sedentary women (age = 21.5 y) were ranked based on the initial quadriceps strength score and trios of subjects were randomly assigned to either an eccentric (n = 14), a concentric (n = 14) or a non-exercising control group (n = 14). Training involved 824, 1.05-rad·s-1 eccentric or concentric quadriceps contractions administered in 4 sets of 6-10 repetitions, 4 times per week for 6 weeks. The concentric group exercised with maximal contractions and the eccentric group exercised at the same force average and thus used submaximal eccentric contractions. Before and after training all subjects were tested for unilateral maximal isometric, eccentric, and concentric contractions at 3 velocities and a 40-repetition eccentric and concentric fatigue series in the left and right quadriceps. Surface electromyo-graphic (EMG) activity of the vastus lateralis and medialis was monitored during testing. Concentric training increased concentric (36%, p<.05), isometric (18%, p<.05), and eccentric strength (13%) and eccentric training increased eccentric (42%, p<.05), isometric (30%, p<.05), and concentric (13%) strength. Eccentric training improved eccentric and isometric strength more (p<.05) than concentric training improved concentric and isometric strength. The EMG adaptations paralleled the strength adaptations. The cross transfer of strength was 6% and neither training modality modified fatigue-resistance. The data suggest that training with submaximal eccentric contractions brings about greater strength adaptations than does training with maximal level concentric contractions in women. This greater adaptation is likely to be mediated by both mechanical and neural factors.