Comparison of the maximum EMG levels recorded in maximum effort isometric contractions at five different knee flexion angles

The effect of muscle length on neural drive (here termed “neural activation”) was investigated from electromyographic activities and activation levels (twitch interpolation). The neural activation was measured in nine men during isometric and concentric (30 and 120°/s) knee extensions for three muscle lengths (35, 55, and 75° knee flexion, i.e., shortened, intermediate, and lengthened muscles, respectively). Long (76°), medium (56°), and short (36°) ranges of motion were used to investigate the effect of the duration of concentric contraction. Neural activation was found to depend on muscle length. Reducing the duration of contraction had no effect. Neural activation was higher with short muscle length during isometric contractions and was weaker for shortened than for intermediate and lengthened muscles performing 120°/s concentric contractions. Muscle length had no effect on 30°/s concentric neural activation. Peripheral mechanisms and discharge properties of the motoneurons could partly explain the observed differences in the muscle length effect. We thus conclude that muscle length has a predominant effect on neural activation that would modulate the angular velocity dependency.

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Neuromuscular Control of the Knee during a Resisted Single-Limb Squat Exercise

The American Journal of Sports Medicine ◽

10.1177/0363546504274150 ◽

2005 ◽

Vol 33 (10) ◽

pp. 1520-1526 ◽

Cited By ~ 49

Author(s):

Richard K. Shields ◽

Sangeetha Madhavan ◽

Emy Gregg ◽

Jennifer Leitch ◽

Ben Petersen ◽

...

Keyword(s):

Body Weight ◽

Knee Joint ◽

Knee Flexion ◽

Neuromuscular Control ◽

Biceps Femoris ◽

Medial Gastrocnemius ◽

Isometric Contractions ◽

Low Resistance ◽

Squat Exercise ◽

Flexion And Extension

Background Closed kinetic chain exercises such as single-limb squats are preferred for knee rehabilitation. A complete understanding of the neuromuscular control of the knee during the single-limb squat is essential to increase the efficiency of rehabilitation programs. Hypothesis Performing a controlled single-limb squat with resistance to knee flexion and extension will increase the coactivation of the hamstring muscle group, thus reducing the quadriceps/hamstrings ratio. Study Design Descriptive laboratory study. Methods A total of 15 healthy human subjects (7 women, 8 men) performed controlled single-limb squats in a custom mechanical device that provided resistance to both flexion and extension. Subjects performed the task at 3 levels of resistance, set as a percentage of body weight. Surface electromyographic recordings from 7 muscles (gluteus medius, rectus femoris, vastus medialis oblique, vastus lateralis, biceps femoris, semitendinosus, and medial gastrocnemius) were collected during the task. Results Biceps femoris activity during knee flexion increased from approximately 12% maximum voluntary isometric contractions during low resistance (0% body weight) to approximately 27% maximum voluntary isometric contractions during high resistance (8% body weight). Although the quadriceps had greater activity than the hamstrings at all levels of resistance, the quadriceps/hamstrings ratio declined significantly with resistance (F2,27 = 29.05; P=. 012) from 3.0 at low resistance to 2.32 at the highest resistance. Conclusions Performing controlled resisted single-limb squats may help to simultaneously strengthen the quadriceps and facilitate coactivation of the hamstrings, thus reducing anterior tibial shear forces. The coactivation may also increase the dynamic control of the knee joint. Clinical Relevance The typical single-limb squat exercise performed in the clinic does not usually control for bidirectional resistance and knee joint excursion. As seen in this study, controlled single-limb squats at increased levels of resistance help to increase the coactivation of the hamstring muscles, which is essential to optimize neuromuscular control of the knee.

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Relationship Between a Modified Thomas Test and Leg Range of Motion in Australian-Rules Football Kicking

Journal of Sport Rehabilitation ◽

10.1123/jsr.12.4.343 ◽

2003 ◽

Vol 12 (4) ◽

pp. 343-350 ◽

Cited By ~ 1

Author(s):

Warren Young ◽

Peter Clothier ◽

Leonie Otago ◽

Lyndell Bruce ◽

David Liddell

Keyword(s):

Range Of Motion ◽

Outcome Measures ◽

Knee Flexion ◽

Flexion Angle ◽

Football Players ◽

Knee Flexion Angle ◽

Australian Rules Football ◽

Maximum Effort ◽

Australian Football ◽

Hip Extension

Context:Flexibility tests are sometimes thought to be related to range of motion in dynamic activities, but such a relationship remains to be determined.Objective:To determine the correlation between flexibility and hip and knee angles in Australian football kicking.Design:Correlation.Setting:Biomechanics laboratory.Participants:16 Australian Rules football players.Main Outcome Measures:Hip and knee angles of the preferred kicking leg in a relaxed position were determined with a modified Thomas test. Maximum hip extension, the knee-flexion angle in this position, the maximum knee-flexion angle, and the hip angle at this position during the swing phase of maximum-effort drop-punt kicks were determined.Results:Significant correlations were found between hip flexibility and maximum hip extension (r = .65, P < .01) and hip angle at the maximum knee-flexion angle (r = .70, P < .01).Conclusions:The data indicate a moderate association between hip flexibility and hip angles during kicking.

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Bilateral deficit is larger for step than for ramp isometric contractions

Journal of Applied Physiology ◽

10.1152/jappl.1993.74.3.1200 ◽

1993 ◽

Vol 74 (3) ◽

pp. 1200-1205 ◽

Cited By ~ 54

Author(s):

T. J. Koh ◽

M. D. Grabiner ◽

C. A. Clough

Keyword(s):

Motor Units ◽

Muscle Group ◽

Isometric Contractions ◽

Healthy Male ◽

Maximum Torque ◽

Bilateral Deficit ◽

Torque Measurements ◽

Torque Generation ◽

Maximum Effort ◽

Male Subjects

The force produced by a muscle group during a maximum effort bilateral task has been reported to be less than that produced during a maximum effort unilateral task. It was hypothesized that if decreased activation of primarily fast motor units is responsible for this bilateral deficit, 1) the bilateral deficit would be larger in step than in ramp isometric contractions and 2) the rate of torque generation in step contractions would be slower in bilateral than in unilateral tasks. Twelve healthy male subjects performed unilateral and bilateral isometric knee joint extension tasks in which maximum torque was generated under ramp and step conditions. The maximum torques produced in the bilateral tasks were 17.0 and 24.6% less than those produced in the unilateral tasks during the ramp and step conditions, respectively (both P < 0.001). The bilateral torque deficit for the step condition was significantly greater than that for the ramp condition (P = 0.012). The rate of torque generation in the step condition was 19.7% slower in the bilateral than in the unilateral task (P = 0.010). Thus the results from these torque measurements support the notion that decreased activation of primarily fast motor units may be responsible for the bilateral deficit.

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Determining the Force-Length-Velocity Relations of the Quadriceps Muscles: II. Maximum Muscle Stress

Journal of Applied Biomechanics ◽

10.1123/jab.15.2.191 ◽

1999 ◽

Vol 15 (2) ◽

pp. 191-199 ◽

Cited By ~ 11

Author(s):

John W. Chow ◽

Warren G. Darling ◽

James C. Ehrhardt

Keyword(s):

Knee Flexion ◽

Isometric Force ◽

Muscle Length ◽

Gravitational Effect ◽

Knee Extension ◽

Cross Sectional ◽

Average Value ◽

Isometric Knee Extension ◽

Maximum Effort ◽

Muscle Stress

The purpose of this study was to determine the maximum muscle stress (σ), defined as the maximum isometric force divided by the physiological cross-sectional area, of the quadriceps muscles for a pilot study involving musculoskeletal modeling. One female subject performed maximum effort isometric knee extension exercises on an isokinetic dynamometer at different attachment arm angles. The gravitational effect was taken into consideration when determining the isometric resultant knee torques at different knee flexion angles. The anatomical and geometric parameters of the quadriceps muscles were obtained from radiography and magnetic resonance imaging taken from the subject. The σ value was computed using me measured knee torques, musculoskeletal parameters data, and information reported in the literature. The computation procedures used in this study represented the first attempt to incorporate the concept of optimal muscle length in the determination of maximum muscle stress. The σ values obtained from the data for nine different knee flexion angles ranged from 21.4 to 30.5 N/cm2. The average value of 25.6 ± 2.6 N/cm2 is notably smaller than the human σ values reported in the literature, but is comparable to the σ values obtained from isolated muscles.

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Motor unit recruitment in human medial gastrocnemius muscle during combined knee flexion and plantarflexion isometric contractions

Experimental Brain Research ◽

10.1007/bf00229364 ◽

1993 ◽

Vol 93 (3) ◽

Cited By ~ 6

Author(s):

B.T. Ballantyne ◽

C.G. Kukulka ◽

G.L. Soderberg

Keyword(s):

Motor Unit ◽

Knee Flexion ◽

Gastrocnemius Muscle ◽

Motor Unit Recruitment ◽

Medial Gastrocnemius ◽

Isometric Contractions ◽

Medial Gastrocnemius Muscle

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Different Lower-Limb Setup Positions Do Not Consistently Change Backstroke Start Time to 10 m

Sports ◽

10.3390/sports8040043 ◽

2020 ◽

Vol 8 (4) ◽

pp. 43

Author(s):

Gordon E. Barkwell ◽

James P. Dickey

Keyword(s):

Lower Limb ◽

Knee Flexion ◽

High Speed ◽

Sagittal Plane ◽

Body Position ◽

Knee Extension ◽

Knee Extensors ◽

Start Time ◽

Performance Impact ◽

Maximum Effort

Backstroke starts involve the athlete starting from a flexed position with their feet against the pool wall and then extending their ankles, knees, hips and back to push off; however, swimmers can start in different positions. The purpose of this study was to evaluate the performance impact of different knee extension angles in the setup position for a backstroke start. Ten backstroke swimmers completed maximum-effort starts in each of two setup positions: one with the knees maximally flexed, and one with the knees less flexed. The start handles and touchpad were instrumented with multi-axial force sensors. Activity of major hip and knee extensors was measured using surface electromyography. Body position in the sagittal plane was recorded using high-speed cameras. There was no overall difference in time to 10 m between the two conditions (p = 0.36, dz = 0.12), but some participants showed differences as large as 0.12 s in time to 10 m between start conditions. We observed that starts performed from a setup position with less knee flexion had an average 0.07 m greater head entry distance (p = 0.07, dz = 0.53), while starts from a setup position with maximal knee flexion had an average 0.2 m/s greater takeoff velocity (p = 0.02, dz = 0.78). Both head entry distance and takeoff velocity are related to start performance, suggesting each position may optimize different aspects of the backstroke start. Coaches should assess athletes individually to determine which position is optimal.

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Evaluating Patellar Kinematics Through Magnetic Resonance Imaging During Open- and Closed-Kinetic-Chain Exercises

Journal of Sport Rehabilitation ◽

10.1123/jsr.19.1.1 ◽

2010 ◽

Vol 19 (1) ◽

pp. 1-11 ◽

Cited By ~ 7

Author(s):

Lílian Ramiro Felicio ◽

Marcelo Camargo Saad ◽

Rogério Ferreira Liporaci ◽

Augusto do Prado Baffa ◽

Antônio Carlos dos Santos ◽

...

Keyword(s):

Magnetic Resonance Imaging ◽

Knee Flexion ◽

Isometric Contractions ◽

Resonance Imaging ◽

Kinetic Chain ◽

Patellar Tilt ◽

Patellar Kinematics ◽

Sulcus Angle ◽

Closed Kinetic Chain ◽

Patellar Tilt Angle

Purpose:To evaluate patellar kinematics of volunteers without knee pain at rest and during isometric contraction in open- and closed-kinetic-chain exercises.Methods:Twenty individuals took part in this study. All were submitted to magnetic resonance imaging (MRI) during rest and voluntary isometric contraction (VIC) in the open and closed kinetic chain at 15°, 30°, and 45° of knee flexion. Through MRI and using medical e-film software, the following measurements were evaluated: sulcus angle, patellar-tilt angle, and bisect offset. The mixed-effects linear model was used for comparison between knee positions, between rest and isometric contractions, and between the exercises.Results:Data analysis revealed that the sulcus angle decreased as knee flexion increased and revealed increases with isometric contractions in both the open and closed kinetic chain for all knee-flexion angles. The patellar-tilt angle decreased with isometric contractions in both the open and closed kinetic chain for every knee position. However, in the closed kinetic chain, patellar tilt increased significantly with the knee flexed at 15°. The bisect offset increased with the knee flexed at 15° during isometric contractions and decreased as knee flexion increased during both exercises.Conclusion:VIC in the last degrees of knee extension may compromise patellar dynamics. On the other hand, it is possible to favor patellar stability by performing muscle contractions with the knee flexed at 30° and 45° in either the open or closed kinetic chain.

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Muscle damage produced by isometric contractions in human elbow flexors

Journal of Applied Physiology ◽

10.1152/japplphysiol.00535.2017 ◽

2018 ◽

Vol 124 (2) ◽

pp. 388-399 ◽

Cited By ~ 8

Author(s):

Trevor J. Allen ◽

Tyson Jones ◽

Anthony Tsay ◽

David L. Morgan ◽

Uwe Proske

Keyword(s):

Muscle Damage ◽

Muscle Soreness ◽

Isometric Exercise ◽

Isometric Contractions ◽

Elbow Flexors ◽

Disuse Atrophy ◽

Half Maximum ◽

Maximum Effort ◽

Damage Process ◽

Voluntary Contractions

Isometric exercise is often prescribed during rehabilitation from injury to maintain muscle condition and prevent disuse atrophy. However, such exercise can lead to muscle soreness and damage. Here we investigate which parameters of isometric contractions are responsible for the damage. Bouts of 30 repetitions of maximum voluntary contractions of elbow flexors in 38 subjects were carried out and peak force, soreness, and tenderness were measured before the exercise, immediately afterwards, at 2 h, and at 24 h postexercise. When one arm was held near the optimum angle for force generation (90°), the force it produced was greater by 28% than by the other arm held at a longer length (155°). However, despite the smaller contraction forces of the muscle held at the longer length, after the exercise it exhibited a greater fall in force that persisted out to 24 h (20% fall) and more delayed soreness than the muscle exercised at 90° (7% fall at 24 h). The result indicates a length dependence of the damage process for isometric contractions at maximum effort. In four additional experiments, evidence was provided that the damage occurred during the plateau of the contraction and not the rising or relaxation phases. The damage had a prompt onset and was cumulative, continuing for the duration of the contraction. We interpret our findings in terms of the nonuniform lengthening of sarcomeres during the plateau of the contractions and conclude that muscle damage from isometric exercise is minimized if carried out at lengths below the optimum, using half-maximum or smaller contractions. NEW & NOTEWORTHY Isometric exercise, where muscle contracts while the limb is held fixed, is often possible for individuals rehabilitating from injury and can help maintain muscle condition. Such exercise has been reported to cause some muscle damage and soreness. We confirm this and show that to minimize damage, exercising muscles should be held at shorter than the optimum length for force and carried out at half-maximum effort or less.

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Poststretch Isometric Contractions of the Hamstrings: Just a Brief Stretch to Achieve Supramaximal Isometric Force

Journal of Applied Biomechanics ◽

10.1123/jab.2020-0236 ◽

2021 ◽

pp. 1-7

Author(s):

Neil Chapman ◽

John William Whitting ◽

Suzanne Broadbent ◽

Zachary Crowley-McHattan ◽

Rudi Meir

Keyword(s):

Isometric Contraction ◽

Knee Flexion ◽

Isometric Force ◽

Biceps Femoris ◽

Isometric Contractions ◽

Maximal Voluntary Isometric Contraction ◽

Hamstring Strain ◽

Long Head ◽

Biceps Femoris Long Head ◽

Knee Flexors

Hamstring strain injuries are common in sport. Supramaximal eccentric or high-intensity isometric contractions are favored in hamstring strain injury prevention. The effect of combining these contraction modes in such prevention programs as a poststretch isometric contraction is unknown. Poststretch isometric contractions incorporate an active stretch and result in greater final isometric force than isometric contractions at comparable joint angles. This study compared torque and muscle activation levels between maximal voluntary isometric contraction and maximal poststretch isometric contractions of the knee flexors. Participants (n = 9) completed baseline maximal voluntary isometric contraction at 150° knee flexion and maximal poststretch isometric contractions at 120° knee flexion actively stretching at 60°/s to 150° knee flexion for final isometric contraction. Torque of the knee flexors and surface electromyography root mean square (sEMGRMS) of biceps femoris long head were simultaneously recorded and compared between baseline and poststretch isometric at 150° knee flexion. Torque was 14% greater in the poststretch isometric condition compared with baseline maximal voluntary isometric contraction (42.45 [20.75] N·m, 14% [22.18%], P < .001) without increase in sEMGRMS of biceps femoris long head (−.03 mV, ±.06, P = .130, d = .93). Poststretch isometric contractions resulted in supramaximal levels of poststretch isometric torque without increased activation of biceps femoris long head.

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