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1.  Mechanisms of quadriceps muscle weakness in knee joint osteoarthritis: the effects of prolonged vibration on torque and muscle activation in osteoarthritic and healthy control subjects 
Arthritis Research & Therapy  2011;13(5):R151.
Introduction
A consequence of knee joint osteoarthritis (OA) is an inability to fully activate the quadriceps muscles, a problem termed arthrogenic muscle inhibition (AMI). AMI leads to marked quadriceps weakness that impairs physical function and may hasten disease progression. The purpose of the present study was to determine whether γ-loop dysfunction contributes to AMI in people with knee joint OA.
Methods
Fifteen subjects with knee joint OA and 15 controls with no history of knee joint pathology participated in this study. Quadriceps and hamstrings peak isometric torque (Nm) and electromyography (EMG) amplitude were collected before and after 20 minutes of 50 Hz vibration applied to the infrapatellar tendon. Between-group differences in pre-vibration torque were analysed using a one-way analysis of covariance, with age, gender and body mass (kg) as the covariates. If the γ-loop is intact, vibration should decrease torque and EMG levels in the target muscle; if dysfunctional, then torque and EMG levels should not change following vibration. One-sample t tests were thus undertaken to analyse whether percentage changes in torque and EMG differed from zero after vibration in each group. In addition, analyses of covariance were utilised to analyse between-group differences in the percentage changes in torque and EMG following vibration.
Results
Pre-vibration quadriceps torque was significantly lower in the OA group compared with the control group (P = 0.005). Following tendon vibration, quadriceps torque (P < 0.001) and EMG amplitude (P ≤0.001) decreased significantly in the control group but did not change in the OA group (all P > 0.299). Hamstrings torque and EMG amplitude were unchanged in both groups (all P > 0.204). The vibration-induced changes in quadriceps torque and EMG were significantly different between the OA and control groups (all P < 0.011). No between-group differences were observed for the change in hamstrings torque or EMG (all P > 0.554).
Conclusions
γ-loop dysfunction may contribute to AMI in individuals with knee joint OA, partially explaining the marked quadriceps weakness and atrophy that is often observed in this population.
doi:10.1186/ar3467
PMCID: PMC3308081  PMID: 21933392
2.  Variations in Neuromuscular Activity of Thigh Muscles During Whole-Body Vibration in Consideration of Different Biomechanical Variables 
The intention of this study was to systematically analyze the impact of biomechanical variables in terms of different vibration frequencies, amplitudes and knee angles on quadriceps femoris and hamstring activity during exposure to whole-body vibration (WBV). 51 healthy men and women (age 55 ± 8 years) voluntary participated in the study and were randomly allocated to five different vibration-frequency groups. Each subject performed 9 static squat positions (3 amplitudes x 3 knee angles) on a side alternating vibration platform. Surface electromyography (EMG) was used to record the neuromuscular activity of the quadriceps femoris and hamstring muscles. Maximal voluntary contractions (MVCs) were performed prior to the measurements to normalize the EMG signals. A three-way mixed ANOVA was performed to analyze the different effects of the biomechanical variables on muscle activity. Depending on the biomechanical variables, EMG muscle activity ranged between 18.2 and 74.1 % MVC in the quadriceps femoris and between 5.2 and 27. 3 % MVC in the hamstrings during WBV. The highest levels of muscle activation were found at high frequencies and large amplitudes. Especially in the quadriceps femoris muscle, a WBV frequency of 30 Hz led to a significant increase in muscle activity compared to the other tested frequencies. However, it seems that knee angle is only relevant for the quadriceps femoris muscle. The results of this study should give more information for developing individual training protocols for WBV treatment in different practical applications.
Key PointsWBV leads to a higher muscle activity of the quadriceps femoris than of the hamstrings.The maximum levels of muscle activity were significantly reached at high amplitude and high frequency.The knee angle only significantly affects the quadriceps femoris.Certain combinations of the biomechanical variables have similar effects on the level of muscle activity.
PMCID: PMC3772586  PMID: 24149149
Vibration training; surface electromyography; muscle strength; muscle tuning
3.  Static stretching does not alter pre and post-landing muscle activation 
Background
Static stretching may result in various strength and power deficiencies. Prior research has not determined, however, if static stretching causes a change in muscle activation during a functional task requiring dynamic stability. The purpose of this study was to determine if static stretching has an effect on mean pre and postlanding muscle (vastus medialis VM, vastus lateralis VL, medial hamstring MH, and biceps femoris BF) activity.
Methods
26 healthy, physically active subjects were recruited, from which 13 completed a 14-day static stretching regimen for the quadriceps and hamstrings. Using the data from the force plate and EMG readings, a mean of EMG amplitude was calculated for 150 msec before and after landing. Each trial was normalized to an isometric reference position. Means were calculated for the VM, VL, MH, and BF from 5 trials in each session. Measures were collected pre, immediately following the 1st stretching session, and following 2 weeks of stretching.
Results
A 14-day static stretching regimen resulted in no significant differences in pre or postlanding mean EMG amplitude during a drop landing either acutely or over a 14-day period.
Conclusions
Static stretching, done acutely or over a 14-day period does not result in measurable differences of mean EMG amplitude during a drop landing. Static stretching may not impede dynamic stability of joints about which stretched muscles cross.
doi:10.1186/1758-2555-3-9
PMCID: PMC3117746  PMID: 21569528
4.  Effects of Prior Hamstring Strain Injury on Strength, Flexibility, and Running Mechanics 
Background
Previous studies have shown evidence of residual scar tissue at the musculotendon junction following an acute hamstring strain injury, which could influence re-injury risk. The purpose of this study was to investigate whether bilateral differences in strength, neuromuscular patterns, and musculotendon kinematics during sprinting are present in individuals with a history of unilateral hamstring injury, and whether such differences are linked to the presence of scar tissue.
Methods
Eighteen subjects with a previous hamstring injury (>5 months prior) participated in a magnetic resonance (MR) imaging exam, isokinetic strength testing, and a biomechanical assessment of treadmill sprinting. Bilateral comparisons were made for peak knee flexion torque, angle of peak torque, and hamstrings:quadriceps strength ratio during strength testing, and muscle activations and peak hamstring stretch during sprinting. MR images were used to measure the volumes of the proximal tendon/aponeurois of the biceps femoris, with asymmetries considered indicative of residual scar tissue.
Findings
A significantly enlarged proximal biceps femoris tendon volume was measured on the side of prior injury. However, no significant differences between the previously injured and uninjured limbs were found in strength measures, peak hamstring stretch, or muscle activation patterns. Further, the degree of asymmetry in tendon volume was not correlated to any of the functional measures.
Interpretation
The results of this study indicate that injury-induced changes in morphology do not seem discernable from strength measures, running kinematics, or muscle activity patterns. Further research is warranted to ascertain whether residual scarring alters localized musculotendon tissue mechanics in a way that may contribute to the high rates of muscle re-injury that are observed clinically.
doi:10.1016/j.clinbiomech.2010.04.015
PMCID: PMC2923440  PMID: 20621753
muscle injury; running; electromyography; magnetic resonance imaging; isokinetic strength
5.  Influence of vibration on delayed onset of muscle soreness following eccentric exercise 
Delayed onset muscle soreness (DOMS), which may occur after eccentric exercise, may cause some reduction in ability in sport activities. For this reason, several studies have been designed on preventing and controlling DOMS. As vibration training (VT) may improve muscle performance, we designed this study to investigate the effect of VT on controlling and preventing DOMS after eccentric exercise.
Methods
Fifty healthy non‐athletic volunteers were assigned randomly into two experimental, VT (n = 25) and non‐VT (n = 25) groups. A vibrator was used to apply 50 Hz vibration on the left and right quadriceps, hamstring and calf muscles for 1 min in the VT group, while no vibration was applied in the non‐VT group. Then, both groups walked downhill on a 10° declined treadmill at a speed of 4 km/hour. The measurements included the isometric maximum voluntary contraction force (IMVC) of left and right quadriceps muscles, pressure pain threshold (PPT) 5, 10 and 15 cm above the patella and mid‐line of the calf muscles of both lower limbs before and the day after treadmill walking. After 24 hours, the serum levels of creatine‐kinase (CK), and DOMS level by visual analogue scale were measured.
Results
The results showed decreased IMVC force (P = 0.006), reduced PPT (P = 0.0001) and significantly increased mean of DOMS and CK levels in the non‐VT group, compared to the VT group (P = 0.001).
Conclusion
A comparison by experimental groups indicates that VT before eccentric exercise may prevent and control DOMS. Further studies should be undertaken to ascertain the stability and effectiveness of VT in athletics.
doi:10.1136/bjsm.2006.031278
PMCID: PMC2465226  PMID: 17138635
vibration training; eccentric exercise; DOMS
6.  Quadriceps and Hamstrings Muscle Dysfunction after Total Knee Arthroplasty 
Background/rationale
Although TKA reliably reduces pain from knee osteoarthritis, full recovery of muscle strength and physical function to normal levels is rare. We presumed that a better understanding of acute changes in hamstrings and quadriceps muscle performance would allow us to enhance early rehabilitation after TKA and improve long-term function.
Questions/purposes
The purposes of this study were to (1) evaluate postoperative quadriceps and hamstrings muscle strength loss after TKA and subsequent recovery using the nonoperative legs and healthy control legs for comparison, and (2) measure hamstrings coactivation before and after TKA during a maximal isometric quadriceps muscle contraction and compare with nonoperative and healthy control legs.
Methods
We prospectively followed 30 patients undergoing TKA at 2 weeks preoperatively and 1, 3, and 6 months postoperatively and compared patient outcomes with a cross-sectional cohort of 15 healthy older adults. Bilateral, isometric strength of the quadriceps and hamstrings was assessed along with EMG measures of hamstrings coactivation during a maximal isometric quadriceps contraction.
Results
There were no differences in strength loss or recovery between the quadriceps and hamstrings muscles of the operative leg throughout the followup, although differences existed when compared with nonoperative and healthy control legs. Hamstrings muscle coactivation in the operative leg during a maximal quadriceps effort was elevated at 1 month (144.5%) compared to the nonoperative leg.
Conclusions
Although quadriceps dysfunction after TKA typically is recognized and addressed in postoperative therapy protocols, hamstrings dysfunction also is present and should be addressed.
Clinical Relevance
Quadriceps and hamstrings muscle strengthening should be the focus of future rehabilitation programs to optimize muscle function and long-term outcomes.
doi:10.1007/s11999-009-1219-6
PMCID: PMC2919870  PMID: 20087703
7.  Treatment of Knee Osteoarthritis in Relation to Hamstring and Quadriceps Strength 
Journal of Physical Therapy Science  2013;25(11):1401-1405.
[Purpose] To assess the effect of hamstring and quadriceps strengthening exercises on pain intensity, gait velocity, maximum isometric strength, and activities of daily living of patients with knee osteoarthritis (OA). [Subjects and Methods] A total of 20 patients with knee OA, 50 to 65 years of age (57.65 ± 4.78 years), received hot packs, strengthening exercises for the quadriceps and the hamstring muscles and stretching exercises for hamstring muscles. Outcome measures included: the Western Ontario and McMaster Universities OA index questionnaire (WOMAC) scores for assessing health status and health outcomes of knee OA; self-reported pain intensity scores, measured using a visual analogue scale; the 50 ft walk test (a measure of gait velocity and function); and handheld dynamometry (a tool used to measure maximum isometric strength of knee extension and flexion). [Results] There was a significant difference between pre- and post-intervention measures of pain intensity, 50 ft walk times, hamstring strength, and quadriceps strength. Significant differences in WOMAC measures were also observed in the subscales of pain, stiffness and physical function, as well as WOMAC total scores. [Conclusion] Strengthening the hamstring muscles in addition to strengthening the quadriceps muscles proved to be beneficial for perceived knee pain, range of motion, and decreasing the limitation of functional performance of patients with knee OA.
doi:10.1589/jpts.25.1401
PMCID: PMC3881465  PMID: 24396198
Hamstring/quadriceps ratio; Knee pain; Osteoarthritis
8.  Thigh Strength and Activation as Predictors of Knee Biomechanics During a Drop Jump Task 
Purpose
To examine whether normalized quadriceps and hamstring strength would predict quadriceps and hamstring muscle activation amplitudes, and whether these neuromuscular factors would predict knee kinematics and kinetics during a drop jump task.
Methods
39 females and 39 males were measured for isometric quadriceps and hamstring strength, and were instrumented to obtain sEMG, kinematic and kinetic measures during the initial landing of a drop jump. Multiple linear regressions first examined the relationship between thigh strength and activation, then examined whether these neuromuscular variables were predictive of hip and knee flexion excursions, knee extensor moments and anterior knee shear forces during the deceleration phase of the drop jump.
Results
Females versus males produced lower normalized thigh strength, and demonstrated greater quadriceps and hamstring activation amplitudes during the drop jump. Lower thigh muscle strength was a weak (males) to moderate (females) predictor of greater quadriceps activation amplitudes. However, thigh strength and activation were poor predictors of hip and knee joint excursions and knee extensor moments. Regardless of sex and thigh strength, anterior shear forces were greater in individuals who demonstrated less hip flexion and greater knee flexion excursions, and greater peak quadriceps activation and internal knee extensor moments during the landing.
Conclusions
While thigh muscle strength explained some of the variance in quadriceps and hamstring activation levels as measured with sEMG, we failed to support the hypothesis that these neuromuscular factors are strong predictors of sagittal plane hip and knee flexion excursions or knee extensor moments. Although greater quadriceps activation amplitude was a significant predictor of greater anterior tibial shear forces, its contribution was relatively small compared to kinematic and kinetic variables.
doi:10.1249/MSS.0b013e3181e3b3f
PMCID: PMC2744399  PMID: 19300140
Quadriceps dominance; ACL risk factors; landing biomechanics; peak torque to body weight
9.  Contribution of Hamstring Fatigue to Quadriceps Inhibition Following Lumbar Extension Exercise 
The purpose of this study was to determine the contribution of hamstrings and quadriceps fatigue to quadriceps inhibition following lumbar extension exercise. Regression models were calculated consisting of the outcome variable: quadriceps inhibition and predictor variables: change in EMG median frequency in the quadriceps and hamstrings during lumbar fatiguing exercise. Twenty-five subjects with a history of low back pain were matched by gender, height and mass to 25 healthy controls. Subjects performed two sets of fatiguing isometric lumbar extension exercise until mild (set 1) and moderate (set 2) fatigue of the lumbar paraspinals. Quadriceps and hamstring EMG median frequency were measured while subjects performed fatiguing exercise. A burst of electrical stimuli was superimposed while subjects performed an isometric maximal quadriceps contraction to estimate quadriceps inhibition after each exercise set. Results indicate the change in hamstring median frequency explained variance in quadriceps inhibition following the exercise sets in the history of low back pain group only. Change in quadriceps median frequency explained variance in quadriceps inhibition following the first exercise set in the control group only. In conclusion, persons with a history of low back pain whose quadriceps become inhibited following lumbar paraspinal exercise may be adapting to the fatigue by using their hamstring muscles more than controls.
Key PointsA neuromuscular relationship between the lumbar paraspinals and quadriceps while performing lumbar extension exercise may be influenced by hamstring muscle fatigue.QI following lumbar extension exercise in persons with a history of LBP group may involve significant contribution from the hamstring muscle group.More hamstring muscle contribution may be a necessary adaptation in the history of LBP group due to weaker and more fatigable lumbar extensors.
PMCID: PMC3818676  PMID: 24198683
Superimposed burst technique; electromyography; spectral median frequency; correlation and regression; low back pain
10.  Influence of Hamstring and Abdominal Muscle Activation on a Positive Ober's Test in People with Lumbopelvic Pain 
Physiotherapy Canada  2013;65(1):4-11.
ABSTRACT
Purpose: To assess the immediate effect of hamstring and abdominal activation on pain levels as measured by the Numeric Pain Scale (NPS) and hip range of motion as measured by Ober's Test in people with lumbopelvic pain. Methods: Thirteen participants with lumbopelvic pain and positive Ober's Tests completed an exercise developed by the Postural Restoration Institute™ to recruit hamstrings and abdominal muscles. Results: There was a significant increase in passive hip-adduction angles (p<0.01) and decrease in pain (p<0.01) immediately after the intervention. Conclusion: Specific exercises that activate hamstrings and abdominal muscles appear to immediately improve Ober's Test measurements and reduce pain as measured by the NPS in people with lumbo-pelvic pain. Hamstring/abdominal activation, rather than iliotibial band stretching, may be an effective intervention for addressing lumbopelvic pain and a positive Ober's Test.
doi:10.3138/ptc.2011-33
PMCID: PMC3563370  PMID: 24381375
hip; iliotibial band syndrome; pelvic pain; Ober's Test; postural balance; restauration posturale; douleurs lombo-pelviennes; syndrome de la bande ilio-tibiale; test d'Ober
11.  Acute Effects of Three Different Stretching Protocols on the Wingate Test Performance 
The purpose of this study was to examine the acute effects of different stretching exercises on the performance of the traditional Wingate test (WT). Fifteen male participants performed five WT; one for familiarization (FT), and the remaining four after no stretching (NS), static stretching (SS), dynamic stretching (DS), and proprioceptive neuromuscular facilitation (PNF). Stretches were targeted for the hamstrings, quadriceps, and calf muscles. Peak power (PP), mean power (MP), and the time to reach PP (TP) were calculated. The MP was significantly lower when comparing the DS (7.7 ± 0.9 W/kg) to the PNF (7.3 ± 0.9 W/kg) condition (p < 0.05). For PP, significant differences were observed between more comparisons, with PNF stretching providing the lowest result. A consistent increase of TP was observed after all stretching exercises when compared to NS. The results suggest the type of stretching, or no stretching, should be considered by those who seek higher performance and practice sports that use maximal anaerobic power.
Key points
The mean power was significantly lower when comparing dynamic stretching.to proprioceptive neuromuscular facilitation.
For peak power, significant differences were observed between more comparisons, with proprioceptive neuromuscular facilitation stretching providing the lowest result.
A consistent increase of time to reach the peak was observed after all stretching exercises when compared to non-stretching.
The type of stretching, or no stretching, should be considered by those who seek higher performance and practice sports that use maximal anaerobic power.
PMCID: PMC3737835  PMID: 24149116
Static stretching; proprioceptive neuromuscular facilitation; dynamic stretching; anaerobic power
12.  Quantitative study of the effect of L-dopa and phenoxybenzamine on the rigidity of Parkinson's disease 
The static and dynamic components of the tonic stretch reflex and shortening reactions have been studied in biceps brachii, triceps, hamstrings, and quadriceps muscles of 19 patients with Parkinson's disease before and during L-dopa therapy. Clinical improvement during L-dopa administration correlated with a reduction in the dynamic component of the tonic stretch reflex in biceps, triceps, and quadriceps but not the hamstrings muscle, and with the static component of the tonic stretch reflex in biceps and triceps but not quadriceps and hamstrings muscles. The only shortening reaction consistently reduced during the L-dopa treatment period was the dynamic shortening reaction of the triceps muscle. The responses of the stretch reflexes to changes in muscle length were not altered by L-dopa therapy. In severely disabled patients the hamstrings and quadriceps stretch reflexes were maximal in a position of partial flexion of the knee joint and this response to muscle length was not altered in these patients despite a dramatic lessening of rigidity in some patients. Shortening reactions commonly remained in some patients after the stretch reflex of the antagonistic muscle was abolished by L-dopa therapy. The administration of phenoxybenzamine to patients being treated with L-dopa reduced the different components of rigidity more than L-dopa alone, and the responses were consistent with phenoxybenzamine suppressing predominantly the descending noradrenergic pathway.
PMCID: PMC494328  PMID: 4714094
13.  Eccentric exercise and delayed onset muscle soreness of the quadriceps induce adjustments in agonist–antagonist activity, which are dependent on the motor task 
This study investigates the effects of eccentric exercise and delayed onset muscle soreness (DOMS) of the quadriceps on agonist–antagonist activity during a range of motor tasks. Ten healthy volunteers (age, mean ± SD, 24.9 ± 3.2 years) performed maximum voluntary contractions (MVC) and explosive isometric contractions of the knee extensors followed by isometric contractions at 2.5, 5, 10, 15, 20, and 30% MVC at baseline, immediately after and 24 h after eccentric exercise of the quadriceps. During each task, force of the knee extensors and surface EMG of the vasti and hamstrings muscles were recorded concurrently. Rate of force development (RFD) was computed from the explosive isometric contraction, and the coefficient of variation of the force (CoV) signal was estimated from the submaximal contractions. Twenty-four hours after exercise, the subjects rated their perceived pain intensity as 4.1 ± 1.2 (score out of 10). The maximum RFD and MVC of the knee extensors was reduced immediately post- and 24 h after eccentric exercise compared to baseline (average across both time points: 19.1 ± 17.1% and 11.9 ± 9.8% lower, respectively, P < 0.05). The CoV for force during the submaximal contractions was greater immediately after eccentric exercise (up to 66% higher than baseline, P < 0.001) and remained higher 24 h post-exercise during the presence of DOMS (P < 0.01). For the explosive and MVC tasks, the EMG amplitude of the vasti muscles decreased immediately after exercise and was accompanied by increased antagonist EMG for the explosive contraction only. On the contrary, reduced force steadiness was accompanied by a general increase in EMG amplitude of the vasti muscles and was accompanied by increased antagonist activity, but only at higher force levels (>15% MVC). This study shows that eccentric exercise and subsequent DOMS of the quadriceps reduce the maximal force, rate of force development and force steadiness of the knee extensors, and is accompanied by different adjustments of agonist and antagonist muscle activities.
doi:10.1007/s00221-011-2942-2
PMCID: PMC3262141  PMID: 22094715
Delayed onset muscle soreness; Eccentric exercise; Muscle damage
14.  Decrease in eccentric hamstring strength in runners in the Tirol Speed Marathon 
British Journal of Sports Medicine  2006;40(10):850-852.
Background
The local muscular endurance of knee flexors, during eccentric work in particular, is important in preventing or delaying kinematic changes associated with fatigue during treadmill running. This result, however, may not be transferable to overground running.
Objective
To test the hypothesis that overground running is associated with eccentric hamstring fatigue.
Methods
Thirteen runners (12 male and one female) performed an isokinetic muscle test three to four days before and 18 hours after a marathon. Both legs were tested. The testing protocol consisted of concentric and eccentric quadriceps and hamstring contractions.
Results
There were no significant differences between peak torque before and after the race, except that eccentric peak hamstring torque (both thighs) was reduced.
Conclusion
Overground running (running a marathon) is associated with eccentric hamstring fatigue. Eccentric hamstring fatigue may be a potential risk factor for knee and soft tissue injuries during running. Eccentric hamstring training should therefore be introduced as an integral part of the training programme of runners.
doi:10.1136/bjsm.2006.028175
PMCID: PMC2465081  PMID: 16825267
fatigue; concentric; eccentric; hamstring; musculoskeletal injury
15.  Alternating Consecutive Maximum Contraction as a Test of Muscle Function in Athletes Following ACL Reconstruction 
The novel test based on isometric alternating consecutive maximal contractions performed by two antagonistic muscles has been recently proposed as a test of muscle function in healthy subjects. The aim of this study was to evaluate reliability and sensitivity of a novel test as a test of knee muscles function in athletes recovering from anterior cruciate ligament reconstruction. Fifteen male athletes with recent ligament reconstruction (4.0 ± 0.1 months following the surgery) and 15 sport and physical education students participated in the study. Peak torques of the quadriceps and hamstring muscles assessed both through the alternating consecutive maximal contractions and standard isokinetic test performed at 60 º/s and 180 º/s served for calculation of the hamstrings-to-quadriceps ratio and the bilateral difference in strength. When applied on individuals recovering from anterior cruciate ligament reconstruction, the novel test revealed a high within-day reliability and sensitivity for detecting imbalances both between antagonistic and between contralateral muscles. The present findings suggest that alternating consecutive maximal contractions could be used as a test of muscle function that is either complementary or alternative to the isokinetic test, particularly in the laboratories where the isokinetic devices are not available. Potential advantages of the novel test could be both a brief testing procedure and a possibility to conduct it using relatively inexpensive devices such as custom made kits containing a single one-axis force transducer.
doi:10.2478/v10078-012-0074-9
PMCID: PMC3588695  PMID: 23487010
Strength; Knee; flexor; extensor; Rehabilitation
16.  The Effect of Stimulus Anticipation on the Interpolated Twitch Technique 
The objective of this study was to investigate the effect of expected and unexpected interpolated stimuli (IT) during a maximum voluntary contraction on quadriceps force output and activation. Two groups of male subjects who were either inexperienced (MI: no prior experience with IT tests) or experienced (ME: previously experienced 10 or more series of IT tests) received an expected or unexpected IT while performing quadriceps isometric maximal voluntary contractions (MVCs). Measurements included MVC force, quadriceps and hamstrings electromyographic (EMG) activity, and quadriceps inactivation as measured by the interpolated twitch technique (ITT). When performing MVCs with the expectation of an IT, the knowledge or lack of knowledge of an impending IT occurring during a contraction did not result in significant overall differences in force, ITT inactivation, quadriceps or hamstrings EMG activity. However, the expectation of an IT significantly (p ¼ 0.0001) reduced MVC force (9.5%) and quadriceps EMG activity (14.9%) when compared to performing MVCs with prior knowledge that stimulation would not occur. While ME exhibited non-significant decreases when expecting an IT during a MVC, MI force and EMG activity significantly decreased 12.4% and 20.9% respectively. Overall, ME had significantly (p ¼ 0.0001) higher force (14.5%) and less ITT inactivation (10.4%) than MI. The expectation of the noxious stimuli may account for the significant decrements in force and activation during the ITT.
Key pointsA single orientation session may not be adequate for a valid estimation of muscle activation using the ITT.The expectation of an electrical stimulation whether delivered or not can impair performance.The validity of the ITT for estimating the extent of full muscle activation must be viewed with caution, since the expectation of IT discomfort may inhibit the individual's ability to exert maximum force, especially with inexperienced participants.
PMCID: PMC3761925  PMID: 24149960
Muscle activation; electromyography; evoked stimulation
17.  Regeneration of hamstring tendons after anterior cruciate ligament reconstruction 
Purpose
Primary aim of the study was analysis of hamstring tendon regeneration after anterior cruciate ligament reconstruction (ACLR). Secondary aim was analysis of isokinetic muscle strength in relation to hamstring regeneration. The hypothesis was that regeneration of hamstring tendons after ACLR occurs and that regenerated hamstring tendons contribute to isokinetic hamstring strength with regeneration distal to the knee joint line.
Methods
Twenty-two patients scheduled for ACLR underwent prospective MRI analysis of both legs. MRI parameters were tendon regeneration and morphology, muscle retraction and muscle cross-sectional area. A double-blind, prospective analysis of isokinetic quadriceps and hamstrings strength was performed.
Results
Regeneration of the gracilis tendon after ACLR occurred in all patients. Regeneration of the semitendinosus tendon occurred in 14 patients. At 1 year, the surface area of the semitendinosus and gracilis muscle decreased compared to both preoperatively (P < 0.01) and the contralateral leg (P < 0.01). The cross-sectional area of the semitendinosus muscle decreased in the absence of tendon regeneration (P = 0.05). The cross-sectional area of the gracilis muscle was greater in case of regeneration distal to the joint line (P = 0.01). Muscle retraction of the semitendinosus muscle was increased in case of nonregeneration (P = 0.02). There was no significant relationship between isokinetic flexion strength and tendon regeneration.
Conclusion
Hamstring tendons regenerated after harvest of both semitendinosus and gracilis tendons for ACLR. There was no relation between isokinetic flexion strength and tendon regeneration.
Level of evidence
Prognostic study, Level II.
doi:10.1007/s00167-012-2125-0
PMCID: PMC3604585  PMID: 22763570
Anterior cruciate ligament reconstruction; Hamstring; Semitendinosus; Gracilis; Regeneration; MRI
18.  Influence of Hip-Flexion Angle on Hamstrings Isokinetic Activity in Sprinters 
Journal of Athletic Training  2012;47(4):390-395.
Context
Hamstrings strains are common and debilitating injuries in many sports. Most hamstrings exercises are performed at an inadequately low hip-flexion angle because this angle surpasses 70° at the end of the sprinting leg's swing phase, when most injuries occur.
Objective
To evaluate the influence of various hip-flexion angles on peak torques of knee flexors in isometric, concentric, and eccentric contractions and on the hamstrings-to-quadriceps ratio.
Design
Descriptive laboratory study.
Setting
Research laboratory.
Patients and Other Participants
Ten national-level sprinters (5 men, 5 women; age = 21.2 ± 3.6 years, height = 175 ± 6 cm, mass = 63.8 ± 9.9 kg).
Intervention(s)
For each hip position (0°, 30°, 60°, and 90° of flexion), participants used the right leg to perform (1) 5 seconds of maximal isometric hamstrings contraction at 45° of knee flexion, (2) 5 maximal concentric knee flexion-extensions at 60° per second, (3) 5 maximal eccentric knee flexion-extensions at 60° per second, and (4) 5 maximal eccentric knee flexion-extensions at 150° per second.
Main Outcome Measure(s)
Hamstrings and quadriceps peak torque, hamstrings-to-quadriceps ratio, lateral and medial hamstrings root mean square.
Results
We found no difference in quadriceps peak torque for any condition across all hip-flexion angles, whereas hamstrings peak torque was lower at 0° of hip flexion than at any other angle (P < .001) and greater at 90° of hip flexion than at 30° and 60° (P < .05), especially in eccentric conditions. As hip flexion increased, the hamstrings-to-quadriceps ratio increased. No difference in lateral or medial hamstrings root mean square was found for any condition across all hip-flexion angles (P > .05).
Conclusions
Hip-flexion angle influenced hamstrings peak torque in all muscular contraction types; as hip flexion increased, hamstrings peak torque increased. Researchers should investigate further whether an eccentric resistance training program at sprint-specific hip-flexion angles (70° to 80°) could help prevent hamstrings injuries in sprinters. Moreover, hamstrings-to-quadriceps ratio assessment should be standardized at 80° of hip flexion.
PMCID: PMC3396298  PMID: 22889654
injury prevention; eccentric exercises; length-tension relationship; hamstrings-to-quadriceps ratio; muscle strains
19.  Contributors to Fatigue Resistance of the Hamstrings and Quadriceps in Cerebral Palsy 
Background
The purpose of this study was to elucidate relationships between quadriceps and hamstrings voluntary muscle fatigue and upper motor lesion impairments in cerebral palsy in order to gain a better understanding of their contribution to the observed fatigue resistance.
Methods
Seventeen ambulatory subjects with cerebral palsy (mean age: 17.0, SD = 4.8 years) were recruited. Quantitative measures of strength, spasticity, cocontraction, and stiffness for both muscle groups were collected on an isokinetic dynamometer and entered in a factor analysis. The resulting factors were used as independent variables in a multiple regression analysis with quadriceps and hamstrings fatigue as dependent variables.
Findings
Five independent factors explained 90% of the variance. In order of loadings, higher hamstring cocontraction and spasticity and lower hamstring strength were associated with lower levels of hamstring fatigue. Higher quadriceps cocontraction and lower quadriceps strength were the most predictive of lower levels of quadriceps fatigue.
Interpretation
Greater motor impairments of the agonist muscle, particularly cocontraction, spasticity, and weakness, were associated with lower rates of muscle fatigue of the same muscle during performance of a voluntary fatigue protocol for the hamstrings and quadriceps. Muscles are highly adaptable; therefore, the results of this study suggest that the observed fatigue resistance may be due to the effect of the primary neural insult on motor unit recruitment and rate modulation or the result of secondary adaptations to spasticity, weakness, or excessive cocontraction.
doi:10.1016/j.clinbiomech.2009.01.012
PMCID: PMC2727679  PMID: 19264384
Muscle Fatigue; Muscle Strength; Rehabilitation; Muscle Spasticity; Cocontraction; Weakness
20.  Fatigue Effects on Knee Joint Stability During Two Jump Tasks in Women 
Dynamic knee joint stability may be affected by the onset of metabolic fatigue during sports participation that could increase the risk for knee injury. The purpose of this investigation was to determine the effects of metabolic fatigue on knee muscle activation, peak knee joint angles, and peak knee internal moments in young women during 2 jumping tasks. Fifteen women (mean age: 24.6 ± 2.6 years) participated in one nonfatigued session and one fatigued session. During both sessions, peak knee landing flexion and valgus joint angles, peak knee extension and varus/valgus internal moments, electromyographic (EMG) muscle activity of the quadriceps and hamstrings, and quadriceps/hamstring EMG cocontraction ratio were measured. The tasks consisted of a single-legged drop jump from a 40-cm box and a 20-cm, up-down, repeated hop task. The fatigued session included a Wingate anaerobic protocol followed by performance of the 2 tasks. Although participants exhibited greater knee injury–predisposing factors during the fatigued session, such as lesser knee flexion joint angles, greater knee valgus joint angles, and greater varus/valgus internal joint moments for both tasks, only knee flexion during the up-down task was statistically significant (p = 0.028). Metabolic fatigue may perhaps predispose young women to knee injuries by impairing dynamic knee joint stability. Training strength-endurance components and the ability to maintain control of body movements in either rested or fatigued situations might help reduce injuries in young women athletes.
doi:10.1519/JSC.0b013e3181c7c5d4
PMCID: PMC2869370  PMID: 20300024
ACL; hop; landing; neuromuscular; drop jump
21.  A Comparison of the Immediate Effects of Eccentric Training vs Static Stretch on Hamstring Flexibility in High School and College Athletes 
Background
A pre-event static stretching program is often used to prepare an athlete for competition. Recent studies have suggested that static stretching may not be an effective method for stretching the muscle prior to competition.
Objective
The intent of this study was to compare the immediate effect of static stretching, eccentric training, and no stretching/training on hamstring flexibility in high school and college athletes.
Methods
Seventy-five athletes, with a mean age of 17.22 (+/- 1.30) were randomly assigned to one of three groups - thirty- second static stretch one time, an eccentric training protocol through a full range of motion, and a control group. All athletes had limited hamstring flexibility, defined as a 20° loss of knee extension measured with the femur held at 90° of hip flexion.
Results
A significant difference was indicated by follow up analysis between the control group (gain = -1.08°) and both the static stretch (gain = 5.05°) and the eccentric training group (gain = 9.48°). In addition, the gains in the eccentric training group were significantly greater than the static stretch group.
Discussion and Conclusion
The findings of this study reveal that one session of eccentrically training through a full range of motion improved hamstring flexibility better than the gains made by a static stretch group or a control group.
PMCID: PMC2953312  PMID: 21522215
22.  Hamstrings Activity During Knee Extensor Strength Testing: Effects of Burst Superimposition 
Quadriceps muscle strength is often used as a criterion for functional progression and return to activity after knee joint injury or surgery. Previous research has demonstrated that noteworthy antagonist activity is present during knee strength testing. the countermoment associated with this antagonist muscle activity may lead to an underestimation of knee strength. the burst superimposition method of strength testing is considered by some to be the current gold standard. the effect of burst super-imposition on antagonist activity is unknown. the purpose of this study was to test the hypothesis that burst superimposition diminishes antagonistic hamstrings activity during knee extensor strength testing. Isometric knee strength testing was performed in 22 (11 males, 11 females) active young people with no history of serious lower extremity injuries using the burst superimposition method. the magnitude of hamstrings muscle activity was assessed just before and after burst superimposition. contrary to our hypothesis, a small, but statistically significant increase in antagonistic medial hamstrings activity was observed with burst superimposition (7.23 vs. 9.62; P < 0.001). Higher lateral hamstrings activity was also observed, but this did not reach statistical significance (15.03 vs. 13.50; P = 0.087). though statistically significant, the small increase in hamstrings activity is unlikely to be clinically meaningful.
PMCID: PMC2603347  PMID: 19223946
23.  Skeletal Muscle Strength and Endurance in Recipients of Lung Transplants 
Purpose: Exercise limitation in recipients of lung transplant may be a result of abnormalities in the skeletal muscle. However, it is not clear whether these abnormalities are merely a reflection of the changes observed in the pretransplant condition. The purpose of this paper was to compare thigh muscle volume and composition, strength, and endurance in lung transplant recipients to people with chronic obstructive pulmonary disease (COPD).
Methods: Single lung transplant recipients (n=6) and people with COPD (n=6), matched for age, sex, and BMI participated in the study. Subjects underwent MRI to determine muscle size and composition, lower extremity strength testing and an isometric endurance test of the quadriceps.
Results: Lung transplant recipients had similar muscle volumes and intramuscular fat infiltration of their thigh muscles and similar strength of the quadriceps and hamstrings to people with COPD who had not undergone transplant. However, quadriceps endurance tended to be lower in transplant recipients compared to people with COPD (15 ± 7 seconds in transplant versus 31 ± 12 seconds in COPD, p = 0.08).
Conclusions: Recipients of lung transplant showed similar changes in muscle size and strength as people with COPD, however muscle endurance tended to be lower in people with lung transplants. Impairments in muscle endurance may reflect the effects of immunosuppressant medications on skeletal muscle in people with lung transplant.
PMCID: PMC2845229  PMID: 20467503
lung transplant; muscle function; MRI
24.  Flexibility is not Related to Stretch-Induced Deficits in Force or Power 
Previous studies have demonstrated that an acute bout of static stretching may cause significant performance impairments. However, there are no studies investigating the effect of prolonged stretch training on stretch-induced decrements. It was hypothesized that individuals exhibiting a greater range of motion (ROM) in the correlation study or those who attained a greater ROM with flexibility training would experience less stretch-induced deficits. A correlation study had 18 participants (25 ± 8.3 years, 1.68 ± 0.93 m, 73.5 ± 14.4 kg) stretch their quadriceps, hamstrings and plantar flexors three times each for 30 s with 30 s recovery. Subjects were tested pre- and post-stretch for ROM, knee extension maximum voluntary isometric contraction (MVIC) force and drop jump measures. A separate training study with 12 subjects (21.9 ± 2.1 years, 1.77 ± 0.11 m 79.8 ± 12.4 kg) involved a four-week, five-days per week, flexibility training programme that involved stretching of the quadriceps, hamstrings and plantar flexors. Pre- and post-training testing included ROM as well as knee extension and flexion MVIC, drop and countermovement jump measures conducted before and after an acute bout of stretching. An acute bout of stretching incurred significant impairments for knee extension (-6.1% to -8.2%; p < 0.05) and flexion (-6.6% to -10.7%; p < 0.05) MVIC, drop jump contact time (5.4% to 7.4%; p < 0.01) and countermovement jump height (-5.5% to -5.7%; p < 0.01). The correlation study showed no significant relationship between ROM and stretch-induced deficits. There was also no significant effect of flexibility training on the stretch-induced decrements. It is probable that because the stretches were held to the point of discomfort with all testing, the relative stress on the muscle was similar resulting in similar impairments irrespective of the ROM or tolerance to stretching of the muscle.
Key PointsA correlation and training study were used to examine the effects of increased range of motion on stretch-induced changes in force and jump measuresAn acute bout of stretching incurred significant impairments for knee extension and flexion MVIC, drop jump contact time and countermovement jump height.Neither study showed any significant relationship between ROM and stretch-induced deficits.
PMCID: PMC3818672  PMID: 24198679
Flexibility; force; jumps; static stretching
25.  Effects of Vibratory Stimulations on Maximal Voluntary Isometric Contraction from Delayed Onset Muscle Soreness 
Journal of Physical Therapy Science  2013;25(9):1093-1095.
[Purpose] The aim of this study was to investigate the effect of vibratory stimulation on maximal voluntary isometric contraction (MVIC) from delayed onset muscle soreness (DOMS). [Subjects] Sixty healthy adults participated in this study. The exclusion criteria were orthopedic or neurologic disease. [Methods] The researchers induced DOMS in the musculus extensor carpi radialis longus of each participant. Subjects in the control group received no treatment. The ultrasound group received ultrasound treatment (intensity, 1.0 W/cm2; frequency 1 MHz; time, 10 minutes). The vibration group received vibration stimulation (frequency, 20 MHz; time, 10 minutes). Maximal voluntary isometric contraction (MVIC) was recorded at baseline, immediately after exercise, and 24, 48, and 72 hours after exercise. [Results] MVIC measurements showed statistically significant differences in the vibration group compared with the control group. [Conclusion] Vibratory stimulation had a positive effect on recovery of muscle function from DOMS.
doi:10.1589/jpts.25.1093
PMCID: PMC3818754  PMID: 24259922
Vibratory stimulation; Ultrasound; Isometric contraction

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