Frost et al

The stated purpose of this study22 was to assess co-contraction of three different age groups of children during walking and jogging and to compare the magnitude of co-contraction among them. The investigation evaluated a total of 30 total healthy, active subjects with 10 subjects in each age group (7–8, 10–12 and 15–16 years). Five different treadmill speeds (two walking and three jogging) were used for each age group, with one walking and two jogging speeds in common between adjacent age groups. Surface electromyography (EMG) electrodes (Ag/AgCl) were placed on the vastus lateralis and “middle of the hamstring group” (no specific muscle was identified in the paper) with an interelectrode distance of 4 cm. Raw EMG was first normalised to the maximum value obtained during either the treadmill trials or maximum voluntary contraction (MVC) trials. A co-contraction index, dimensionless value, was calculated based on overlaying the linear envelopes of the vastus lateralis and hamstring, calculating the area of overlap and dividing by the number of data points. Frost et al.22 found when comparing the running speeds at the same relative metabolic intensity (% VO_2max) the co-contraction index was higher in younger compared with older age groups. They concluded that co-contraction was an important component of age-related differences in VO₂, which was possibly used to enhance joint stability at the younger age.

Croce et al

This study24 examined the differences between prepubescent and postpubescent male and female subjects. The authors stated that different stabilisation patterns might be a causative factor related to ACL injuries. Two groups of subjects were studied based on age and were classified as prepubescent (7–10-year-old girls, and 8–11-year-old boys) or post-pubescent (19–29-year-old men and women). A two-foot vertical jump (50% of maximum) and landing was analysed with the subject landing with the dominant foot on a force platform. EMG surface electrodes (Ag/AgCl, pre-gelled) were placed on the biceps femoris, semimembranosus/semitendinosus and vastus medialis. The electrodes were located 2.5 cm apart, parallel to the muscle fibres and over the midline between the motor end plate and tendon. The data from the hamstring muscle sites were averaged. Mean amplitude root mean square (RMS) was calculated at three different time intervals during the trials (100 ms before contact, 100 ms after contact and from contact to maximum knee flexion). The EMG signal was normalised to the highest signal during the landing phase of each trial. Co-contraction ratio was calculated by dividing hamstring by quadricep EMG activity. Co-contraction ratio was higher in the postpubescent subjects before landing; however, after landing, the co-contraction ratio was higher in prepubescent subjects. No gender differences were found in co-contraction during the vertical jump. The authors concluded that post-pubescent subjects rely more on hamstring activation before landing (preactivation), whereas prepubescent subjects rely more on hamstring activation during landing (reflexive activation pattern). This would seem to relate to a motor learning strategy throughout skill development of preactivation to stabilise the joint before the high ground reaction forces and joint load, which exist during landing.

Hamstra-Wright et al

The purposes of this study6 were to assess dynamic neuromuscular restraint differences between high-skilled and low-skilled prepubescent children and to determine the contributions of sport experience and physical characteristics to motor skill. The authors hypothesised that high-skilled and male subjects would have greater co-contraction (hamstrings and quadriceps) than low skilled and female subjects. In total, 36 prepubescent children were enrolled in this study (19 girls and 17 boys). High skill and low skill were determined based on performance of battery of 12 fundamental motor skills on the Test of Gross Motor Development, second edition (TGMD-2). Subjects above the overall mean were classified as high-skilled, while subjects below the mean were classified as low-skilled. Surface EMG electrodes (bipolar Ag/AgCl) were placed on the vastus medialis and medial hamstring. EMG was normalised to the highest activity during each trial (150 ms before ground contact to 250 ms after, inclusive). Preparatory co-contraction was determined as the medial hamstring area divided by the vastus medialis area 150 ms before ground contact during a drop jump (24 cm). Vertical leg stiffness was also determined during the drop jumps based on a simple spring-mass model from the vertical ground reaction force measures. Low-skilled subjects had significantly greater (48%) preparatory co-contraction during the drop jump compared with high-skilled subjects. No differences were found in vertical leg stiffness between skill or gender groups. The authors concluded, based on the higher co-contraction in low-skilled subjects, that, in addition to the lack of gender differences, skill level affects neuromuscular control. They further suggest that girls may develop risk factors that predispose them to knee injuries between pre-puberty and post-puberty. The authors state that this was an “unrefined motor skill” that may compromise knee joint stability. This conclusion is in direct contrast to the study hypothesis that high-skilled athletes would exhibit greater preparatory co-contraction.

da Fonseca et al

This study21 compared muscular co-contraction levels among male and female athletic and sedentary subjects during walking and landing from a jump (30 cm). They hypothesised that females (athletic and sedentary) would have lower co-contraction levels than males. The hypothesis was based on both the higher ACL injury rates in females and the possible joint protective mechanism that co-contraction may have due to increased joint stiffness. Nine subjects in each group participated, with their activity level classified based on the Cincinnati Knee Rating Scale (level I, participate in sports activities 4–7 days a week; level IV, do not participate in any sports activities). Surface EMG electrodes (bipolar Ag/AgCl) were placed over the vastus lateralis and biceps femoris with co-contraction index calculated similar to Frost et al22 during both walking and landing from a jump. EMG was filtered with a bandpass filter (10 and 500 Hz) and normalised to MVC collected from a dynamometer. Co-contraction index was only calculated for the 150 ms time period before initial contact. There were no differences in preparatory co-contraction index during landing movements between genders or activity levels. However, during walking, the sedentary females had higher co-contraction levels compared with athletic females. The authors suggest that sedentary women may compensate for weakness with higher co-contraction levels to generate appropriate joint stability.

Sigward and Powers

This study23 evaluated the effect of soccer experience on knee kinematics, kinetics and muscle activation patterns during side step cuts. One hypothesis of this study was that novice female athletes would exhibit increased quadriceps activation and decreased hamstring activation. Thirty young female athletes (14–16 years old) were classified as experienced (n = 15) or novice (n = 15) based on years of experience playing soccer. Surface EMG electrodes were placed on the vastus lateralis, biceps femoris and semimembranousus. EMG was filtered with a band-pass Butterworth filter (20–500 Hz) and a 60 Hz notch filter. The data were processed with RMS smoothing (75 ms window), and normalised to maximum voluntary isometric contraction collected before the cutting trials. Co-contraction was calculated, based on Besier et al,26 by dividing the average hamstring activation by the average quadriceps activation during the initial 20% of the cutting manoeuvre stance phase (early deceleration). Knee joint kinematics and kinetics were also assessed during the same period. During early deceleration, the novice athletes had a significantly larger co-contraction index than the experienced athletes. However, the internal knee flexor moment was significantly increased in the experienced athletes compared with the novice athletes. There was also a negative correlation (R =  −0.32) between years of experience and co-contraction ratio. The authors concluded that with experience, the side-step cutting (kinetics and muscle co-contraction) task is performed in a more at-risk pattern for a non-contact knee injury. The relationship between skill acquisition (increased co-contraction) and potential at-risk patterns (decreased co-contraction) is difficult to address in one study and should be investigated in conjunction with neuromuscular training programmes.

Russell et al

In a follow-up study25 to another previously reviewed24, Russel et al evaluated knee muscular activation differences in children and adults (male and female) during landing from a normal and perturbed vertical jump. The authors hypothesised that co-contraction gender differences would be evident with the addition of a perturbed landing in this study25, in contrast to their previous work,24 which found no gender differences during vertical jump landing. They also hypothesised that adults would exhibit a higher co-contraction ratio during the preparatory landing phase and that children would have higher co-contraction during the landing, as previously found.24 EMG surface electrodes (Ag/AgCl pre-gelled) were placed on the biceps femoris, semimembranosus/semitendinosus and vastus medialis. The electrodes were located 2.5 cm apart, parallel to the muscle fibres and over the midline between the motor end plate and tendon. The data from the hamstring muscle sites were averaged. Mean amplitude RMS was calculated at three different time intervals during the trials (100 ms before contact, 100 ms after contact, and from contact to maximum knee flexion). The EMG signal was normalised to the highest signal during the landing phase of each trial. Co-contraction ratio was calculated by dividing hamstring by quadriceps EMG activity. A vertical jump was performed at 50% of the subject’s maximum jump height during a normal condition (ball placed at subject’s midline) and perturbed condition (ball offset 45.7 cm for adults and 30.5 cm for children from midline). The results of the study indicated that during the preparatory phase (100 ms before initial contact) the co-contraction ratio was higher in adults compared with children. This was explained by higher hamstring activity relative to quadriceps in adults before landing. No gender differences were found with or without the landing perturbation. The authors suggest that increased co-contraction during the preparatory phase is probably a learned feed-forward mechanism, as the children did not exhibit similar co-contraction levels.

Methodological and technical considerations

Muscular co-contraction results vary among studies based on the population and movement task. However, it is clear that technically, the calculation of a co-contraction index or ratio can be extremely complex and vary markedly among studies. Typically, surface EMG is used to obtain agonist and antagonist muscular activation patterns. The EMG signal is the electrical representation of neuromuscular activation focused at the neuromuscular endplate related to a contracting muscle.2 Both extrinsic and intrinsic factors may influence the EMG signal that is detected and recorded.28 Extrinsic factors may include the electrode configuration, electrode location relative to motor points and lateral edge of muscle and electrode orientation with respect to muscle fibres, among other potential factors.28 Intrinsic factors, such as the number of active motor units, muscle fibre type, blood flow in the muscle, fibre diameter, depth and location of active muscle fibres, and the amount of tissue between surface of muscle and electrode are other important factors that can affect detectable signal intensity.28

In addition to the number of anatomical, physiological and technical factors that may affect recorded EMG signals, the specific processing and analysis of the signal are also important to the reported index or ratio.29 30 Kellis30 reviewed several factors specifically related to quantification of antagonist activation around the knee. The method of normalising an EMG signal is an important factor that is often performed differently among studies.29 30 Antagonist EMG is typically normalised to reduce variability and/or to relate the signal contribution to the resultant joint moment.30 Kellis30 provided discussion in his review of how the method of normalisation can alter reported results and suggested that standardised methods be considered; a careful interpretation of normalised EMG data was recommended.

Alternative methods

Alternative methods of examining co-contraction have used joint moment analyses, leg and vertical stiffness calculations and model-based estimation of muscle forces. Calculation of joint moments through inverse dynamics incorporates the net forces that act about the joint.7 One limitation of net joint moment analysis is that it does not indicate which muscles are active or what are the individual muscle forces generated at any specific point in time. Therefore, cautious interpretation of the joint moment to the actual muscle forces is necessary. Joint stiffness calculations involve the resistance of a mechanical stretch by an applied force.31 Joint stability requires muscle stiffness and may relate to musculoskeletal injury potential.31 Padua et al.31 found that female subjects had lower leg stiffness values and higher quadriceps to hamstring co-contraction compared with male subjects Although once normalised to mass, the stiffness values were not different, female subjects exhibited greater quadriceps activation than did male subjects.

Other techniques estimate individual muscle forces through computational models. The agonist and antagonist muscle groups can then be further investigated to address co-contraction during different movements. These computer models represent varying degrees of complexity and require detailed information regarding the muscle and skeletal systems.32 Erdemir et al reviewed the various techniques for this type of modelling and provided recommendations for clinical applications33 These authors suggest that when a clinical problem involves co-contraction and muscle forces cannot be adequately interpreted based on EMG or joint torques, that model-based estimation of individual muscle forces may be necessary.33 A combination of methods is likely to be necessary to gain a better understanding of how the neuromuscular system uses co-contraction for joint stabilisation and coordinated movements in a timely manner.

Conclusions and further recommendations

Longitudinal study designs should be used to address motor control and learning adaptations that may occur throughout development related to co-contraction of hamstring and quadriceps musculature for both dynamic joint stability and efficient movement patterns. Neuromuscular injury prevention studies may also be beneficial to help address the changes that occur in co-contraction during dynamic activities. The relationship between adequate dynamic joint stability and efficient movement patterns is complex. In high joint force and torque motions, where the hamstring are activated to increase joint stiffness and stabilise the knee, the effectiveness of the quadriceps may be decreased, requiring greater work and reduction of the efficiency of movement. If individual muscle forces can be estimated during these dynamic movements, then the movement control and loading on the ligaments may be better understood.33

From the systematic review we conclude the following.

(1) Antagonistic co-contraction of hamstring musculature is evident during dynamic closed chained movements.

(2) Conflicting relationships of co-contraction to age and skill have been reported.

(3) Standardisation of EMG-based methods is an important component for measures of co-contraction based on technical and practical limitations. Alternative methods should be considered and validated.