Quadriceps and hamstrings coactivation in exercises used in prevention and rehabilitation of hamstring strain injury in young soccer players

This study aimed to study the co-activation of hamstring-quadriceps muscles during submaximal strength exercises without the use of maximum voluntary isometric contraction testing and compare (i) the inter-limb differences in muscle activation, (ii) the intra-muscular group activation pattern, and (iii) the activation during different phases of the exercise. Muscle activation was recorded by surface electromyography of 19 elite male youth players. Participants performed five repetitions of the Bulgarian squat, lunge and the squat with an external load of 10 kg. Electrical activity was recorded for the rectus femoris, vastus medialis, vastus lateralis, biceps femoris and semitendinosus. No significant inter-limb differences were found (F1, 13=619; p=0.82; partial η2=0.045). Significant differences were found in the muscle activation between different muscles within the muscle group (quadriceps and hamstrings) for each of the exercises: Bulgarian squat (F1,18=331: p<0.001; partial η2=0.80), lunge (F4,72=114.5; p<0.001; partial η2=0.86) and squat (F1,16=247.31; p<0.001; partial η2=0.93).Differences were found between the concentric, isometric and eccentric phases of each of the exercises (F2, 26=52.27; p=0.02; partial η2=0.80). The existence of an activation pattern of each of the muscles in the three proposed exercises could be used for muscle assessment and as a tool for injury recovery.

99 2) Muscle activation is different according to the phases of movement.
100 3) There is a different pattern of intra-hamstrings and intra-quadriceps muscle group co-101 activation for each exercise.
102 Material and methods 103 Participants 104 Nineteen soccer players participated in the study ( The players who were included in the study did not sustain leg injury in the last 111 six months prior to the day of testing. All participants gave their written informed consent 112 to participate in the study. The study followed the guidelines of the Declaration of 113 Helsinki and has been approved by the ethics committee of the **blinded for peer 114 review**.
115 Study Design

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In the Bulgarian squat exercise (Fig 2), significant differences were found between 216 phases (F 10,58 =12.18; p<0.001; partial η 2 =0.68). In all quadriceps muscles, the isometric 217 phase was the one that registered the greatest electrical activity (p=0.01). The ascent 218 phase had greater activation than the descent phase (p=0.03). In the hamstrings, the ascent 219 phase was greater than the other two (p=0.02) in both biceps femoris and semitendinosus. In the lunge exercise (Fig 3), significant differences were found between the 3 224 phases (F 10,54 =16.85; p<0.001; partial η 2 =0.76). In the rectus femoris, vastus medialis and 225 vastus lateralis, the isometric phase had greater activation than the descent and ascent 226 phase (p<0.001), the ascent phase had greater activation than the descent phase (p<0.001).
227 In the biceps femoris, the ascent phase had greater activation than the isometric phase and 228 the descent phase (p<0.001); the descent phase had greater activation than the isometric 229 phase (p<0.001). In the semitendinosus, descent and ascent phase had similar activation 230 (p=0.07). In the squat exercise (Fig 4) there were significant differences between the phases 234 (F 10,46 =8.04; p<0.001; partial η 2 =0.64). In the rectus femoris, the isometric phase had 235 greater activation than the descent and ascent phase (p=0.04), there were no significant 236 differences between ascent phase and descent phase (p=0.09). In the vastus medialis the 237 isometric phase had greater activation than the descent phase (p<0.001), there were no 238 significant differences between the isometric phase and the ascent phase (p=0.25), the 239 ascent phase had greater activation than the descent phase (p=0.04). In the vastus lateralis, 240 the isometric phase had greater activation than the descent phase (p= 0.02), the ascent 241 phase had greater activation than the descent phase (p=0.03), there was no difference 242 between the isometric and ascent phase (p=0.87). In biceps femoris and in the 243 semitendinosus, the ascent phase had the highest activation (p<0.001). In the hamstrings, the greater intra-muscular activation (Fig 1) of the 282 semitendinosus muscle compared to that of the biceps femoris account for its 283 predominance (59, 60). A balance in the synergy of the hamstring muscles is important 284 in the reduction of risk of a potential injury (60). This synergy helps to reduce the tension 285 on the biceps femoris (19, 61), which plays a fundamental role in the hamstring injury 286 since biceps femoris is affected in 80% of all hamstring lesions (62). A low hamstring to 287 quadriceps ratio is believed to be a risk factor for injuries (63) and many rehabilitation 288 and prevention programs focus on strengthening the hamstrings. The heterogeneity of 289 activation pattern of different exercises must be taken into consideration when developing 290 hamstring strength (30), and these exercises help in the coactivation of the hamstring and 291 quadriceps muscle groups.
292 Activation during the movement phases 293 Differences were found between the different phases of the 3 exercises (Fig 2-4), 294 for quadriceps muscles, the greatest activation was recorded in the isometric phase, while 295 in the hamstrings the greatest activation was in the ascent phase, or knee extension phase.

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In the quadriceps, in all the muscles and exercises (Fig 2-4)

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The isometric phase showed that rectus femoris presented the least activation in 310 the quadriceps in all exercises, giving predominance in the stabilization of movement to 311 the vastus medialis and vastus lateralis (67).

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For hamstrings muscles, there was greater activation in all phases in the Bulgarian 313 squat (Fig 2) than in the normal squat (Fig 4), which may be due to a greater demand for 314 co-activation of the posterior leg musculature to keep joints and the body stable (70).