– van den Tillaar R, Andersen V, Saeterbakken AH
13 resistance-training males (aged 24.2 ± 2.0 years, body mass 81.5 ± 9.1 kg, height 1.78 ± 0.06 m) with 6 ± 3 years of resistance-training experience conducted squats with 30%–100% of 1-RM
– Barbell kinematics and electromyographic (EMG) activity of the vastus lateralis, vastus medialis, rectus femoris, semitendinosus, biceps femoris, and gluteus maximus were measured in the upward phase of each load
1RM Before Testing
– Before starting the tests using the different loads, 1-RM in free-weight back squat was performed
– After the final warm-up set, the load was increased to approximately 95% of the participants’ self-reported 1-RM. The load was then increased by 2.5–5.0 kg until failure
– Failure was defined by the following criteria: 1) the participants failed to complete a lift, 2) the participants could not complete the lift with proper technique, or 3) both the participant and the test leader agreed that the participant would not be able to lift 2.5 kg more
– The 1-RM was achieved within 2–4 attempts
– Each attempt was separated by a pause of 4–5 minutes
– After the final 1-RM attempt, a 10-minute pause was given before starting the testing using the different loads
– The protocol consisted of 15 repetitions at 30%, 10 repetitions at 50%, and 6 repetitions at 80% of the participants’ self-reported 6-RM loads in squatting
– Using a self-paced but controlled tempo, the participants lowered themselves to 80° knee flexion (180° fully extended knee) measured with a protractor (femur–fibula)
– When the participants had the correct knee angle, a horizontal elastic band was adjusted [2021]
– The participants had to touch the band (mid-thigh) in every repetition before starting the concentric phase
– The loads began from 30%, with 10% increments until 100% of 1-RM, which was based on 1-RM achieved by each participant
– Importantly, the participants were instructed to accelerate the loads in the entire concentric movement, which resulted in a jump using the lowest loads (i.e. 30%-60% of 1-RM)
– The different loads were randomized for each participant, with random order determined by a random number generator
– Two repetitions per load from 30% to 60% were conducted, while from 70% to 100%, 1 repetition per load was performed
– A rest of 3–5 minutes was given between each attempt 
– With increasing loads, the barbell velocity decreased, the upward phase duration increased, and the peak velocity occurred later
– The muscle activation in all muscles increased with increasing loads but was not linear
– In general, similar muscle activation in the prime movers was observed for loads between 40% and 60% of 1-RM and between 70% and 90% of 1-RM, with 100% of 1-RM being superior to the other loads when the loads were lifted at maximal intended velocity
– The timing of maximal muscle activations was not affected by the different loadings for the quadriceps, but the timing was sequential and independent of loading (rectus femoris before vastus medial before vastus lateral)
– The maximal activation in the gluteus maximus and semitendinosus increased with increasing loads
– In general, the muscle activation in all muscles increased with increasing loads but was not linear
EX: there were no differences between loads 30%–90% of 1-RM in vastus lateral, 40%–60% and 70%–90% in vastus medial, and 50%–90% in rectus femoris. However, in the quadriceps, greater muscle activation was observed performing 1-RM compared to the other loads.
– Greater muscle activation was observed for the loads 80%–100% of 1-RM, but only compared to the loads 30%–60% of 1-RM
– These results corroborate those by Yavuz and Erdag [17] and Gomes [18], who also found increases in gluteus maximus activity with increasing loads
– The gluteus maximus demonstrated differences in the timing of maximal activation between 30% and 50% of 1-RM and from 50% and 90% of 1-RM
– The change in timing may be the result of lower lifting velocity with increasing loads
– The participants were more dependent on the contributions and coordination between the different prime movers, in contrast to lighter loads where the participants had a rapid acceleration from the lowest position
– The results for the vastus lateral were in contrast to previous findings
– Gomes [18] reported an increase comparing 60%–90% of 1-RM loads, and Yavuz and Erdag [17] reported increases in the vastus medial between 80% and 90% of 1-RM
– The discrepancy in findings with these previous studies could be the result of experience (3 yrs vs. 6 yrs of resistance training experience) and strength level (107 and 120 kg as 1-RM compared with 130 kg in 1-RM in the present study)
– One could speculate that greater performance level in the present study could be related to longer training experience. This might suggest a better muscle recruitment and firing frequency strategy testing the spectrum of different loads [3637] and explain the inconsistent results compared to previous studies [1718]
– This is one of the few studies examining muscle activation with increasing loads in squats where the lift was a maximal intended velocity over a large spectrum of loads
– Our study corroborates results found by McBride [4], who examined vastus lateral activation in squats using 70%, 80%, and 90% of 1-RM; The participants were instructed to lift at maximal intended velocity; Even though the aim of that study was not to compare the muscle activation between loads, the difference between 70% and 90% of 1-RM was only 1.3%
– We found a sequential and significant difference in maximal peak activation between the quadriceps starting with rectus femoris, vastus medial, and then vastus lateral. The peak activation pattern was independent of loads and fairly constant (see Fig 4). Peak activation occurred at approximately 85°–103° knee flexion, as shown by van den Tillaar [15].
– The findings were partly supported by a previous study by Escamilla et al. [40]. They demonstrated a peak activation at approximately 100°–110° knee flexion for the quadriceps muscles, examining 12-RM loads among experienced participants; However, the 12-RM loads were lifted in a slow and continuous manner (1–1.5 seconds in the upward phase), which may explain the minor variation in peak activation; Yet Escamilla et al. [40] did not report any differences in maximal timing between the quadriceps muscles
– The quadriceps muscles component may provide a different contribution to knee extensor torque due to their anatomical structure [3031]
– The rectus femoris has a bi-articular function as a hip flexor and knee extensor [3941] and may therefore be the first muscle to activate to stabilize the hip
– A later timing of peak activation may thereby increase the torque of the hip.
– For the antagonist biceps femoris and semitendinosus in the upward movement, there were no differences between the loads 60%–100% of 1-RM
– Still, lifting 70%–100% of 1-RM demonstrated greater muscle activation than the lowest load (30% of 1-RM)
– The results were not surprising in terms of hamstring muscles being an antagonist in the upward movement and therefore to a lesser extent being affected by the loading
– Increased muscle activation above 70% 1-RM may be the result of co-contraction to stabilize the knee and pelvis in the turnover from eccentric to concentric movement
– The hamstring muscles contribute to avoiding a forward rotation of the pelvis
– While increased activation of the rectus femoris would increase hip flexor torque, hamstring activation may be of more importance as loads increase and lifting velocity decreases
– Several studies have examined the peak hamstring muscles (biceps femoris and semitendinosus) and reported the peak to be between 110° and 130° knee flexion [3940]
– The findings of the present study support these previous studies
– However, the biceps femoris demonstrated similar maximal timing across the loads, while the semitendinosus had significant later maximal timing between 50% and 80% of 1-RM
– Greater coactivation with the heaviest loads (> 80% of 1-RM) may avoid a hip flexion torque caused by the rectus femoris activation with increasing loads [3942]
– The study included resistance-trained males, and the results may therefore not be generalized to other populations
– Resistance-trained athletes may decrease the loads but have similar muscle activity when lifting with maximal lifting velocity
– By decreasing the loads, the mechanical stress decreases and time to recover is reduced
– Using lower loads with maximal lifting velocity may therefore allow athletes to increase the total volume without increasing the risk of injuries
– With the exception of the heaviest load (1-RM), the prime movers (quadriceps and gluteus maximus) have similar muscle activations between 70% and 90% of 1-RM and between 40% and 60% of 1-RM
– Therefore, athletes and trainers could vary the loading within the load windows and expect the same effect
– This is important in regard to accommodating athletes’ preferences
– The force requirement differs during different tasks/sports, and a variation in the loading could help to deal with this differentiation

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