We found that neuromuscular activation of trunk stabilizer muscles and vastus lateralis increased in a single set of back squats to volitional fatigue at 85% SM. This was significant for all muscle sites and phases apart from RA in the eccentric phase. Neuromuscular activation was higher in NG compared to EG in EO, LSES and ULES in eccentric phase and LSES in concentric phase. Participants in each group were significantly different according to squat training experience (NG: 2.4 vs EG: 6.4 years), absolute squat 1RM test performance (NG: 96.2 vs EG: 156.3 kg), relative squat 1RM test performance (NG: 1.2 vs EG: 1.8 kg.kg−1) and test load at 85% SM (NG: 71.3 vs EG: 121.3 kg) representing the wide range of potential athletes and exercisers in the applied training environment. Despite this, there were no group differences in number of completed repetitions to failure (NG: 12 vs EG: 10 reps) nor RPE (NG: 8.2 vs EG: 8.5 RPE) after performing a single set to volitional fatigue at 85% SM.
RMS in a single fatiguing set at 85% SM
Understanding acute neuromuscular response of trunk stabilizer muscles in a fatiguing set of barbell back squat repetitions has particular relevance for applied strength training. We know that activation of prime movers increases with acute fatigue during the squat to maintain power (Pick and Becque
2000; Brandon et al.
2014) and this study shows that the same applies for the trunk stabilizers. It is also established that stronger participants elicit increasingly higher prime mover activation across the duration of a squat set to failure (Pick and Becque
2000). In contrast, activation of trunk stabilizers was higher in weaker squat participants and this was significant in EO, LSES & ULES in the eccentric phase and LSES in the concentric phase. Furthermore, we did not find a group difference for vastus lateralis activation, as reported by Pick and Becque (
2000). There were study design differences that may explain the contrast in findings. These authors normalized RMS in the single set to failure to RMS obtained in squat 1RM test conducted on a separate day, which required re-application of electrodes (Pick and Becque
2000). We normalized RMS in the fatiguing set to RMS in the concentric phase of the warm-up set at 65% SM in the same session according to Balshaw and Hunter (
2012). Re-application of electrodes for between-day EMG capture increases EMG variance compared to within-day measurement (Dankaerts et al.
2004).
We have previously shown lower mean activation of trunk muscles in stronger compared to weaker participants during three repetitions of the squat at moderate (65 and 75% SM) and heavy (95% SM) relative loads (Clark et al.
2020). This was significant for all loads in eccentric phase and at 95% SM in the concentric phase. Absolute and relative squat loads were significantly higher in the stronger group who had a significantly greater mean squat training age of 4 years. It has been shown that increases in back squat strength from medium to long term progressive load squat training is achieved primarily through increased prime mover strength (Pick and Becque
2000), enhanced trunk stabilization at lower neuromuscular activation levels contributes to the improved squat performance. Trunk muscle adaptation ensures that trunk integrity is maintained at the increased absolute squat loads by preventing trunk flexion, especially in the more demanding concentric phase of the exercise. This study demonstrated that this efficiency, acquired through training translates to submaximal multiple repetitions to volitional fatigue, similar to the configuration in applied resistance training. This differs to the impact of training status on the response of prime movers under the same submaximal fatiguing conditions (Pick and Becque
2000). In both prime movers and trunk stabilizers, activation increases with fatigue; however, in stronger participants prime mover activation is greater than weaker participants throughout the set. Progressive strength training increases the capacity to activate a greater percentage of motor units in agonist muscles which results in increased absolute force producing capability (Brandon et al.
2014). This translates to greater relative submaximal force and greater relative performance in a fatiguing set of resistance training repetitions to failure. Under these conditions the role of trunk stabilizers is to maintain trunk stiffness and an upright posture to ensure that centre of mass remains over the base of support. This study confirms that acquired squat strength through training increases this capability with reduced trunk muscle activation.
The challenge placed on the trunk in both squat phases is predominantly to maintain extension of lumbar spine and avoid flexion in thoracic vertebral region (Myer et al.
2014). This corresponds with the reported high activation levels of the posterior trunk stabilizers in both phases of squats and in response to load increments. However, the role and therefore activation of RA and EO are not that obvious. Our findings demonstrating activation of both muscles (RA and EO) increase with load in both phases and is higher in the concentric phase, suggests that anterior and lateral trunk muscles contribute to stabilizing the spine and trunk in both the squat descent and ascent (Clark et al.
2016,
2019b,
2020). As a result, it is surprising that RA activation in this study did not increase in the eccentric phase during the repetitions to volitional fatigue.
In this study we found no group differences between experienced and novice participants in vastus lateralis RMS previously reported (Pick and Becque
2000), but our findings were similar in that vastus lateralis activation in both groups did increase for the duration of the fatiguing repetitions. A fundamental difference between the studies was the lower absolute squat strength of both our groups compared to Pick and Becque (
2000). However, our study comprised more than double the number of participants a far wider range of squat strength and, therefore, more power. Furthermore, this is in agreement with Shimano (
2006) who reported no difference in completed repetitions at a range relative intensities in the back squat based on resistance training status (Shimano et al.
2006).
Vastus lateralis eccentric RMS at 85% SM was lower than concentric RMS in the set to failure and lower than the submaximal reference value calculated in the concentric phase at 65% SM used for normalization. Eccentric vastus lateralis RMS in the set to failure was normalized to the mean concentric RMS for three repetitions at 65% SM during the warm-up. It is well established that concentric RMS is higher than eccentric RMS for any load of the squat exercise (Escamilla et al.
1998; Balshaw and Hunter
2012), and the leg press (Komi et al.
1987; Sarto et al.
2020). Sarto et al. (
2020) demonstrated that eccentric RMS was the same for leg press at 70 and 80% 1RM and the only method of significantly increasing eccentric activation was by overloading the eccentric phase by 150% (Sarto et al.
2020). In the squat, we have previously shown that absolute unnormalized eccentric RMS (mV) is 42% lower than concentric RMS at 80% SM (Balshaw and Hunter
2012). The anomaly may be explained by higher activation in the concentric phase at 65% SM due to the novelty of the first loaded squat repetitions in the warm-up compared to fatigue-induced lower recruitment of larger motor units in the single set to failure at 85% SM.
Previous work from our laboratory showed that well-trained participants were not able to maintain initial barbell velocity in heavy load (85% SM) back squat as repetitions as sets progressed (Brandon et al.
2014). They demonstrated that this decrement in velocity and, therefore, power occurred despite a significant increase in activation of vastus lateralis. We demonstrated a similar increase in activation of the trunk stabilizer muscles also under these conditions; however, the effect of training status was the opposite, and stronger participants had lower activation than weaker participants at the same relative load. Or more importantly, reported adaptations in prime movers in response progressive load squat training is now supported by evidence of adaptations in trunk stabilizers to explain improved squat performance. Furthermore, we have shown that participants with higher squat strength produced significantly higher squat and countermovement jump heights with significantly lower activation of the trunk stabilizers during the concentric and flight phases (Clark et al.
2019a). Trunk stabilizer adaptation to progressive load squat training contributes significantly to improved squat performance by increasing trunk stability and stiffness at more efficient levels of neuromuscular activation.
Percentage 1RM is a common method of manipulating acute resistance training intensity and 85% 1RM is reported to relate to approximately 6RM (Haff and Triplett
2016). We found an average of 11 repetitions to failure (Range: 5–22) at 85% 1RM back squat for all participants combined (NG and EG) with no significant difference between groups (Table
1). The wide range of repetitions completed to failure at 85% 1RM challenges the traditional relationship between percentage 1RM and RM, and it appears training status does not affect accuracy. Others, however, have found a significant difference in completed squat repetitions at 85% 1RM between trained (10 repetitions) and untrained groups (7 repetitions) (Pick and Becque
2000). They had fewer participants (Trained:
n = 9 vs 19, Untrained:
n = 7 vs 21), who were significantly stronger according to mean squat 1RM, (Trained: 184 vs 156 kg, Untrained: 120 vs 96 kg). The contributes to the growing challenge directed at the repetition maximum continuum (Haff and Triplett
2016; Fisher et al.
2020), specifically the accepted relationship between load calculated as a percentage 1RM and the repetition maximum method of load assignment (Hoeger Werner et al.
1990; Shimano et al.
2006). Our data also support evidence that training status does not influence the relationship between the relative 1RM and RM method of load calculation in the squat exercise (Hoeger Werner et al.
1990; Shimano et al.
2006; Mann et al.
2010).