The progressive barbell-back squat program
During the familiarization session, participants performed exercises under the guidance of two trained instructors. All subjects attended two training sessions in the first week to get familiarized with the equipment and squat techniques, and to ensure they understood the proper form. The instructors used a barbell (20 kg) and weight plates with different loads to measure each participant's one-repetition maximum (1RM) in the second week. Using a multiple-repetitions testing procedure, participants were required to perform 5 sets of 5 repetitions of their maximum load of squats to estimate their 1RM. A jump box was placed behind participants to ensure proper squat form, with their hips and thighs close to the box and their thighs parallel to the ground, while their knees did not extend beyond their toes. The barbell squat movement adopted a high-bar back squat action Before the test, participants had their barbell-back squat technique critiqued and corrected during the first and second familiar sessions in the first week, and became familiar with the equipment. The test was preceded by a warm-up set of 8 repetitions of barbell squats without plates. Participants then performed sets at progressively increasing loads until they failed to complete a valid repetition, judged by their inability to complete the full range of motion. Ideally, subjects failed within 3–5 repetitions during the last and heaviest setAfter the initial familiarization period, each exercise session from the third week onward consisted of six sets of barbell-back squats. Prior to the first set, participants performed a warm-up set of 12 repetitions, starting at 60% of their predicted 1RM, progressing to 10 repetitions at 70% of predicted 1RM in the second set, and finally 8 repetitions at 80% of 1RM in the last four sets. We encouraged participants to aim for one extra repetition in each of the last four sets, and the number of repetitions per set was based on the quality and intensity of their performance as well as their perceived exertion. The total training volume was calculated by multiplying the number of repetitions by the sets performed in 12 training sessions over 6 weeks. A rest period of 2 min was allowed between subsequent sets. For example, in the first formal intervention session (session 1), participant A used barbell weights of 10/12/14/14/14/14 kg, and the repetitions performed under each weight were 12/10/8 + 1/8 + 1/8 + 1/8 + 1. If she completed an extra repetition in each of the last four sets, her next training session would involve using weights of 12/10/10/10/10/10 kg and performing 12/10/10/10/10/10 repetitions. If she was able to achieve this for two consecutive sessions, the weights were increased by 2.5 kg for each set, resulting in weights of 12.5/14.5/16.5/16.5/16.5/16.5 kg and 12/10/8/8/8/8 repetitions per set. All participants’ progression loads were determined according to the progressive resistance training for healthy adults, with the content of the first to fourth sessions in six weeks shown in Table Barbell back squat progression.Note: Repetitions × Sets = R × S, weight unite (kilogram); 1–7 represents each participant; The table shows the training volume (intensity, number of sets and times) completed by 7 participants in the barbell squat group in the first four training sessions.