Improvements in Maximal Strength Are Not Associated With Improvements in Sprint Time or Jump Height in High-level Female Football Players: A Cluster-randomized Controlled Trial

In male football players, studies have reported a favorable effect on both 1RM, sprint and jumping performance following MST. Although females and males possess diverse levels of anabolic hormones, they do in general respond similarly after training interventions in most strength outcomes. However, there are reports of a larger relative increase in females compared to males when the same ST is applied.

In this cluster-randomized controlled trial, two football teams (playing at level 2 and 3 in Norway) was invited to participate. The study were conducted during the last part of the pre-season preparations, ending one week before first seasonal competition. The training group (TG) performed MST training carried out as free-barbell squats twice a week over five weeks in addition to the planned pre-season training, while the control group (CG) was instructed to perform their originally planned pre-season training.

The total sample comprised 46 players aged 15-26 years, where two separate football teams were cluster-randomized to either TG or CG. The two teams played at level two and three in Norway, where level two is a national league and level three a regional league in Northern Norway. Inclusion criteria was that the players perceived themselves as injury free and able to complete the strength training. Players were only excluded if having injuries that made strength training, running and jumping unachievable. The players carried out ̴ 6.5 h training per week with their team. Four players were injured, two did not complete the required amount of training, one withdrew due to time limitations and five withdrew without providing any reason resulting in 19 participants in TG and 15 in CG that completed both pre- and posttests, and were included in the analyses for training effect.

According to the declaration of Helsinki, all participants were fully informed of the potential benefits and risks of the study, both orally and written, before signing an informed consent. For participants under 16 years, both the players and their parents gave their written informed consent. The participants were fully informed of their rights to withdraw from the study at any time without providing any reason.

All testing and training sessions were conducted in an exercise training laboratory at Alfheim Stadium, Tromsø. Prior to the intervention, the players underwent baseline tests over two test days, with a 72 hour washout period to avoid any detrimental effects from the preceding test day: day 1) measurement of body mass and body height, 5-, 10- and 15m sprint time and a counter-movement jump (CMJ), day 2) 1RM in a free-barbell squat exercise with partial 90° knee angle range of motion (ROM).