Anterior-posterior ground reaction forces in free-weight vs. Smith machine squats: Are differences modulated by load and volume?

The back squat is likely the preferred choice for increasing lower-body maximal strength. However, there is limited research on how movement restriction during strength training (e.g. free-weight [FW] vs. Smith machine [SM]) affects the anterior-posterior component of ground reaction forces (GRFs). Thirty-five young volunteers (14 female) completed two randomized sessions involving a single set of 10 back squat repetitions with two loads (≈45% and 70% of 1-repetition maximum), differing only in the use of FW in one session and SM in the other. GRFs were assessed using a force plate. Participants generated larger posterior impulse in the SM squat compared to the FW squat during both the braking (-64.9 ± 51.4 vs. -8.1 ± 5.2 Ns) and propulsive (-143.8 ± 95.1 vs. -18.0 ± 8.8 Ns) phases, and this trend was consistent across all ranges of repetitions. The horizontal angle of the GRFs significantly differed between both squat types. The differences between the FW and SM squats became more pronounced with higher loads and throughout the set and were most significant during the propulsive phase. These results suggest that FW squats may favour over SM squats for coaches aiming to improve vertical jump performance.