Determining concentric and eccentric force–velocity profiles during squatting

Fifteen resistance trained males (age 24 ± 2 years, body mass 79.8 ± 9.1 kg, height 177.5 ± 6 cm, training age 3.5 ± 1.5 years) volunteered for this study. All participants could demonstrate a good squatting technique, as determined by a qualified strength and conditioning coach, and frequently (> 1 × per week) performed the squat (or variation) within their habitual resistance training practice. Resistance-trained participants were selected for this study to limit the known negative effects of the eccentric neural activation strategy in untrained participants (Aagaard 2018). Prior to participation, written informed consent was completed and this study received ethical approval from Liverpool John Moores University research ethics committee (19/SPS/038).

Participants reported to the Liverpool John Moores laboratories on three occasions. The first and second visits were used for participant familiarisation with the experimental protocols, and to measure body mass (to the nearest 0.1 kg, on electronic scales; SECA, Germany), and height (to the nearest 0.5 cm, with a stadiometer; SECA, Germany). Participants completed a standardised warmup following the RAMP protocol (Jeffreys 2006), which was concluded with several progressively heavier isotonic squats on the Kineo Training System on which all experimental trials were also completed. Following the warmup, participants underwent a familiarisation session inclusive of concentric and eccentric isovelocity squatting. Squat stance was standardised with feet shoulder-width apart and externally rotated ~ 20°. Squatting range of motion was determined, whereby the eccentric phase started with the participant standing with hips and knees fully extended and lasted until the participant had squatted down to a depth where the top of the thigh was parallel to the ground. The concentric phase began after the eccentric phase finished and until the participant had fully extended the hips and knees (Fig. 1). Squatting depth was confirmed by analysis of cable displacement during experimental trials. Pilot data (n = 4) from our laboratory identified that following two familiarisation sessions, forces produced during the − 0.5 m s⁻¹ eccentric trail produced a variation of 1.9%, this is in line with previous research and recommendations (Hahn 2018; McMaster et al. 2014), and therefore, sufficient familiarisation was achieved.

Experimental data were collected during the third visit, 4–10 days following the last familiarisation session. Participants refrained from strenuous physical activity for 48 h prior to testing and were asked to arrive in a fed and hydrated state. Following the standardised warmup, participants completed a total of six maximum effort isovelocity trials at 0.75, 0.5, 0.25, − 0.25, − 0.5, and − 0.75 m s⁻¹, whereby positive and negative values were indicative of concentric and eccentric directions, respectively, with three repetitions per trial.

During concentric trials, participants began by standing with the hips and knees extended, performed a submaximal (~ 80% perceived effort) eccentric isovelocity squat at − 0.25 m s⁻¹, immediately followed by a maximum effort isovelocity concentric squat at the prescribed trial velocity (Fig. 2). Participants were provided visual feedback to ensure they produced an effort of 80% during the submaximal eccentric phase, with this value having been identified during the second familiarisation session.

During eccentric trials, participants began by standing with the hips and knees extended, performed a submaximal eccentric isovelocity squat (~ 80% perceived effort, − 0.25 m s⁻¹), followed by a near-maximal concentric isovelocity squat (~ 90% perceived effort, 0.25 m s⁻¹), before performing the maximal effort isovelocity eccentric squat for which data were recorded (Fig. 3). Visual feedback was provided as per the concentric trials. The near-maximal concentric effort immediately prior to the maximal eccentric effort ensured preload on the musculature, which is required for maximal eccentric efforts (Hahn 2018; Linnamo et al. 2006). During the maximal eccentric trial, the participant maximally resisted the downwards displacement of the external cable at the respective velocity until the end of the range of motion. Three repetitions were completed at each velocity, with 5-min passive rest between each trial.

During all trials, ground reaction forces (N) under each foot were collected via a dual force plate system (9287c, Kistler, Switzerland), sampling at 2000 Hz. Analogue signals were amplified and converted to a digital signal prior to being collected in Qualisys Track Manager (Qualisys, Sweden) and then exported to Visual 3D (C-Motion, USA) for subsequent analysis. The greatest peak vertical ground reaction forces from each of the six experimental conditions (Concentric; 0.75, 0.5, and 0.25 m s⁻¹, Eccentric; − 0.25, − 0.5, & − 0.75 m s⁻¹) were used for analysis. Ground reaction forces were then processed via a fourth-order Butterworth filter with a cutoff frequency of 6 Hz; then, the forces of the dominant and non-dominant limb were summed together.

During these trials, only forces that occurred during the isovelocity phase of the squat were used, which were defined from the measured movement velocity profile. To confirm actual squat velocity for each defined trial, reflective markers were placed on the cables which attached the participant to the Kineo Training System, and monitored by three 3D motion capture cameras (Opus 3 series, Qualisys, Sweden), sampling at 200 Hz.

Forces were plotted against the target velocity to create F–V relationships for each participant. Forces were normalised against a predicted isometric force. A joint-angle specific maximum isometric force could not be measured as peak forces occur at different joint angles during the concentric and eccentric phase (Melo et al. 2016), and between participants. Instead, isometric force was calculated for zero velocity from a cubic polynomial regression equation fitted to each participant’s measured force–velocity profile. Calculating isometric force in this manner has been previously used (Morin and Samozino 2016; Samozino et al. 2010) and shown to be robust.

All data were statistically analysed using SPSS (version 26, IBM, USA). A one-way repeated measures ANOVA with six factor levels was used to test for differences in the peak force from each velocity. As there was a violation of sphericity (P < 0.001), a Greenhouse–Geisser correction was used (Atkinson 2001). A two-way repeated measures ANOVA (2 × 6) was used to test for differences between the dominant and non-dominant limbs. Finally, a one-way repeated measures ANOVA was used to tests for differences in the squat depth (%) at which peak force occurred (whereby 0% is the position at which the hips and knees are fully extended, and 100% is the position at which the thighs were parallel to the ground). Significance was accepted when P < 0.05. Statistically significant results underwent a Holm–Bonferroni post hoc analysis. All data are presented as mean ± SD, unless otherwise stated. Correlation analysis was performed to determine if maximal concentric strength influenced eccentric force production. Absolute and normalised peak force from all eccentric trials (− 0.25, − 0.5, and − 0.75 m s⁻¹) were correlated against the trial in which the greatest concentric force was produced (0.25 m s⁻¹).

Coefficients of variation and intraclass correlation coefficients were performed to identify the reliability of ground reaction forces between repetitions at each velocity. Intraclass correlation coefficient was interpreted in line with recent guidelines (Koo and Li 2016). Lastly, Bland–Altman analyses (Bland and Altman 1986) and limits of agreement (LOA) were used to identify if the measured velocity differed from the target velocity.

There was large inter-participant variability between the eccentric forces produced. Analyses of individual data revealed that some participants did not produce eccentric forces greater than isometric (Fig. 5). Table 1 summarises the characteristic differences between those individuals who had no eccentric increase (normalised eccentric force ≤ 1.0 across all trials) and those who had an eccentric increase (> 1.0). No significant differences were found between groups (P = 0.059–0.971), although the no eccentric-increase group tended to be taller and heavier.

In addition, there was a modest to high positive correlation between absolute peak concentric force (0.25 m s⁻¹) and absolute peak eccentric force (− 0.75 m s⁻¹; r₁₅ = 0.544, 95% CI of Δ = 0.04 to 0.83, P = 0.036, − 0.5 m s⁻¹; r₁₅ = 0.745, 95% CI of Δ = 0.38 to 0.91, P = 0.001, − 0.25 m s⁻¹; r₁₅ = 0.738, 95% CI of Δ = 0.36–0.91, P = 0.002) (Fig. 6A). However, there was no significant correlation between absolute peak concentric force (0.25 m s⁻¹) and the isometric-normalised eccentric force (− 0.75 m s⁻¹; P = 0.757, − 0.5 m s⁻¹; P = 0.19, − 0.25 m s⁻¹; P = 0.628) (Fig. 6B).

Analysis of repetition-to-repetition variation of vertical ground reaction forces within each velocity identified acceptable coefficients of variation (6.1–9.2%) and intraclass correlation coefficients (0.84–0.93) (McMaster et al. 2014). These values are similar to those reported for both traditional (Fairus et al. 2016), and isometric squats (Palmer et al. 2018). Finally, measured cable velocity was, on average ~ 0.02 m s⁻¹ greater than that of each target velocity (P < 0.001, LOA = 0.002:0.037) [exact velocities (and percentage difference) are as follows; − 0.767 m s⁻¹ (2.2%), − 0.519 m s⁻¹ (3.7%), − 0.261 m s⁻¹ (4.3%), 0.268 m s⁻¹ (6.9%), 0.522 m s⁻¹ (4.3%), and 0.769 m s⁻¹ (2.5%)]. Therefore, the conclusions would be the same if we had used measured velocity rather than target velocity in our calculations.

When exploring our data, it becomes evident that a greater variation existed amongst the eccentric trials than amongst the concentric trials (see the standard deviations in Fig. 4). At − 0.75 m s⁻¹, the normalised force ranged from 0.84 to 1.62 around a mean of 1.1, indicating that although many individuals generated the physiologically expected eccentric force above isometric (Fig. 5B), some did not (Fig. 5A), even though all participants were familiar with resistance training and the squat. This large inter-participant variability was still apparent even when excluding the participant who achieved 1.6 × isometric. Variability between individuals in the ability to produce eccentric moments has been reported previously during knee extension/flexion (Hahn 2018). Group mean knee moments were reported as 1.2 × isometric, however, some individuals were shown to be capable of producing moments 1.8 × isometric (Hahn 2018). Therefore, our measurement of a maximal eccentric force of 1.6 × the isometric agrees with the limited previous data.

We assessed whether the individual ability to generate eccentric forces was associated with overall squatting ability. However, the normalised eccentric force was not correlated with absolute concentric force (Fig. 6B), and so we find that strength itself did not determine whether an individual produced eccentric forces greater or less than isometric. This is supported by previous research that has shown that we cannot accurately predict eccentric strength from a concentric strength test (Harden et al. 2019). Therefore, we reject hypothesis 2. Previous training interventions have demonstrated that eccentric-specific resistance training causes an increase in eccentric force production (Seger et al. 1998; Spurway et al. 2000), probably due to movement-specific improvements in muscle activation and a greater lengthening of muscle fascicles (Franchi et al. 2014). Therefore, we would expect individuals with a history of eccentric-specific resistance training to display a greater normalised eccentric force. However, although all participants had a history of resistance training (3.5 ± 1.5 years) and could squat a minimum of 1.4 × body weight (Table 1), none of these participants had a notable history of eccentric-specific resistance training, so this does not explain the variances found in this study.

There are other factors that may influence eccentric squatting force that this study did not specifically assess, but comparison of the sub-groups (Table 1) may offer some insight. Although all participants squatted to a depth where the centre of the hip was below the centre of the knee, squatting technique varies between individuals (Myer et al. 2014). Although not statistically significant, the group that had no eccentric increase was taller. Taller individuals may adopt a more hip-dominant squatting technique to counter balance (Myer et al. 2014), and this technique can change between the concentric and eccentric phase. This may result in differing hip, knee, and ankle joint ranges of motion, which would influence the amount of force exerted into the ground (Beckham et al. 2018), and thus the profile of the F–V relationship. Future research may explore the effects of height and limb lengths on the joint contributions and their effect on exploiting the greater strength capacity of eccentric squatting.

Another factor worth considering is the ability to activate the musculature during eccentric squatting. This could explain the large inter-participant variability. Although it was not assessed in this study, the ability to activate skeletal muscle has been shown to correlate with force production (Folland et al. 2014). Eccentric contractions have a unique neurological activation, compared to concentric/isometric (Enoka 1996) and activation capacity is known to differ between individuals (Avrillon et al. 2021), furthermore eccentric activation can be trained (Aagaard et al. 2000). Therefore, measurements of eccentric activation should be included in future research.

Regardless of the reasons why certain individuals were able to produce more or less eccentric normalised force, practitioners and researchers need to be aware that this variability is real and of a large magnitude, and future research on eccentric-specific resistance training should take this into consideration.

Our data suggest that maximal concentric strength does not influence the ability to maximise relative eccentric force production, and therefore practitioners should attempt to measure eccentric capability prior to prescribing eccentric-specific resistance training rather than relying on standardised loads relative to concentric maximums (Harden et al. 2020). This lack eccentric-specific assessment, and thus individualisation of training programs, may explain why the efficacy of eccentric-specific resistance training (e.g. accentuated eccentric loading) has been debated in the past (Douglas et al. 2018).

In many applied settings, eccentric-specific assessment is achieved under external load (Harden et al. 2019), which will dictate movement velocity, rather than the imposition of isovelocity movements. This presents practical challenges if done using traditional weightlifting techniques. In this study, however, the Kineo training system proved effective in delivering the fast eccentric squatting efforts required to identify a plateau in eccentric force, allowing individualised F–V profiles to be developed. However, for practitioners that do not have access to this equipment, field-based assessment of eccentric capabilities may need to be developed to individualise eccentric-specific resistance training. Future research should also examine the effects of different eccentric protocols on movement velocity, and subsequent force production, and the ability of resistance training interventions to train and improve these aspects of performance.

Although this study focussed on establishing underpinning knowledge of the multi-joint force velocity curve, some findings may be extrapolated to applied practice. In applied settings, AEL is often coupled with a slower eccentric velocity, which results in an eccentric phase duration of 3–4 s (Harden et al. 2020), equivalent to a velocity slower than − 0.25 m s⁻¹. However, our data suggest that at this slow eccentric velocity, AEL squatting may only provide a 2% benefit in terms of peak forces imposed on the body, when compared to maximal effort traditional squatting. In contrast, the data reported here demonstrate that many trained individuals are able to generate larger forces and experience greater training loads in faster eccentric trials, which may provide an accentuated stimulus for adaptation. However, future studies will need to be performed to confirm this.

Lastly, there are some limitations to this study. First, this study did not assess the kinematics of squatting, and as such, we do not know the joint angles at which peak ground reaction force occurred, nor do we know if taller individuals adopted a different squatting technique (as proposed in the individual differences section). An analysis of squatting kinematics could solve these limitations, and therefore, should be included in future research. Second, this study only assessed three eccentric velocities, and found that peak eccentric force occurred between − 0.5 and − 0.75 m s⁻¹. Therefore, future research should examine the effects of a greater range of velocities (targeted at the faster velocities) on ground reaction forces, and attempt to identify the participant characteristics that determine individual differences.