Feasibility of Velocity Loss-Based Volume Prescription During Constant-Load and Chain-Based Smith Machine Back Squats

The goal of this observational study was to evaluate the feasibility of percentage of velocity loss (%VL)-based approaches for prescribing training volume during constant-load and chain-based Smith machine back squats. The study aimed to answer whether %VL can be expressed as the maximum number of repetitions (%Rep) in constant-load and chain-based Smith machine squats. Twenty-six resistance-trained men performed repetitions-to-failure tests under variable (VRT) and constant resistance training (CRT). The %Rep attained before reaching different levels of effort were compared between VRT and CRT across various loads and sessions. Furthermore, this study evaluated the influence of different factors on the predictive accuracy of the %Rep-%VL relationships. These findings will provide evidence-based evidence in prescribing and monitoring intra-set volume.

Study Overview

• Status: Completed
• Conditions: Loading Configuration; Levels of Effort
• Intervention / Treatment: Other: variable resistance training; Other: constant resistance training

Detailed Description

The goal of this observational study was to evaluate the feasibility of percentage of velocity loss (%VL)-based approaches for prescribing training volume during constant-load and chain-based Smith machine back squats. The study aimed to answer whether %VL can be expressed as the maximum number of repetitions (%Rep) in constant-load and chain-based Smith machine squats. Twenty-six resistance-trained men performed repetitions-to-failure tests at 85%, 75%, and 65% of their constant-load one-repetition maximum (1RM) under variable (VRT; 40% of the total external load provided by chains) and constant resistance training (CRT; 0% chains). The %Rep attained before reaching low (10% VL), moderate (20% VL), and high (30% VL) levels of effort were compared between VRT and CRT across various loads and sessions. Furthermore, this study evaluated the influence of regression type (individual vs. general), load specificity (load-specific vs. multiple-load), and loading configuration (CRT vs. VRT) on the predictive accuracy of the %Rep-%VL relationships established in session 1 for estimating %Rep in session 2. These findings will provide evidence-based evidence in prescribing and monitoring intra-set volume.

• Study Type: Observational
• Enrollment (Actual): 26

Contacts and Locations

Study Locations

• China
  • Guangdong
    • Guangzhou, Guangdong, China, 510006
      • School of Physical Education and Sports Science, South China normal university

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: Yes

• Sampling Method: Non-Probability Sample

Study Population

Participant characteristics (mean ± standard deviation [range]) included: age, 20.4 ± 2.1 (18-25) years; body height, 1.78 ± 0.05 (1.68-1.87) m; body mass, 71.7 ± 7.5 (60.0-91.4) kg; training experience, 3.5 ± 1.8 (1-10) years; and constant-load 1RM, 135.4 ± 17.4 (95-163) kg, corresponding to 1.89 ± 0.21 (1.58-2.28) times body mass.

Description

Inclusion Criteria:

• at least 1 year of resistance training experience, performing back squats at least once per week.
• proficiency in the Smith machine back squat executed at maximal intended velocity.
• a constant-load 1RM of ≥ 1.5 times their body mass.
• no reported musculoskeletal injuries in the past three months.

Exclusion Criteria:

• Voluntary withdrawal or injury.
• Not satisfied with inclusion label.

Study Plan

How is the study designed?

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
The %Rep attained before reaching the 10%, 20%, and 30% VL thresholds	The %Rep attained before reaching the 10%, 20%, and 30% VL thresholds was recorded across all loads, sessions, and loading configurations.	April 1st, 2026
The %rep predictive accuracy	To assess predictive accuracy, second-order polynomial regression equations were generated from the first session of each loading configuration using all repetitions within a set, setting %Rep as the dependent variable and %VL as the independent variable. These equations were constructed using different regression types (individual vs. general), load specificities (load-specific vs. multiple-load), and loading configurations (CRT vs. VRT). During the second session, the actual %VL recorded after reaching the 10%, 20%, and 30% VL thresholds was substituted into the corresponding first-session equations to derive the predicted %Rep. Then, the absolute estimation error was calculated as (|actual %Rep - predicted %Rep|). In cases where muscular failure occurred before a specific %VL threshold was reached (e.g., 30% VL), the %VL of the final completed repetition was recorded, and the corresponding actual %Rep was defined as 100%.	April 1st, 2026

Collaborators and Investigators

• Sponsor: South China Normal University

Publications and helpful links

Helpful Links

• Related Info

Study record dates

Study Major Dates

• Study Start (Actual): October 7, 2025
• Primary Completion (Actual): January 10, 2026
• Study Completion (Actual): May 1, 2026

Study Registration Dates

• First Submitted: July 15, 2026
• First Submitted That Met QC Criteria: July 18, 2026
• First Posted (Actual): July 22, 2026

Study Record Updates

• Last Update Posted (Actual): July 22, 2026
• Last Update Submitted That Met QC Criteria: July 18, 2026
• Last Verified: July 1, 2026

More Information

Terms related to this study

• Keywords: training volume; velocity-based training; variable resistance training; level of effort; lifting velocity
• Other Study ID Numbers: SCNU-SPT-2025-055

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: YES
• IPD Plan Description: The data that support the findings of this study are available from the corresponding author upon reasonable request.
• IPD Sharing Time Frame: Individual participant data and supporting documents will become available after publication of the study results and will remain available for 5 years.
• IPD Sharing Access Criteria: Individual participant data (IPD) will be shared in anonymized form with qualified researchers for scientific purposes. The shared data will include lifting velocity of each repetition completed.
• IPD Sharing Supporting Information Type: STUDY_PROTOCOL; SAP; CSR

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No